Meiosis

Meiosis introduces genetic variation partly through recombination between homologous chromosomes, and the timing of this exchange is precise. During prophase I the homologous chromosomes pair through synapsis, and at the pachytene sub-stage the non-sister chromatids of the paired homologues exchange segments by crossing over at points called chiasmata, a sequence described in NCERT Class 11, Chapter 10 (Cell Cycle and Cell Division). Anaphase II is incorrect because at that stage the sister chromatids of already separated chromosomes move apart, an event that distributes but does not create new allele combinations. Metaphase II is wrong as well, since it merely involves alignment of chromosomes at the equator before the second division. Telophase I marks the reorganisation of nuclei after homologues have separated and so comes after recombination has already occurred. Prophase I is itself subdivided into leptotene, zygotene, pachytene, diplotene and diakinesis, and the exchange specifically follows the complete synapsis achieved at zygotene, which is why pachytene rather than any earlier sub-stage is the site of crossing over. The recombined chromatids first become visible as chiasmata when the homologues begin to separate at diplotene. A logical consistency check confirms the answer: new combinations of alleles can arise only when homologous chromatids physically exchange parts, and this crossing over is restricted to the pachytene stage of prophase I.