Describe various subphases of prophase - I of Meiosis - I.

Explanation

Prophase. I of Meiosis - I : It is longer and more complex. It has been sub divided into five sub phases based on chromosomal behaviour.

(i) Leptotene : This is a first phase of meiosis. During this phase the chromosomes become gradually visible. The compaction of hromosomes continues throughout Leptotene.
Each chromosome is made up of two chromatids and centromere attached to them. But chromatids are tangled with each other so its dual form is not seen.
(ii) Zygotene : During this stage, chromosomes start pairing together and this process of association is called Synapsis. Such paired chromosomes are called homologous chromosomes.
This process proceeds further like a Zipper. Signal strands of chromosomes entangled or interwind around each other at some places. The complex formed by a pair of synapsed homologous chromosomes is called a Bivalent. Infact it is a tetrads.
(iii) Pachytene : During this stage bivalent chromosomes clearly appears as tetrads. Single chromatid of the homologous chromosomes are entangled with each other.
This stage is characterised by the appearance of recombination nodules.
Crossing over occurs between non-sister chromatids of the homologous chromosomes.
Crossing over is the exchange of genetic material between two homologous chromosomes.
Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.
Crossing over leads to recombination of genetic material on the two chromosomes.
Crossing over takes place at the exchange place of genes. This way new combinations are formed.
(iv) Diplotene : The beginning of diplotene is recognized by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossing over.
These X-shaped structures are called Chiasmata.
The number of Chiasmata depend on length of chromosomes. They are more in long chromosomes. Exchange of genes takes place at site of formation of chiasmata.
(v) Diakinesis : Bispindle fibre separating homologous chromosomes is formed, single chromatids are separated at the site of formation of chiasmata.
By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Each chromosome is made up of two single chromatids and centromere attaching to it