Question

Crossing over takes place in
a) Zygotene
b) Diakinesis
c) Pachytene or diplotene
d) Diplotene

Answer 1

Hint :This stage begins with the completion of synapsis and is associated with further thickening and shortening of chromosomes.

Complete answer : During the production of gametes, there is an exchange of chromosome segments between non-sister chromatids, this is called crossing over. It is also known as recombination. Crossing over takes place between the chromatids of two homologous chromosomes. It occurs during meiosis. Pachytene stage was the substage in the prophase of meiosis 1. During this stage, the homologous chromosomes thicken and become recombinant. In this, each chromosome of each bivalent begins to split longitudinally into two similar chromatids. As a result each bivalent now contains four chromatids, which are called tetrads. The chromatids belonging to the same chromosomes are called sister chromatids and those belonging to different chromosomes are called non-sister chromatids. Non-sister chromatids in each bivalent exchange their segments during pachytene. This is known as crossing over. This results in the exchange of chromosome segments between homologous chromosomes. This leads to genetic recombination. The points at which crossing over takes place are known as chiasmata.

Additional information :-The homologous chromosomes come closer and begin to pair, this pairing is called synapsis.
-It happens as the result of sexual reproduction and not because of non-sexual reproduction.
-The main advantage of crossing over is the greater variation in offspring. So, this will increase the chance of offspring to survive and reproduce.
So, the correct answer is pachytene.

Note: -Meiosis is a type of cell division, where the reduction of chromosomes occurs. In this, the number of chromosomes in the parent cell reduces by half and produces four gamete cells. Meiosis is very much required to produce egg and sperm cells for the sexual mode of reproduction.