AP BIOLOGY

Detailed explanation-1: -Crossing over is an enzyme-mediated process, where the exchange of genetic material between non-sister chromatids of homologous chromosomes takes place. It occurs at the pachytene stage of prophase I of meiosis.

Detailed explanation-2: -Crossing over occurs only during prophase I. The complex that temporarily forms between homologous chromosomes is only present in prophase I, making this the only opportunity the cell has to move DNA segments between the homologous pair.

Detailed explanation-3: -Crossing over takes place at the pachytene stage of prophase I of Meiosis. Crossing over includes the symmetrical division of chromatids, and the reciprocal exchange and crosswise assembly of segments between non-sister chromatids, often breaking linkage. This results in the recombination of genes.

Detailed explanation-4: -So, the correct answer is Pachytene.

Detailed explanation-5: -Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad.