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Meiosis I

2018-12-06T18:35:16Z

170319036: Missed one reference

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]] ===

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.

==== '''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]].  

==== '''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|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.

==== '''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 ===

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. 

=== 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. 

=== 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. 

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: ===

<references />

Meiosis I

2018-12-06T18:33:31Z

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

<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]]. 

=== [[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.

==== '''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. 

==== '''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]].

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.

==== '''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 ===

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. 

=== 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. 

=== 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. 

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: ===

<references />