Biology : PU-CET (U.G) 2015 Panjab University Entrance Exam With Answers

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27.

Crossing over occurs at :

A: Two Strand Stage

B: Four Strand Stage

C: Three Strand Stage

D: One Strand Stage

The answer is: B

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Explanation

Crossing over occurs at the "Four Strand Stage". Crossing over is the exchange of genetic material between homologous chromosomes during meiosis. It occurs during the prophase I stage of meiosis, specifically at the four-strand stage of meiotic prophase I.

During this stage, the replicated homologous chromosomes pair up and form tetrads (a group of four chromatids). At this stage, crossing over can occur between non-sister chromatids of the homologous chromosomes, resulting in the exchange of genetic material. This process increases genetic diversity by creating new combinations of alleles on the chromosomes.