Question

Briefly describe crossing over.

Hint:

Recall that it happens at pachytene between non-sister chromatids of paired homologous chromosomes.

Answer 1

Understanding the Question:
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis.

Detailed Explanation:
It happens at the pachytene stage of prophase I, when homologous chromosomes pair up closely (synapsis) and form a tetrad of four chromatids.
At certain points, called chiasmata, the non-sister chromatids break and rejoin with each other, swapping segments of DNA.
Because the swapped chromatids now carry a new combination of alleles from both parental chromosomes, crossing over is the main source of genetic recombination, producing gametes with combinations of traits different from either parent.

Final Answer:
Crossing over is the exchange of chromatid segments between homologous chromosomes at pachytene, creating new allele combinations and driving genetic variation.