Meiosis

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Meiosis is a type of cell division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]]. This contrasts to mitosis which can occur in both [[Haploid|haploid]] and [[Diploid|diploid]] cells, producing only two identical daughter cells. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus diving twice but chromosome replication only occuring once. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase.&nbsp;<br>
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Meiosis is a type of cell division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]].This contrasts to mitosis which can occur in both [[Haploid|haploid]] and [[Diploid|diploid]] cells, producing only two identical daughter cells. Therefore meiosis results in variation where as mitosis produces exact copies of the parent cell. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus dividing twice but chromosome replication only occuring once. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase.&nbsp;<br>  
  
 
= Meiosis I <ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref>  =
 
= Meiosis I <ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref>  =
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== Prophase I  ==
 
== Prophase I  ==
  
During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, cetrioles move to opposite poles and spindle fibres start to form.<br>
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During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, cetrioles move to opposite poles and spindle fibres start to form.<br>  
  
 
=== Leptotene  ===
 
=== Leptotene  ===
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=== Pachytene  ===
 
=== Pachytene  ===
  
Synapsis is now complete and the a [[Bivalent|bivalent]] (pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma)&nbsp;form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs.<br>
+
Synapsis is now complete and the a [[Bivalent|bivalent]] (pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma)&nbsp;form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs.<br>  
  
=== Diplotene<br> ===
+
=== Diplotene<br> ===
  
The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.<br>
+
The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.<br>  
  
=== Diakensis<br> ===
+
=== Diakensis<br> ===
  
During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase<br>
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During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase<br>  
  
== Metaphase I<br> ==
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== Metaphase I<br> ==
  
Bivalents or tetrads (four chromotids)&nbsp;align on on the metaphase plate (the equator of the cell)&nbsp;and spindle fibres attach to the kinetichores, protein structures located at the centromeres.<br>
+
Bivalents or tetrads (four chromotids)&nbsp;align on on the metaphase plate (the equator of the cell)&nbsp;and spindle fibres attach to the kinetichores, protein structures located at the centromeres.<br>  
  
== Anaphase I<br> ==
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== Anaphase I<br> ==
  
Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles. <br>
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Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles. <br>  
  
== Telophase I<br> ==
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== Telophase I<br> ==
  
Nuclear envelope may form around the dyads and cytokinesis (cell division)&nbsp;occurs.<br>
+
Nuclear envelope may form around the dyads and cytokinesis (cell division)&nbsp;occurs.<br>  
  
== Interphase<br> ==
+
== Interphase<br> ==
  
Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA&nbsp;replication but growth and biosynthetic activities can still occur. <br>
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Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA&nbsp;replication but growth and biosynthetic activities can still occur. <br>  
  
 
= Meiosis II<br>&nbsp;  =
 
= Meiosis II<br>&nbsp;  =
  
= Prophase II<br> =
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= Prophase II<br> =
  
Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centroiles move to opposing poles and spindle fibres start to form.<br>
+
Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centroiles move to opposing poles and spindle fibres start to form.<br>  
  
 
== Metaphase II  ==
 
== Metaphase II  ==
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== Telophase II  ==
 
== Telophase II  ==
  
The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes.<br>
+
The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes.<br>  
  
 
= References  =
 
= References  =
  
 
<references /><br>
 
<references /><br>

Revision as of 14:59, 23 November 2012

Meiosis is a type of cell division. It can only occur in diploid cells, resulting in four unidentical haploid daughter cells.This contrasts to mitosis which can occur in both haploid and diploid cells, producing only two identical daughter cells. Therefore meiosis results in variation where as mitosis produces exact copies of the parent cell. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus dividing twice but chromosome replication only occuring once. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase. 

Contents

Meiosis I [1]

Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, cetrioles move to opposite poles and spindle fibres start to form.

Leptotene

Leptotene is the first stage of Prophase I. During the Leptotene stage chromosomes coil and condense. This is whne the chromosomes first become visible.

Zygotene

During Zygotene the homologous chromosomes pair up and the synaptonemal complex between the homologous chromosomes start forming.

Pachytene

Synapsis is now complete and the a bivalent (pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma) form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs.

Diplotene

The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.

Diakensis

During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase

Metaphase I

Bivalents or tetrads (four chromotids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the kinetichores, protein structures located at the centromeres.

Anaphase I

Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles.

Telophase I

Nuclear envelope may form around the dyads and cytokinesis (cell division) occurs.

Interphase

Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA replication but growth and biosynthetic activities can still occur.

Meiosis II
 

Prophase II

Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centroiles move to opposing poles and spindle fibres start to form.

Metaphase II

The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.

Anaphase II

The dyads are pulled apart by spindle fibres and the myads arrive at opposite poles.

Telophase II

The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes.

References

  1. Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers

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