Question

What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?

Answer 1

Prophase-I

Prophase-I of meiosis is subdivided into five phases which are as follows:-

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene

Leptotene is characterised by the initiation of chromatin compaction. Chromosomes become visible during this stage under the light microscope.

Zygotene

Zygotene is characterised by the pairing of homologous chromosomes (bivalent), an event called synapsis. It is accompanied by the formation of a complex structure called the synaptonemal complex.

Pachytene

The pachytene stage is characterised by the distinction of the recombination nodules and crossing over (exchange of non-sister chromatids of homologous chromosomes). It is accomplished by the enzyme recombinase. It leads to the recombination along with the formation of an X-shaped structure called chiasmata.

Diplotene

In the diplotene stage, the X-shaped structure where the region of non-sister chromatids of two homologous chromosomes remains attached, called chiasmata, becomes visible as the dissolution of the rest of the part of the synaptonemal complex occurs.

Diakinesis

Diakinesis is the final stage of prophase-I. In this phase, chiasmata gets terminalised and chromatin gets completely condensed. Nuclear envelope disintegrates, and the nucleolus disappears in this phase.