Meiosis I - Revision history

Nnjm2 at 16:40, 7 December 2018

2018-12-07T16:40:30Z

← Older revision		Revision as of 16:40, 7 December 2018
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	The First Meiotic Division (Meiosis~~ ~~I), sometimes called the reductional division,~~ ~~is the first of two successive~~ ~~nuclear divisions~~ ~~in the process called~~ ~~[[Meiosis\|Meiosis]] to produce 4 genetically different~~ ~~[[Haploid\|haploid]]~~ ~~sex~~ ~~cells. [[Meiosis\|Meiosis]] can only~~ ~~occur in~~ ~~[[Diploid\|diploid]]<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>.~~ ~~Before Meiosis I takes place, the [[DNA\|DNA]]~~ ~~is replicated so single arm [[Chromosomes\|chromosomes]]~~ ~~are now identical and in a pair, one which is inherited from the father and the other inherited from the mother,~~ ~~are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]]~~ ~~are now~~ ~~called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]]~~ ~~to make a pair, joined at the [[Centromere\|centromere]]<ref>~~ALBERT~~, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in~~ ~~[[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the~~ ~~[[Homologous chromosomes\|homologous chromosomes ]](each containing two identical~~ ~~[[Sister chromatids\|sister chromatids]]~~ ~~from the same parent)~~ ~~are~~ ~~separated so both [[Sister chromatids\|sister chromatids]] of each~~ ~~[[Chromosome\|chromosome]]~~ ~~pair~~ ~~goes to each daughter cell. They, therefore, do~~ ~~not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA~~ ~~replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another~~ ~~[[DNA replication\|DNA~~ ~~replication]]~~ ~~and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]]<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] are now identical and in a pair, one which is inherited from the father and the other inherited from the mother, are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]<ref>Albert, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	=== [[Meiosis prophase 1\|Prophase I]] ===		=== [[Meiosis prophase 1\|Prophase I]] ===

	This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]].~~ ~~[[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref>. This is key to producing genetic variability.		This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]]. [[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref>. This is key to producing genetic variability.

	==== ~~'''~~Lepotene~~:'''~~ ====		==== Lepotene ====

	The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.		The duplicated [[Chromosome\|chromosomes]] start to become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

	==== ~~'''~~Zygotene~~:'''~~ ====		==== Zygotene ====

	The [[Homologous chromosomes\|homologous pairs]] start to line up laterally~~ ~~so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure,~~ ~~forms between them called a [[Synapsis\|synapsis]]. The structure the paired~~ ~~[[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]]~~ ~~or bivalent.~~ ~~		The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	==== ~~'''~~Pachytene~~:''' ~~ ====		==== Pachytene ====

	In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete~~ ~~[[Synapsis\|synapsis]], start to [[Crossing over\|cross over]]. ~~ ~~		In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].

	==== ~~'''~~Diplotene~~:'''~~ ====		==== Diplotene ====

	The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].		The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

	In some organisms, prophase I can take years to complete. For example, in female [[Homo sapiens\|homo sapiens]], the development of egg cells start during embryo development, but then pauses after the Diplotene stage until~~ ~~it~~ ~~enters the menstrual cycle many years later~~ ~~to finish development. When the cell pauses for that period of time, the [[Chromosomes\|chromosomes]] de-condense.~~ ~~When they finish development years later, they~~ ~~then re-condense to maximum capacity with the onset of Diakinesis.		In some organisms, prophase I can take years to complete. For example, in female [[Homo sapiens\|homo sapiens]], the development of egg cells start during embryo development but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the [[Chromosomes\|chromosomes]] de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

	The number bivalents is equal to the number of [[Chromosomes\|chromosomes]] in a [[Haploid\|haploid cell of]] that organism. More than one [[Chiasma\|chiasma]] indicates more than one [[Crossing over\|crossing over]] has occurred.		The number bivalents is equal to the number of [[Chromosomes\|chromosomes]] in a [[Haploid\|haploid cell of]] that organism. More than one [[Chiasma\|chiasma]] indicates more than one [[Crossing over\|crossing over]] has occurred.

	==== ~~'''~~Diakinesis~~:'''~~ ====		==== Diakinesis ====

	The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still ~~in tact~~. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.~~ ~~		The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still intact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.

