Meiosis prophase 1

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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].

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'[5], this can be compared to a zipper structure, due to the coils of chromatin[6]

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[7] 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.

Diplotene

During this phase the two homologous chromosomes begin to migrate apart as the 'synaptonemal complex'[8] 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[9].

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[10].

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. European informatics institute, 2011, "Pachytene", available at: http://www.ebi.ac.uk/QuickGO/GTerm?id=GO:0000239
  8. Med., June 1996, "Synaptonemal Complexes: Structure and Function", http://www.ncbi.nlm.nih.gov/pubmed/8743892
  9. Dr. William H. Heidcamp, 22/5/1995, "Chapter 11: Cell Cycles - Introduction", http://homepages.gac.edu/~cellab/chpts/chpt11/intro11.html
  10. 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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