Question

Among the different phases of meosis cell division, chromosomes pairing occurs in _________.

• A. Pachytene
• B. Zygotene
• C. Anaphase II
• D. Interphase II

Hint:

To remember the sequence and events of Prophase I:
Leptotene (threads) \(\rightarrow\) Zygotene (Zipping/pairing) \(\rightarrow\) Pachytene (Packing/crossing over) \(\rightarrow\) Diplotene (parting/chiasmata) \(\rightarrow\) Diakinesis (moving apart).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four haploid daughter cells.
During Prophase I of Meiosis I, several highly coordinated stages (Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis) occur to facilitate genetic recombination and homologous chromosome pairing.

Step 2: Detailed Explanation:
Let us trace the specific chromosomal behaviors during Prophase I to identify when pairing occurs:
- Leptotene: Chromosomes begin to condense and become visible as thin threads inside the nucleus.
- Zygotene: During this stage, homologous chromosomes start pairing up side-by-side along their entire length.
This precise pairing process of homologous chromosomes is called synapsis, and the paired chromosomes are held together by a protein structure called the synaptonemal complex, forming bivalents.
Thus, chromosome pairing occurs during the Zygotene stage.
- Pachytene: Once pairing is complete, crossing over (exchange of genetic material between non-sister chromatids of homologous chromosomes) occurs, facilitated by the recombination nodules.
- Anaphase II: Sister chromatids separate and move to opposite poles of the cell, which occurs in Meiosis II.
- Interphase II: A brief resting phase between Meiosis I and Meiosis II, where no DNA replication occurs.

Step 3: Final Answer:
Chromosomes pairing (synapsis) occurs during the Zygotene stage of Prophase I (Option B).