Q.Some Mendelian hybridization experimental results were shown to the students. Teacher informed that there are two genes located on the same chromosome. He enquired if they will be ever separated from each other?

Prophase I of meiosis is divided into five sub-stages (Leptotene -> Zygotene -> Pachytene -> Diplotene -> Diakinesis), during which homologous chromosomes pair, recombine, and prepare to separate.

1. Leptotene:

Chromosomes begin to condense and become visible under the microscope as long, thin, single threads. Compaction of chromatin starts.

2. Zygotene:

Homologous chromosomes start to pair up — this pairing is called synapsis. A protein structure, the synaptonemal complex, forms between the paired homologues. Each pair of synapsed homologous chromosomes is called a bivalent or tetrad (as it consists of 4 chromatids).

3. Pachytene:

Chromosomes continue to condense further. Crossing over occurs — non-sister chromatids of the homologous chromosomes exchange genetic material at points called chiasmata, mediated by the enzyme complex recombinase, aided by recombination nodules. This stage is responsible for genetic recombination/variation.

4. Diplotene:

The synaptonemal complex dissolves, and the homologous chromosomes of each bivalent start to separate from each other, but they remain connected at the points of crossing over (chiasmata). This gives a characteristic cross-shaped appearance. …