Question

During the pachytene stage in crossing over, the exchange of genetic material occurs between?
A. Two non-homologous chromosomes
B. Two homologous chromosomes
C. Sister chromatids of non-homologous chromosomes
D. Non-sister chromatids of non-homologous chromosomes

Answer 1

Hint: Pachytene is one of the five substages of prophase I of meiosis.

Complete Answer:
- Meiosis is one type of cell division where the offsprings acquire half the number of chromosomes from the parents. Hence it is called a reductional division.
- Chromosomes pair that are similar having the same genes, but with different alleles, with each set inherited from the parents are called homologous chromosomes.
- During replication, each chromosome contains two DNA molecules. The two DNA molecules form two halves of the chromosomes known as chromatids. They can be sister chromatids or non-sister chromatids.
- Sister chromatids are formed by the same chromosome while non-sister chromatids are formed by either of the homologous pair of chromosomes.
- The important stages of meiosis are prophase, metaphase, anaphase and telophase. It occurs as meiosis I and meiosis II.
- The main phases of prophases are leptotene, zygotene, pachytene, diplotene and diakinesis.

1) In leptotene, the chromosomes begin to condense and compactly arranged.
2) In zygotene, homologous chromosomes pair up this process is known as chromosome synapsis. This synapsed chromosome forms tetrad.
3) At the pachytene stage, chromosomal crossing over occurs in the two non-sister chromatids of homologous chromosomes. Here, the chromosomes remain linked.
4) In Diplotene, synaptonemal complex and separation of the two non-sister homologous chromosomes occurs leaving crossover sites and forms X-shaped structures called chiasmata. 5) Diakinesis is the terminal process of prophase which assembles the meiotic spindles to separate the chromosomes and break down the nuclear envelope. Generally, the non-homologous chromosomes do not undergo the crossing over process.

Hence, the answer is option b) Two homologous chromosomes.

Note: Crossing over refers to the exchange of genetic information between two non-sister chromatids. The crossing occurs only between the two homologous non-sister chromatids and not between the two non-homologous non-sister chromatids because they have different genes and alleles.