	=== Metaphase I ===		=== Metaphase I ===

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]]~~ ~~going~~ ~~across the cell to and from~~ ~~the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]]~~ ~~of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The [[Tetrad\|tetrads are]] randomly orientated on the [[Metaphase plate\|metaphase plate so]] either the paternal or maternal [[Homologous\|homologue]] can go to the opposite pole. They are also aligned so a [[Homologous\|homologous]] [[Chromatid\|chromatid]] pair face the opposite end.		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The [[Tetrad\|tetrads are]] randomly orientated on the [[Metaphase plate\|metaphase plate so]] either the paternal or maternal [[Homologous\|homologue]] can go to the opposite pole. They are also aligned so a [[Homologous\|homologous]] [[Chromatid\|chromatid]] pair face the opposite end.

	[[Chiasma\|Chiasmas]] help position and stabilise the [[Tetrad\|tetrads]] on the [[Metaphase plate\|metaphase plate]]. If [[Crossing over\|crossing over did]] not~~ ~~occur, then a [[Chiasma\|chiasma]] has not manifested. This is essential to the lining of [[Tetrads\|tetrads]] on the [[Metaphase plate\|metaphase plate and]] so, therefore, they might not be able to separate properly. ~~<br>~~		[[Chiasma\|Chiasmas]] help position and stabilise the [[Tetrad\|tetrads]] on the [[Metaphase plate\|metaphase plate]]. If [[Crossing over\|crossing over did]] not occur, then a [[Chiasma\|chiasma]] has not manifested. This is essential to the lining of [[Tetrads\|tetrads]] on the [[Metaphase plate\|metaphase plate and]] so, therefore, they might not be able to separate properly.

	=== Anaphase I ===		=== Anaphase I ===

	The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads.The 2 pairs of~~ ~~[[Sister chromatids\|sister chromatids]] ,making up~~ ~~each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.~~<br>~~		The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads. The 2 pairs of [[Sister chromatids\|sister chromatids]] , making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.

	=== Telophase I ===		=== Telophase I ===

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly~~ ~~around each of these sets~~ ~~of [[Chromosome\|chromosomes]] and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.~~<br>~~		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of [[Chromosome\|chromosomes]] and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.

	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both~~ ~~containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.~~<br>~~		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.

	=== ~~References~~: ===		=== Reference: ===

	<references />		<references />

170319036: Missed one reference

2018-12-06T18:35:16Z

Missed one reference

← Older revision		Revision as of 18:35, 6 December 2018
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	~~<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>~~The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]]~~ meiocytes cells in the testis~~ and~~ ovaries~~. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] are now identical and in a pair, one which is inherited from the father and the other inherited from the mother, are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]<ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].~~<br>~~		The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]]<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] are now identical and in a pair, one which is inherited from the father and the other inherited from the mother, are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]<ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	=== [[Meiosis prophase 1\|Prophase I]] ===		=== [[Meiosis prophase 1\|Prophase I]] ===

170319036: I corrected the references and some grammar to make it make sense and be clearer

2018-12-06T18:33:31Z

I corrected the references and some grammar to make it make sense and be clearer

← Older revision		Revision as of 18:33, 6 December 2018
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	<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] in a pair, one inherited from the father and the other inherited from the mother, are duplicated. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]~~. ~~<ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp; Francis Group</ref>However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].<br>		<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]] meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] are now identical and in a pair, one which is inherited from the father and the other inherited from the mother, are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]<ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].<br>

	=== [[Meiosis prophase 1\|Prophase I]] ===		=== [[Meiosis prophase 1\|Prophase I]] ===

	This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]]. [[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination.<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref> This is key to producing genetic variability.		This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]]. [[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref>. This is key to producing genetic variability.

			==== '''Lepotene:''' ====

	~~==== '''Lepotene:''' ====~~
	The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.		The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

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	The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.		The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	==== '''Pachytene:'''  ====		==== '''Pachytene:'''  ====

	In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].		In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].
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	=== Anaphase I ===		=== Anaphase I ===

	The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.<br>		The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.<br>

	=== Telophase I ===		=== Telophase I ===

Nnjm2 at 02:31, 24 November 2014

2014-11-24T02:31:41Z

Show changes

130055435 at 22:56, 23 November 2014

2014-11-23T22:56:34Z

← Older revision		Revision as of 22:56, 23 November 2014
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	<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated so single arm [[Chromosomes\|chromosomes]] in a pair, one inherited from the father and the other inherited from the mother, are duplicated. When they are duplicated, the single chromosomes are now called chromatids and a they join together with their replicated sister chromatid to make a pair, joined at the centromere. <ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor & Francis Group</ref>However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] in a pair, one inherited from the father and the other inherited from the mother, are duplicated. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister_chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]. <ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp; Francis Group</ref>However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].


