AP BIOLOGY

Detailed explanation-1: -The chiasma is a structure that forms between a pair of homologous chromosomes by crossover recombination and physically links the homologous chromosomes during meiosis.

Detailed explanation-2: -During meiosis, crossing-over occurs at the pachytene stage, when homologous chromosomes are completely paired. At diplotene, when homologs separate, the sites of crossing-over become visible as chiasmata, which hold the two homologs of a bivalent together until segregation at anaphase I.

Detailed explanation-3: -The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual “crossing-overs” of genetic material are thought to occur during the previous pachytene stage.

Detailed explanation-4: -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.