AP BIOLOGY

Detailed explanation-1: -Crossing over occurs during pachytene stage between non sister chromatids of a pair of homologous chromosomes. Diplotene is characterised by appearance of chiasma marking the locations at which crossing over has already occurred in the pachytene stage.

Detailed explanation-2: -Crossing over occurs during the pachytene stage of prophase in meiosis, however the chiasma are visible during. No worries!

Detailed explanation-3: -During meiosis, crossing-over occurs at the pachytene stage, when homologous chromosomes are completely paired.

Detailed explanation-4: -Pachytene is the third stage of meiosis prophase-1 (a five-stage process). It is preceded by the zygotene stage, where synapsis, i.e. pairing of homologous chromosomes take place. Recombination or crossing over takes place at this stage.