Line 5:		Line 5:
	===== [[Meiosis prophase 1\|Prophase I]] =====		===== [[Meiosis prophase 1\|Prophase I]] =====

	This ~~phased~~ is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination.<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref> This is key to producing genetic variability.		This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing_over\|crossing over]]. [[Crossing_over\|Crossing over ]]is the physical exchange of [[DNA\|DNA]] between [[Homologous_chromosomes\|homologous chromosomes ]]and results in genetic recombination.<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref> This is key to producing genetic variability.

	'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.		'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.
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	'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].		'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

	In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.		In some organisms, prophase I can take years to complete. For example, in female [[Homo_sapiens\|homo sapiens]], the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the [[Chromosomes\|chromosomes]] de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

	The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.		The number bivalents is equal to the number of [[Chromosomes\|chromosomes]] in a [[Haploid\|haploid cell ]]of that organism. More than one [[Chiasma\|chiasma]] indicates more than one [[Crossing_over\|crossing over]] has occurred.

	'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.		'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.
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	===== Metaphase I =====		===== Metaphase I =====

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The tetrads are randomly orientated on the metaphase plate so either the paternal or maternal homologue can go to the opposite pole. They are also aligned so a homologous chromatid pair face the opposite end.		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The [[tetrad\|tetrads ]]are randomly orientated on the [[Metaphase_plate\|metaphase plate ]]so either the paternal or maternal [[Homologous\|homologue]] can go to the opposite pole. They are also aligned so a [[Homologous\|homologous]] [[Chromatid\|chromatid]] pair face the opposite end.

	Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly. <br>		[[Chiasma\|Chiasmas]] help position and stabilise the [[tetrad\|tetrads]] on the [[Metaphase_plate\|metaphase plate]]. If [[Crossing_over\|crossing over ]]did not occur, then a [[Chiasma\|chiasma]] has not manifested. This is essential to the lining of [[tetrads\|tetrads]] on the [[Metaphase_plate\|metaphase plate ]]and so, therefore, they might not be able to separate properly. <br>

	<br>		<br>
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	===== Anaphase I =====		===== Anaphase I =====

	The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.<br>		The [[Microtubules\|microtubules]] of the [[Spindle\|spindle ]]start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.<br>


Line 39:		Line 39:
	===== Telophase I =====		===== Telophase I =====

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of [[Chromosome\|chromosomes]] and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>

	<br>		<br>

130055435 at 22:44, 23 November 2014

2014-11-23T22:44:15Z

130055435 at 22:00, 23 November 2014

2014-11-23T22:00:03Z

← Older revision		Revision as of 22:00, 23 November 2014
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	The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells~~<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones and Bartlett Learning</ref>~~. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated ~~and~~ so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes ~~into~~ each ~~[[Daughter cells\|~~daughter cell,~~]] they are~~ not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division ~~with no~~ [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	~~===== [[Meiosis prophase 1\|Prophase I]] =====~~

	~~This stage is commonly split into 5 sub-stages to describe the appearance of~~ [[~~Chromosome~~\|~~chromosomes~~]] ~~in each stage of [[Crossing over\|crossing over in]] order to exchange [[DNA\|DNA]]. This is key to produce genetic variability.~~		===== [[Meiosis prophase 1\|Prophase I]] =====

	~~Lepotene: The duplicated~~ [[Chromosome\|chromosomes]] ~~start become visible as they condense into~~ the ~~[[Sister chromatids\|sister chromatids]] structure~~.		This phased is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination. This is key to producing genetic variability.

