Meiosis prophase 1

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=== Leptotene  ===
 
=== Leptotene  ===
  
Leptotene is the first of five stages of Prophase 1 and consists of the condensing of the already replicated [[Chromosomes|chromosomes]], this procedure continues throughout Prophase 1. The appearance of the chromosomes at this stage of Prophase 1 is likened to 'a string with beads'<ref>Dr. William H. Heidcamp, from 1995 onwards, "Cell Cycles Introduction", available at: http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>, these beads are called chromomeres. Each sister [[Chromatid|chromatid]] is attached to the nuclear envelope and are so close together that they can be mistaken for only one chromosome&nbsp;<ref>Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html</ref>.<br>  
+
Leptotene is the first of five stages of Prophase 1 and consists of the condensing of the already replicated [[Chromosomes|chromosomes]], this procedure continues throughout Prophase 1. The appearance of the chromosomes at this stage of Prophase 1 is likened to 'a string with beads'<ref>Dr. William H. Heidcamp, from 1995 onwards, "Cell Cycles Introduction", available at: http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>, these beads are called chromomeres. Each sister [[Chromatid|chromatid]] is attached to the nuclear envelope and are so close together that they can be mistaken for only one chromosome&nbsp;<ref>Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html</ref>. This is a very short stage of Prophase 1.<br>  
  
 
=== Zygotene  ===
 
=== Zygotene  ===
  
Zygotene is the sub-stage where synapsis between homologous chromosomes begins. These synapsis can form up and down the chromosomes allowing numerous points of contact called 'synaptonemal complex'<ref>Pub Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892</ref>, this can be compared to a zipper structure, due to the coils of chromatin<ref>Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html</ref>.&nbsp;  
+
Zygotene is the sub-stage where synapsis between homologous chromosomes begins. It is also known as zygonema. These synapsis can form up and down the chromosomes allowing numerous points of contact called 'synaptonemal complex'<ref>Pub Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892</ref>, this can be compared to a zipper structure, due to the coils of chromatin<ref>Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html</ref>. After the homologous pairs synapse they are either called tetrads or bivalents. Bivalent is more commonly used at an advanced level as it is a better choice due to similar names for similar states (a single homolog is a 'univalent', and three homologs are a 'trivalent').&nbsp;<ref>McCarthy, E.M. MacroEvolution Available at: http://www.macroevolution.net/prophase-details.html [Accessed November 21, 2014]</ref><br>
  
 
=== Pachytene  ===
 
=== Pachytene  ===
  
Once the synapse is formed it is called a bivalent (where a chromatid of one pair is synapsed/attached to the chromatid in a homologous chromosomes<ref>European informatics institute, 2011, "Pachytene", available at: http://www.ebi.ac.uk/QuickGO/GTerm?id=GO:0000239</ref>&nbsp;and crossing over can occur. Subsequently the synapses snap completing the crossing over of the genetic information. As a result the variation in genetic material has been increased significantly, because up and down the chromosome there has been an exchanged of the mother and father's genetic material.  
+
Once the synapse is formed it is called a bivalent (where a chromatid of one pair is synapsed/attached to the chromatid in a homologous chromosomes<ref>European informatics institute, 2011, "Pachytene", available at: http://www.ebi.ac.uk/QuickGO/GTerm?id=GO:0000239</ref>&nbsp;and crossing over can occur. Subsequently the synapses snap completing the crossing over of the genetic information. As a result the variation in genetic material has been increased significantly, because up and down the chromosome there has been an exchanged of the mother and father's genetic material. The two sister chromatids separate from each other, but the homologous chromosomes remain attached.This makes the complex look much thicker. <ref>McCarthy, E.M. MacroEvolution\r\nAvailable at: http://www.macroevolution.net/prophase-details.html [Accessed November 21, 2014]</ref>&nbsp;The synaptonemal complex is complete, allowing chiasma to form. This is what allows the crossing over alleles to occur as this is a process that only happens over a small region of the chromosomes.
  
 
=== Diplotene  ===
 
=== Diplotene  ===
  
During this phase the two homologous chromosomes begin to migrate apart as the&nbsp;'synaptonemal complex'<ref>Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892</ref> disintegrates between the two chromosomal arms and they begin to repel one another. This allows the two chromosome to move apart, held only by the chiasma(ta). Whilst this process occurs the chromosome begin to uncoil, contrary to the natural progression of [[Prophase|Prophase]], however they are still coiled enough to allow a distinct image of a chiasma formation under a microscope<ref>Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>.  
+
During this phase the two homologous chromosomes begin to migrate apart as the&nbsp;'synaptonemal complex'<ref>Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892</ref> disintegrates between the two chromosomal arms and they begin to repel one another. This allows the two chromosome to move apart, held only by the chiasma(ta). Whilst this process occurs the chromosome begin to uncoil, contrary to the natural progression of [[Prophase|Prophase]], however they are still coiled enough to allow a distinct image of a chiasma formation under a microscope<ref>Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>.&nbsp;The chiasma move towards the end of the chromatids in a process known as terminalization.&nbsp;<ref>Nokkala, S. &amp; Nokkala, C., 1996. The absence of chiasma terminalization and inverted meiosis in males and females of Myrmus miriformis. Available at http://www.nature.com/hdy/journal/v78/n5/abs/hdy199787a.html [Accessed 21 November, 2014]</ref>
  
