Question

In meiosis, crossing over is initiated at
A. Pachytene
B. Leptotene
C. Zygotene
D. Diplotene

Answer 1

Pachytene stage is defined by the formation of the synaptonemal complex. It allows the exchange of genetic material between two homologous chromosomes by a process called crossing over. This also enables the generation of recombinants.

Complete answer:
Option A- Pachytene
Pachytene or pachynema is one of the five substages of Prophase I in meiosis. It is the third substage and follows leptotene and zygotene. It precedes diplotene and diakinesis, beginning after synapsis is complete. During pachytene, the homologous chromosomes thicken and become recombinant.

Option B- Leptotene
The leptotene stage or leptonema is the first of five substages of prophase I in meiosis. The term leptonema is derived from Greek words which means "thin threads". A cell fated to become a gamete enters the leptotene stage only after its chromosomes are duplicated during interphase.

Option C- Zygotene
Zygotene refers to the phase where the homologous chromosomes pair or come together in the synapse. This pairing or coming together of homologous chromosomes is known as synapsis. It may be facilitated by the synaptonemal complex. The term zygotene is derived from Greek words meaning paired threads.

Option D- Diplotene
Prophase refers to the first stage of cell division in both mitosis and meiosis. Starting after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin and the disappearance of the nucleolus.

So, the correct option is A- Pachytene.

Note: Meiosis refers to the process of a single cell dividing twice to produce four cells that contain half the original amount of genetic information. These cells are sex cells of humans– sperm in males and eggs in females. During meiosis, one cell divides twice to form four daughter cells.