	~~Zygotene~~: The [[~~Homologous~~ chromosomes~~\|homologous pairs~~]] start ~~to line up so a~~ [[~~Synaptonemal Complex~~\|~~synaptonemal complex~~]]~~, a protein~~ structure~~, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]]~~.		'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

	~~Pachytene~~:~~ In this stage, the~~ [[~~Chromatids~~\|~~chromatids~~]] ~~from each~~ [[~~Chromosome~~\|~~chromosome~~]] ~~on the inside of the~~ [[~~Tetrad~~\|~~tetrad,~~]] ~~that are forming~~ the [[~~Synapsis~~\|~~synapsis~~]]~~, start to~~ [[~~Crossing over~~\|~~cross over~~]]		'''Zygotene:''' The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	~~Diplotene~~: ~~The~~ [[~~Synaptonemal Complex~~\|~~synaptonemal complex~~]] ~~in the~~ [[~~Synapsis~~\|~~synapsis~~]]~~, between~~ the [[~~Chromatid~~\|~~chromatid~~]]~~, starts to break down so as~~ the ~~separate~~ [[~~Chromosomes~~\|~~chromosomes~~]] ~~pull away~~, ~~the point where the [[Chromatids\|chromatids]]~~ [[Crossing over\|cross over~~]] is more visible. This point of connection is called a [[Chiasma\|chiasma~~]].		'''Pachytene:''' In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].

	Diakinesis: The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.~~<br>~~		'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

			In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

			The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.

			'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.



	===== Metaphase I =====		===== Metaphase I =====

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole, attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]], of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell).<br>		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The tetrads are randomly orientated on the metaphase plate so either the paternal or maternal homologue can go to the opposite pole. They are also aligned so a homologous chromatid pair face the opposite end.

			Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly. <br>



	===== Anaphase I =====		===== Anaphase I =====

	The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. the ~~[[Tetrad\|tetrad]]~~, ~~therefore, splits and~~ genetic information has been fully exchanged.<br>		The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.<br>



	===== Telophase I =====		===== Telophase I =====

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]].<br>		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>



	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>

	=== References: ===		=== References: ===

	<references />		<references />

Nnjm2 at 06:45, 11 November 2014

2014-11-11T06:45:04Z

← Older revision		Revision as of 06:45, 11 November 2014
Line 1:		Line 1:
	~~<ref name="null">Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones & Bartlett Learning</ref>~~ The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones and Bartlett Learning</ref>. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	===== [[Meiosis prophase 1\|Prophase I]] =====		===== [[Meiosis prophase 1\|Prophase I]] =====
Line 13:		Line 13:
	Diplotene: The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection is called a [[Chiasma\|chiasma]].		Diplotene: The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection is called a [[Chiasma\|chiasma]].

	Diakinesis: The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.		Diakinesis: The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.<br>

	<br>

	===== Metaphase I =====		===== Metaphase I =====

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole, attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]], of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell).		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole, attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]], of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell).<br>

	<br>

	===== Anaphase I =====		===== Anaphase I =====

	The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. the [[Tetrad\|tetrad]], therefore, splits and genetic information has been fully exchanged.		The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. the [[Tetrad\|tetrad]], therefore, splits and genetic information has been fully exchanged.<br>

	<br>

	===== Telophase I =====		===== Telophase I =====

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]].		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]].<br>

	<br>

	~~These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.~~

	<br>		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>

	=== References: ===		=== References: ===

	<references />		<references />

130055435 at 17:22, 9 November 2014

2014-11-09T17:22:20Z

← Older revision		Revision as of 17:22, 9 November 2014
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	<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA: Jones &~~amp;amp;~~amp; Bartlett Learning</ref> The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref name="null">Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones & Bartlett Learning</ref> The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	===== [[Meiosis prophase 1\|Prophase I]] =====		===== [[Meiosis prophase 1\|Prophase I]] =====
Line 35:		Line 35:
	<br>		<br>

	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.~~<ref>Hartl D. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones & Bartlett Learning</ref>~~		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.

	<br>		<br>

130055435 at 17:21, 9 November 2014

2014-11-09T17:21:03Z

← Older revision		Revision as of 17:21, 9 November 2014
Line 1:		Line 1:
	<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA: Jones &amp; Bartlett Learning</ref> The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA: Jones &amp;amp; Bartlett Learning</ref> The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated and so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes into each [[Daughter cells\|daughter cell,]] they are not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division with no [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	===== [[Meiosis prophase 1\|Prophase I]] =====		===== [[Meiosis prophase 1\|Prophase I]] =====
Line 35:		Line 35:
	<br>		<br>

	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<ref>Hartl D. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones & Bartlett Learning</ref>