 
=== Diakinesis  ===
 
=== Diakinesis  ===
  
Diakinesis is the final step of Prophase 1 and is the termination of the condensing of the chromosomes, this allows the chiasmata and bivalent structure to be seen more clearly under an electron microscope. The rest of this phase is setting up the cell to make way for metaphase 1. Therefore, the [[Nucleolus|nucleolus]] disappears, the [[Nuclear envelope|nuclear envelope]] disintegrates and the [[Centrioles|centrioles ]]([[Centrosome|centrosome]]) move to the equator, whilst the mitotic spindles migrate<ref>Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>.  
+
Diakinesis is the final step of Prophase 1 and is the termination of the condensing of the chromosomes, this allows the chiasmata and bivalent structure to be seen more clearly under an electron microscope. The rest of this phase is setting up the cell to make way for metaphase 1. Therefore, the [[Nucleolus|nucleolus]] disappears, the [[Nuclear envelope|nuclear envelope]] disintegrates and the [[Centrioles|centrioles ]]([[Centrosome|centrosome]]) move to the equator, whilst the mitotic spindles migrate<ref>Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html</ref>.&nbsp;<br>
  
 
=== References  ===
 
=== References  ===
  
 
<references />
 
<references />

Revision as of 14:00, 21 November 2014

Prophase 1 of Meiosis is the first stage of meiosis and is defined by five different phases; Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis[1] (in that order). Prophase 1 is essentially the crossing over and recombination of genetic material between non sister chromatids[2] - this results in the genetically unidentical, haploid daughter chromatid cells.

Contents

Leptotene

Leptotene is the first of five stages of Prophase 1 and consists of the condensing of the already replicated chromosomes, this procedure continues throughout Prophase 1. The appearance of the chromosomes at this stage of Prophase 1 is likened to 'a string with beads'[3], these beads are called chromomeres. Each sister chromatid is attached to the nuclear envelope and are so close together that they can be mistaken for only one chromosome [4]. This is a very short stage of Prophase 1.

Zygotene

Zygotene is the sub-stage where synapsis between homologous chromosomes begins. It is also known as zygonema. These synapsis can form up and down the chromosomes allowing numerous points of contact called 'synaptonemal complex'[5], this can be compared to a zipper structure, due to the coils of chromatin[6]. After the homologous pairs synapse they are either called tetrads or bivalents. Bivalent is more commonly used at an advanced level as it is a better choice due to similar names for similar states (a single homolog is a 'univalent', and three homologs are a 'trivalent'). [7]

Pachytene

Once the synapse is formed it is called a bivalent (where a chromatid of one pair is synapsed/attached to the chromatid in a homologous chromosomes[8] and crossing over can occur. Subsequently the synapses snap completing the crossing over of the genetic information. As a result the variation in genetic material has been increased significantly, because up and down the chromosome there has been an exchanged of the mother and father's genetic material. The two sister chromatids separate from each other, but the homologous chromosomes remain attached.This makes the complex look much thicker. [9] The synaptonemal complex is complete, allowing chiasma to form. This is what allows the crossing over alleles to occur as this is a process that only happens over a small region of the chromosomes.

Diplotene

During this phase the two homologous chromosomes begin to migrate apart as the 'synaptonemal complex'[10] disintegrates between the two chromosomal arms and they begin to repel one another. This allows the two chromosome to move apart, held only by the chiasma(ta). Whilst this process occurs the chromosome begin to uncoil, contrary to the natural progression of Prophase, however they are still coiled enough to allow a distinct image of a chiasma formation under a microscope[11]. The chiasma move towards the end of the chromatids in a process known as terminalization. [12]

Diakinesis

Diakinesis is the final step of Prophase 1 and is the termination of the condensing of the chromosomes, this allows the chiasmata and bivalent structure to be seen more clearly under an electron microscope. The rest of this phase is setting up the cell to make way for metaphase 1. Therefore, the nucleolus disappears, the nuclear envelope disintegrates and the centrioles (centrosome) move to the equator, whilst the mitotic spindles migrate[13]

References

  1. Biology online, 26/10/2011, "Prophase 1", Available at: http://www.biology-online.org/dictionary/Prophase_i
  2. Biology online, 27/09/2011, “Meiosis” available at: http://www.biology-online.org/dictionary/Meiosis
  3. Dr. William H. Heidcamp, from 1995 onwards, "Cell Cycles Introduction", available at: http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html
  4. Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html
  5. Pub Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892
  6. Macroevolution, "Sub stages of Prophase 1 (Meiosis)", http://www.macroevolution.net/prophase-details.html
  7. McCarthy, E.M. MacroEvolution Available at: http://www.macroevolution.net/prophase-details.html [Accessed November 21, 2014]
  8. European informatics institute, 2011, "Pachytene", available at: http://www.ebi.ac.uk/QuickGO/GTerm?id=GO:0000239
  9. McCarthy, E.M. MacroEvolution\r\nAvailable at: http://www.macroevolution.net/prophase-details.html [Accessed November 21, 2014]
  10. Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892
  11. Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html
  12. Nokkala, S. & Nokkala, C., 1996. The absence of chiasma terminalization and inverted meiosis in males and females of Myrmus miriformis. Available at http://www.nature.com/hdy/journal/v78/n5/abs/hdy199787a.html [Accessed 21 November, 2014]
  13. Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html
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