In which substage of meiotic Prophase I does crossing over (genetic recombination) occur between non-sister chromatids of homologous chromosomes?
Answer and explanation
Correct answer: C. Pachytene
- Step 1: Prophase I of meiosis comprises five sequential substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
- Step 2: In Pachytene, the paired bivalent chromosomes are fully condensed and visible as four distinct chromatids (tetrads).
- Step 3: Crossing over between non-sister chromatids occurs during pachytene, mediated by recombination nodules containing the enzyme recombinase.
Answer
Pachytene.
Why not the others: Leptotene is characterized by initial chromatin condensation into fine threads; Zygotene involves pairing of homologous chromosomes (synapsis) to form the synaptonemal complex; Diplotene is marked by dissolution of the synaptonemal complex and appearance of chiasmata.
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