	<br>		<br>

← Older revision		Revision as of 16:40, 7 December 2018
Line 1:		Line 1:
	The First Meiotic Division (Meiosis~~ ~~I), sometimes called the reductional division,~~ ~~is the first of two successive~~ ~~nuclear divisions~~ ~~in the process called~~ ~~[[Meiosis\|Meiosis]] to produce 4 genetically different~~ ~~[[Haploid\|haploid]]~~ ~~sex~~ ~~cells. [[Meiosis\|Meiosis]] can only~~ ~~occur in~~ ~~[[Diploid\|diploid]]<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>.~~ ~~Before Meiosis I takes place, the [[DNA\|DNA]]~~ ~~is replicated so single arm [[Chromosomes\|chromosomes]]~~ ~~are now identical and in a pair, one which is inherited from the father and the other inherited from the mother,~~ ~~are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]]~~ ~~are now~~ ~~called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]]~~ ~~to make a pair, joined at the [[Centromere\|centromere]]<ref>~~ALBERT~~, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in~~ ~~[[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the~~ ~~[[Homologous chromosomes\|homologous chromosomes ]](each containing two identical~~ ~~[[Sister chromatids\|sister chromatids]]~~ ~~from the same parent)~~ ~~are~~ ~~separated so both [[Sister chromatids\|sister chromatids]] of each~~ ~~[[Chromosome\|chromosome]]~~ ~~pair~~ ~~goes to each daughter cell. They, therefore, do~~ ~~not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA~~ ~~replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another~~ ~~[[DNA replication\|DNA~~ ~~replication]]~~ ~~and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]]<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>. Before Meiosis I takes place, the [[DNA\|DNA]] is replicated so single arm [[Chromosomes\|chromosomes]] are now identical and in a pair, one which is inherited from the father and the other inherited from the mother, are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes\|chromosomes]] are now called [[Chromatids\|chromatids]] and a they join together with their replicated [[Sister chromatids\|sister chromatid]] to make a pair, joined at the [[Centromere\|centromere]]<ref>Albert, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor &amp;amp;amp;amp;amp; Francis Group</ref>. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	=== [[Meiosis prophase 1\|Prophase I]] ===		=== [[Meiosis prophase 1\|Prophase I]] ===

	This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]].~~ ~~[[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref>. This is key to producing genetic variability.		This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of [[Crossing over\|crossing over]]. [[Crossing over\|Crossing over is]] the physical exchange of [[DNA\|DNA]] between [[Homologous chromosomes\|homologous chromosomes and]] results in genetic recombination<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref>. This is key to producing genetic variability.

	==== ~~'''~~Lepotene~~:'''~~ ====		==== Lepotene ====

	The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.		The duplicated [[Chromosome\|chromosomes]] start to become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

	==== ~~'''~~Zygotene~~:'''~~ ====		==== Zygotene ====

	The [[Homologous chromosomes\|homologous pairs]] start to line up laterally~~ ~~so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure,~~ ~~forms between them called a [[Synapsis\|synapsis]]. The structure the paired~~ ~~[[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]]~~ ~~or bivalent.~~ ~~		The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	==== ~~'''~~Pachytene~~:''' ~~ ====		==== Pachytene ====

	In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete~~ ~~[[Synapsis\|synapsis]], start to [[Crossing over\|cross over]]. ~~ ~~		In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].

	==== ~~'''~~Diplotene~~:'''~~ ====		==== Diplotene ====

	The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].		The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

	In some organisms, prophase I can take years to complete. For example, in female [[Homo sapiens\|homo sapiens]], the development of egg cells start during embryo development, but then pauses after the Diplotene stage until~~ ~~it~~ ~~enters the menstrual cycle many years later~~ ~~to finish development. When the cell pauses for that period of time, the [[Chromosomes\|chromosomes]] de-condense.~~ ~~When they finish development years later, they~~ ~~then re-condense to maximum capacity with the onset of Diakinesis.		In some organisms, prophase I can take years to complete. For example, in female [[Homo sapiens\|homo sapiens]], the development of egg cells start during embryo development but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the [[Chromosomes\|chromosomes]] de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

	The number bivalents is equal to the number of [[Chromosomes\|chromosomes]] in a [[Haploid\|haploid cell of]] that organism. More than one [[Chiasma\|chiasma]] indicates more than one [[Crossing over\|crossing over]] has occurred.		The number bivalents is equal to the number of [[Chromosomes\|chromosomes]] in a [[Haploid\|haploid cell of]] that organism. More than one [[Chiasma\|chiasma]] indicates more than one [[Crossing over\|crossing over]] has occurred.

