Question

Crossing over occurs in

• A. Four strand stage
• B. Three strand stage
• C. Two strand stage
• D. Single strand stage

Answer 1

The correct option is A Four strand stage

During the zygotene and pachytene stages of the prophase I of meiosis, there is pairing or synapsis between homologous chromosomes. In this pairing, similar parts of the chromosomes remain side by side by mutual attraction between allelic genes. This is known as a two-strand stage or bivalent. Further, in the diplotene stage of prophase I of meiosis, each chromosome splits longitudinally into two chromatids. Now bivalent is composed of four chromatids. This stage is known as the four-strand stage or tetrad. The chromatids evolving from the same chromosome are known as the sister chromatids. Crossing over is detected at the four-strand stage.

So, the correct answer is 'Four strand stage'.