Question

In the pachytene stage of meiosis, the chromosomes appear
A. Single-stranded
B. Double-stranded
C. Three stranded
D. Four stranded

Answer 1

Hint: Meiosis is a type of reductional division that occurs in gamete cells. It consists of two major stages called meiosis I and meiosis II. Each stage consists of five further phases called prophase, metaphase, anaphase, telophase, and last cytokinesis. Pachytene is the third stage of the prophase-I stage of meiosis I. Bivalent structures of chromosomes forms during pachytene.

Complete step-by-step answer:
The prophase-I of meiosis consists of the condensation of chromosomes. It is a five-stage process. Leptotene being the first stage is followed by zygotene, pachytene, diplotene, and diakinesis. Chromatin condensation occurs in the leptotene stage. Synaptonemal complex formation between two homologous chromosomes occurs in the zygotene stage. The nuclear envelope degenerates and this stimulates centrioles to form microtubules on both sides of the cell. Microtubules attach to the kinetochores in the centromeres of chromosomes. After this, chromosomes pair up with their homologous pairs in the pachytene stage which is the third stage. In the fourth stage, the disappearance of the synaptonemal complex takes place. In the fifth stage of prophase I, the chromosomes become fully condensed and the nuclear membrane fully degenerates.

The pairing of homologous chromosomes results in the formation of bivalent structures called tetrads. Due to this, the chromosomes appear four-stranded. The four chromatids become clear with each tetrad having four kinetochores. This pairing is necessary for recombination to take place.

Thus, from the above discussion, we can conclude that option D is correct.

Note: Another feature of pachytene is recombination or crossing over between non-sister chromatids of homologous chromosomes. Exchange of genetic material occurs between the two which later results in variations in the zygote. The two homologous chromosomes reshuffle and rearrange their genetic make-up. An enzyme called recombinase regulates this process of crossing-over.