	==== ~~'''~~Diakinesis~~:'''~~ ====		==== Diakinesis ====

	The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still ~~in tact~~. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.~~ ~~		The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still intact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.

	=== Metaphase I ===		=== Metaphase I ===

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]]~~ ~~going~~ ~~across the cell to and from~~ ~~the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]]~~ ~~of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The [[Tetrad\|tetrads are]] randomly orientated on the [[Metaphase plate\|metaphase plate so]] either the paternal or maternal [[Homologous\|homologue]] can go to the opposite pole. They are also aligned so a [[Homologous\|homologous]] [[Chromatid\|chromatid]] pair face the opposite end.		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The [[Tetrad\|tetrads are]] randomly orientated on the [[Metaphase plate\|metaphase plate so]] either the paternal or maternal [[Homologous\|homologue]] can go to the opposite pole. They are also aligned so a [[Homologous\|homologous]] [[Chromatid\|chromatid]] pair face the opposite end.

	[[Chiasma\|Chiasmas]] help position and stabilise the [[Tetrad\|tetrads]] on the [[Metaphase plate\|metaphase plate]]. If [[Crossing over\|crossing over did]] not~~ ~~occur, then a [[Chiasma\|chiasma]] has not manifested. This is essential to the lining of [[Tetrads\|tetrads]] on the [[Metaphase plate\|metaphase plate and]] so, therefore, they might not be able to separate properly. ~~<br>~~		[[Chiasma\|Chiasmas]] help position and stabilise the [[Tetrad\|tetrads]] on the [[Metaphase plate\|metaphase plate]]. If [[Crossing over\|crossing over did]] not occur, then a [[Chiasma\|chiasma]] has not manifested. This is essential to the lining of [[Tetrads\|tetrads]] on the [[Metaphase plate\|metaphase plate and]] so, therefore, they might not be able to separate properly.

	=== Anaphase I ===		=== Anaphase I ===

	The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads.The 2 pairs of~~ ~~[[Sister chromatids\|sister chromatids]] ,making up~~ ~~each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.~~<br>~~		The [[Microtubules\|microtubules]] of the [[Spindle\|spindle start]] to contract to pull apart the tetrads. The 2 pairs of [[Sister chromatids\|sister chromatids]] , making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the [[Homologous\|homologous]] chromatid pairs separate completely, genetic information has been fully exchanged.

	=== Telophase I ===		=== Telophase I ===

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly~~ ~~around each of these sets~~ ~~of [[Chromosome\|chromosomes]] and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.~~<br>~~		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of [[Chromosome\|chromosomes]] and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.

	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both~~ ~~containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.~~<br>~~		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.

	=== ~~References~~: ===		=== Reference: ===

	<references />		<references />

← Older revision		Revision as of 22:00, 23 November 2014
Line 1:		Line 1:
	The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells~~<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones and Bartlett Learning</ref>~~. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated ~~and~~ so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes ~~into~~ each ~~[[Daughter cells\|~~daughter cell,~~]] they are~~ not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division ~~with no~~ [[DNA replication\|DNA replication]]. [[Meiosis\|Meiosis]] goes through 4 stages similar to [[Mitosis\|mitosis]] which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th Edition - Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated so [[Chromosomes\|chromosomes]] are duplicated to produce two [[Sister chromatids\|sister chromatids]] joined at the [[Centromere\|centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].

	~~===== [[Meiosis prophase 1\|Prophase I]] =====~~

	~~This stage is commonly split into 5 sub-stages to describe the appearance of~~ [[~~Chromosome~~\|~~chromosomes~~]] ~~in each stage of [[Crossing over\|crossing over in]] order to exchange [[DNA\|DNA]]. This is key to produce genetic variability.~~		===== [[Meiosis prophase 1\|Prophase I]] =====

	~~Lepotene: The duplicated~~ [[Chromosome\|chromosomes]] ~~start become visible as they condense into~~ the ~~[[Sister chromatids\|sister chromatids]] structure~~.		This phased is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination. This is key to producing genetic variability.

