Question

In which stage of the first meiotic division two sister chromatids are formed?
A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Answer 1

Hint: This question is related to meiosis. Meiosis is generally divided into Meiosis I and Meiosis II on the basis of sequential cycles of cell division and nuclear division.
Meiosis I occurs when the replication occurs in the chromosomes of parents and recombination also occurs between them.

Complete answer:
At the time of division in the pachytene stage, the chromosomes clearly appear as tetrads. The pachytene stage is identified when the recombination nodules appear, the crossing over occurs between the non sister chromatids of the homologous chromosome at the site. Crossing over results in the recombination of genetic material which occurs on the two chromosomes. Crossing over also involves the enzyme recombinase that’s why it is also an enzyme mediated process. At the end of the pachytene stage the recombination process is completed between the homologous chromosomes and leaves the chromosomes linked at the site where crossing over is done.

Additional information:
Meiosis is defined as the production of offspring with the help of sexual reproduction, when two gametes are fused together and each gamete has a haploid chromosome. Gametes generally form from diploid cells. Cell division occurs in the diploid cells and it reduces to half and haploid daughter cells are produced. At the time of meiotic division haploid cells are produced whereas at the time of fertilization diploid phase is restored. Meiosis takes place during gametogenesis in both plants and animals.

So, the correct option is (C) Pachytene.

Note: The prophase I is the process of meiotic division and the process took more time and the process is also more complex whereas the process of prophase in mitotic division is simple. The prophase involves the five phases which are strictly based on the chromosomal behavior. These are Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.