Q.Write about Pachytene and Diplotene of Prophase I

Step 1. Pachytene follows zygotene, once synapsis is complete; at this stage the paired bivalents are clearly visible as tetrads, each consisting of 4 chromatids held by 2 centromeres.

Step 2. Recombination nodules appear during pachytene at the specific sites where crossing over will occur between non-sister chromatids of the homologous chromosomes, and this genetic recombination - mediated by the enzyme recombinase - is completed by the end of the stage, although the chromosomes remain physically linked at these crossover sites.

Step 3. Diplotene begins as the synaptonemal complex that had held the homologues tightly together disassembles and dissolves.

Step 4. The homologous chromosomes then begin to separate from each other, but remain attached at one or more points corresponding to where crossing over took place; these X-shaped attachment points are called chiasmata, and they physically hold the homologues together even as the rest of each chromosome pulls apart, while sister chromatids of each homologue stay closely associated throughout. …

Describe each substage in sequence: what structure is visible, what molecular event occurs, a …

• Reversing which stage forms chiasmata (diplotene) versus which stage carries out the actual recombination (pachytene). …