	~~Zygotene~~: The [[~~Homologous~~ chromosomes~~\|homologous pairs~~]] start ~~to line up so a~~ [[~~Synaptonemal Complex~~\|~~synaptonemal complex~~]]~~, a protein~~ structure~~, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]]~~.		'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

	~~Pachytene~~:~~ In this stage, the~~ [[~~Chromatids~~\|~~chromatids~~]] ~~from each~~ [[~~Chromosome~~\|~~chromosome~~]] ~~on the inside of the~~ [[~~Tetrad~~\|~~tetrad,~~]] ~~that are forming~~ the [[~~Synapsis~~\|~~synapsis~~]]~~, start to~~ [[~~Crossing over~~\|~~cross over~~]]		'''Zygotene:''' The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	~~Diplotene~~: ~~The~~ [[~~Synaptonemal Complex~~\|~~synaptonemal complex~~]] ~~in the~~ [[~~Synapsis~~\|~~synapsis~~]]~~, between~~ the [[~~Chromatid~~\|~~chromatid~~]]~~, starts to break down so as~~ the ~~separate~~ [[~~Chromosomes~~\|~~chromosomes~~]] ~~pull away~~, ~~the point where the [[Chromatids\|chromatids]]~~ [[Crossing over\|cross over~~]] is more visible. This point of connection is called a [[Chiasma\|chiasma~~]].		'''Pachytene:''' In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].

	Diakinesis: The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.~~<br>~~		'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

			In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

			The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.

			'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.



	===== Metaphase I =====		===== Metaphase I =====

	Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole, attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]], of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell).<br>		Like in [[Mitosis\|Mitosis]], the [[Microtubules\|microtubule]] components of the [[Spindle fibres\|spindle fibre]] going across the cell to and from the [[Centrioles\|centrioles]] at each pole attach to the [[Kinetochore\|kinetochore]], a protein structure in the [[Centromere\|centromere]] of the [[Tetrad\|tetrads]]. Therefore, the [[Tetrad\|tetrads]] align themselves along the [[Metaphase plate\|metaphase plate]] (the 'equator' of the cell). The tetrads are randomly orientated on the metaphase plate so either the paternal or maternal homologue can go to the opposite pole. They are also aligned so a homologous chromatid pair face the opposite end.

			Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly. <br>



	===== Anaphase I =====		===== Anaphase I =====

	The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. the ~~[[Tetrad\|tetrad]]~~, ~~therefore, splits and~~ genetic information has been fully exchanged.<br>		The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.<br>



	===== Telophase I =====		===== Telophase I =====

	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]].<br>		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>



	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>

	=== References: ===		=== References: ===

	<references />		<references />

← Older revision		Revision as of 22:44, 23 November 2014
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	<ref>Hartyl D. L. - Ruvolo M. - (2012) - Genetics: Analysis of Genes and Genomes, 8th ~~Edition -~~ Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated so [[Chromosomes\|chromosomes]] are duplicated ~~to produce two [[Sister~~ chromatids\|sister ~~chromatids]]~~ joined at the ~~[[Centromere\|~~centromere]]. However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two [[Sister chromatids\|sister chromatids]]) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].		<ref name="null">Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning</ref>The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called [[Meiosis\|Meiosis]] to produce 4 genetically different, [[Haploid\|haploid]] sex cells. [[Meiosis\|Meiosis]] can only occur in [[Diploid\|diploid]], meiocytes cells in the testis and ovaries. Before Meiosis I takes place, the DNA is replicated so single arm [[Chromosomes\|chromosomes]] in a pair, one inherited from the father and the other inherited from the mother, are duplicated. When they are duplicated, the single chromosomes are now called chromatids and a they join together with their replicated sister chromatid to make a pair, joined at the centromere. <ref>ALBERT, B., BRAY, D., HOPKIN, K., JOHNSON, A., LEWIS, J., RAFF, M., ROBERTS, K., WALTER, P. (2004) Essential Cell Biology. 2nd Ed. New York and London: Garland Science Taylor & Francis Group</ref>However, in contrast to segregation in [[Mitosis\|Mitosis]] and [[Meiosis II\|Meiosis II]], the [[Homologous chromosomes\|homologous chromosomes ]](each containing two identical [[Sister chromatids\|sister chromatids]] from the same parent) are separated so both [[Sister chromatids\|sister chromatids]] of each [[Chromosome\|chromosome]] pair goes to each daughter cell. They, therefore, do not split by the [[Centromere\|centromere]]. [[DNA replication\|DNA replication]] only occurs before the first division of Meiosis I, [[Meiosis II\|Meiosis II continues]] after the first division without another [[DNA replication\|DNA replication]] and chromosomes remain condensed. [[Meiosis\|Meiosis]] goes through 4 stages, similar to [[Mitosis\|mitosis]], which are called [[Meiosis prophase 1\|Prophase I]], Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in [[Meiosis II\|Meiosis II]].



