Q.The reciprocal exchange of segments of genetic material between non-sister chromatids of a bivalent, a major source of new genetic combinations, takes place during which sub-stage of prophase I?

Meiosis I is the first, reductional division of meiosis, the specialised two-division process restricted to germ cells that ultimately produces four haploid daughter cells from one diploid parent cell. It is meiosis I, not meiosis II, that actually halves the chromosome number, from diploid (2n) to haploid (n), because at anaphase I whole homologous chromosomes -- not sister chromatids -- separate from one another. Meiosis I is distinguished by a dramatically elaborated prophase I, divided into five sub-stages: leptotene (chromosomes become visible as separate, thin threads), zygotene (homologous chromosomes undergo synapsis, pairing precisely along their length via the synaptonemal complex to form bivalents/tetrads), pachytene (crossing over -- reciprocal exchange of genetic material between non-sister chromatids -- occurs at recombination nodules), diplotene (the synaptonemal complex dissolves and homologues begin separating, re …