Meiosis I

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The First Meiotic Division (Meiosis&nbsp;I), sometimes called the reductional division,&nbsp;is the first of two successive&nbsp;nuclear divisions&nbsp;in the process called&nbsp;[[Meiosis|Meiosis]] to produce 4 genetically different&nbsp;[[Haploid|haploid]]&nbsp;sex&nbsp;cells. [[Meiosis|Meiosis]] can only&nbsp;occur in&nbsp;[[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>.&nbsp;Before Meiosis I takes place, the [[DNA|DNA]]&nbsp;is replicated so single arm [[Chromosomes|chromosomes]]&nbsp;are now identical and in a pair, one which is inherited from the father and the other inherited from the mother,&nbsp;are duplicated to give 2 copies of each. When they are duplicated, the single [[Chromosomes|chromosomes]]&nbsp;are now&nbsp;called [[Chromatids|chromatids]] and a they join together with their replicated [[Sister chromatids|sister chromatid]]&nbsp;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&nbsp;[[Mitosis|Mitosis]] and [[Meiosis II|Meiosis II]], the&nbsp;[[Homologous chromosomes|homologous chromosomes ]](each containing two identical&nbsp;[[Sister chromatids|sister chromatids]]&nbsp;from the same parent)&nbsp;are&nbsp;separated so both [[Sister chromatids|sister chromatids]] of each&nbsp;[[Chromosome|chromosome]]&nbsp;pair&nbsp;goes to each daughter cell. They, therefore, do&nbsp;not split by the [[Centromere|centromere]]. [[DNA replication|DNA&nbsp;replication]] only occurs before the first division of Meiosis I, [[Meiosis II|Meiosis II continues]] after the first division without another&nbsp;[[DNA replication|DNA&nbsp;replication]]&nbsp;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]].
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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;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]].&nbsp;[[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&nbsp;so a [[Synaptonemal Complex|synaptonemal complex]], a protein structure,&nbsp;forms between them called a [[Synapsis|synapsis]]. The structure the paired&nbsp;[[Chromosomes|chromosomes]] forms is called a [[Tetrad|tetrad]]&nbsp;or bivalent.&nbsp;
+
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:'''&nbsp;  ====
+
==== Pachytene   ====
  
In this stage, the [[Chromatids|chromatids]] from each [[Chromosome|chromosome]] on the inside of the [[Tetrad|tetrad,]] that are forming the complete&nbsp;[[Synapsis|synapsis]], start to [[Crossing over|cross over]]. &nbsp;
+
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&nbsp;it&nbsp;enters the menstrual cycle many years later&nbsp;to finish development. When the cell pauses for that period of time, the [[Chromosomes|chromosomes]] de-condense.&nbsp;When they finish development years later, they&nbsp;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.&nbsp;
+
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]]&nbsp;going&nbsp;across the cell to and from&nbsp;the [[Centrioles|centrioles]] at each pole attach to the [[Kinetochore|kinetochore]], a protein structure in the [[Centromere|centromere]]&nbsp;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&nbsp;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&nbsp;[[Sister chromatids|sister chromatids]] ,making up&nbsp;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&nbsp;around each of these sets&nbsp;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&nbsp;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 />

Latest revision as of 16:40, 7 December 2018

The First Meiotic Division (Meiosis I), sometimes called the reductional division, is the first of two successive nuclear divisions in the process called Meiosis to produce 4 genetically different haploid sex cells. Meiosis can only occur in diploid[1]. Before Meiosis I takes place, the DNA is replicated so single arm 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 are now called chromatids and a they join together with their replicated sister chromatid to make a pair, joined at the centromere[2]. However, in contrast to segregation in Mitosis and Meiosis II, the homologous chromosomes (each containing two identical sister chromatids from the same parent) are separated so both sister chromatids of each chromosome pair goes to each daughter cell. They, therefore, do not split by the centromere. DNA replication only occurs before the first division of Meiosis I, Meiosis II continues after the first division without another DNA replication and chromosomes remain condensed. Meiosis goes through 4 stages, similar to mitosis, which are called Prophase I, Metaphase I, Anaphase I, Telophase I to distinguish from the phases it goes through in Meiosis II.

Contents

Prophase I

This phase is particularly long and complex. This stage is commonly split into 5 sub-stages to describe the appearance of chromosomes in each stage of crossing over. Crossing over is the physical exchange of DNA between homologous chromosomes and results in genetic recombination[3]. This is key to producing genetic variability.

Lepotene

The duplicated chromosomes start to become visible as they condense into the sister chromatids structure.

Zygotene

The homologous pairs start to line up laterally so a synaptonemal complex, a protein structure, forms between them called a synapsis. The structure the paired chromosomes forms is called a tetrad or bivalent.

Pachytene

In this stage, the chromatids from each chromosome on the inside of the tetrad, that are forming the complete synapsis, start to cross over.

Diplotene

The synaptonemal complex in the synapsis, between the chromatid, starts to break down so as the separate chromosomes pull away, the point where the chromatids cross over is more visible. This point of connection and where crossing over just took place is called a 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 start to separate as much as possible with the chiasmas still intact. The chromatids are at their maximum point of condensation. At the end of Prophase I, the nuclear envelope fragments and the spindle starts to form.

Metaphase I

Like in Mitosis, the microtubule components of the spindle fibre going across the cell to and from the centrioles at each pole attach to the kinetochore, a protein structure in the centromere of the tetrads. Therefore, the tetrads align themselves along the 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.

Anaphase I

The microtubules of the spindle start to contract to pull apart the tetrads. The 2 pairs of sister chromatids , making up each half of the tetrad, split and move to opposite poles of the cell. As the homologous chromatid pairs separate completely, genetic information has been fully exchanged.

Telophase I

A set of chromosomes (2n) are now at each pole of the cell. Nuclear membrane forms briefly around each of these sets of chromosomes and the spindle breaks down. The cell divides and the cells enter The Second Meiotic Division. The chromosomes do not de-condense.

These 2 daughter cells are genetically different to each other and to the parent and are both containing diploid set of chromosomes and recombinant DNA.

Reference:

  1. Hartyl, D. L., Ruvolo, M.(2012) Genetics: Analysis of Genes and Genomes. 8th Ed. Burlington, MA, USA: Jones and Bartlett Learning
  2. 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
  3. 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.
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