	===== [[Meiosis prophase 1\|Prophase I]] =====		===== [[Meiosis prophase 1\|Prophase I]] =====

	This phased is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination. This is key to producing genetic variability.		This phased is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of [[Chromosome\|chromosomes]] in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination.<ref>BECKER, W. M., POENIE., M. F., REECE, J. B (1996) The World of the Cell. 3rd Ed. Menlo Park, California: The Benjamin/Cummings Publishing Company.</ref> This is key to producing genetic variability.

	'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.		'''Lepotene: '''The duplicated [[Chromosome\|chromosomes]] start become visible as they condense into the [[Sister chromatids\|sister chromatids]] structure.

	'''Zygotene:''' The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.		'''Zygotene:''' The [[Homologous chromosomes\|homologous pairs]] start to line up laterally so a [[Synaptonemal Complex\|synaptonemal complex]], a protein structure, forms between them called a [[Synapsis\|synapsis]]. The structure the paired [[Chromosomes\|chromosomes]] forms is called a [[Tetrad\|tetrad]] or bivalent.

	'''Pachytene:''' In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].		'''Pachytene:''' In this stage, the [[Chromatids\|chromatids]] from each [[Chromosome\|chromosome]] on the inside of the [[Tetrad\|tetrad,]] that are forming the complete [[Synapsis\|synapsis]], start to [[Crossing over\|cross over]].

	'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].		'''Diplotene:''' The [[Synaptonemal Complex\|synaptonemal complex]] in the [[Synapsis\|synapsis]], between the [[Chromatid\|chromatid]], starts to break down so as the separate [[Chromosomes\|chromosomes]] pull away, the point where the [[Chromatids\|chromatids]] [[Crossing over\|cross over]] is more visible. This point of connection and where crossing over just took place is called a [[Chiasma\|chiasma]].

	In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.		In some organisms, prophase I can take years to complete. For example, in female homo sapiens, the development of egg cells start during embryo development, but then pauses after the Diplotene stage until it enters the menstrual cycle many years later to finish development. When the cell pauses for that period of time, the chromosomes de-condense. When they finish development years later, they then re-condense to maximum capacity with the onset of Diakinesis.

	The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.		The number bivalents is equal to the number of chromosomes in a haploid cell of that organism. More than one chiasma indicates more than one crossing over has occurred.

	'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.		'''Diakinesis:''' The [[Homologous chromosomes\|homologous chromosomes]] start to separate as much as possible with the [[Chiasma\|chiasmas]] still in tact. The [[Chromatids\|chromatids]] are at their maximum point of condensation. At the end of [[Meiosis prophase 1\|Prophase I]], the [[Nuclear envelope\|nuclear envelope fragments]] and the [[Spindle\|spindle]] starts to form.



	===== Metaphase I =====		===== Metaphase I =====
Line 29:		Line 29:
	Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly. <br>		Chiasmas help position and stabilise the tetrads on the metaphase plate. If crossing over did not occur, then a chiasma has not manifested. This is essential to the lining of tetrads on the metaphase plate and so, therefore, they might not be able to separate properly. <br>

			<br>

	===== Anaphase I =====		===== Anaphase I =====
Line 35:		Line 35:
	The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.<br>		The microtubules of the spindle start to contract to pull apart the tetrads.The 2 pairs of [[Sister chromatids\|sister chromatids]] ,making up each half of the [[Tetrad\|tetrad]], split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.<br>



	===== Telophase I =====		===== Telophase I =====
Line 41:		Line 41:
	A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>		A set of [[Chromosomes\|chromosomes]] (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the [[Spindle\|spindle breaks]] down. The cell divides and the cells enter [[Meiosis II\|The Second Meiotic Division]]. The chromosomes do not de-condense.<br>

			<br>

	These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>		These 2 [[Daughter cells\|daughter cells]] are genetically different to each other and to the parent and are both containing [[Diploid\|diploid]] set of chromosomes and recombinant DNA.<br>