A Level Biology Practice Exam

What does crossing over involve during meiosis?

1. Sharing of identical genetic material between sister chromatids

2. Exchange of genetic material between homologous chromosomes

3. Separation of chromatids to opposite poles of the cell

4. Replication of DNA before cell division

The correct answer is: Exchange of genetic material between homologous chromosomes

Crossing over is a crucial process that occurs during prophase I of meiosis, where homologous chromosomes pair up closely together in a structure known as a tetrad. This process involves the exchange of segments of genetic material between these homologous chromosomes, leading to genetic recombination. This exchange increases genetic diversity within gametes, which is essential for evolution and adaptability in populations. The significance of crossing over lies in the generation of new combinations of alleles, which contributes to the uniqueness of offspring. Because the exchange occurs between non-sister chromatids of homologous chromosomes, the resultant chromosomes inherit different segments of DNA from each parent, enhancing genetic variation. This process is distinct from sharing genetic material between sister chromatids, which do not involve recombination. It also differs from the separation of chromatids during anaphase II of meiosis, where the sister chromatids are pulled apart, and from the replication of DNA that occurs during the S phase of the cell cycle prior to meiosis. Thus, the correct understanding of crossing over specifically pertains to the exchange of genetic segments between homologous chromosomes.