Explain five stages of prophase - I of meiosis
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Meiosis prophase 1
Prophase 1 of Meiosis is the first stage of meiosis and is defined by five different phases; Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis (in that order). Prophase 1 is essentially the crossing over and recombination of genetic material between non sister chromatids - this results in the genetically unidentical, haploid daughter chromatid cells.
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 chromosomes become visible by using electron microscopy, which can distinguish between sister chromatids[3]. The appearance of the chromosomes at this stage of Prophase 1 is likened to 'a string with beads', 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. 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', this can be compared to a zipper structure, due to the coils of chromatin. The synaptonemal complex facilitates synapsis by holding the alligned chromosomes together. 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').
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 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. 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' 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. The chiasma are fully visible at this stage, so can be seen to move towards the end of the chromatids in a process known as terminalization.
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 chromosomes are at their most condensed form during diakinesis. The homologous chromosomes in a bivalent are still connected by at least 1 chiasma. 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.