Cell Cycle Stages and Key Events — NEET Biology

Correct answer: A — A-4-ii, B-1-iii, C-3-i, D-2-iv

This question tests the detailed understanding of the sequence of events and characteristic features of each sub-stage of Prophase I of Meiosis. A. Zygotene: Characterized by the pairing of homologous chromosomes (synapsis), which is accompanied by the formation of the synaptonemal complex. Thus, A matches 4 and ii. NCERT states: "During this stage [zygotene], chromosomes start pairing together and this process of association is called synapsis. ...chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex." B. Pachytene: During this stage, recombination nodules appear, where crossing over (recombination of genetic material) occurs between non-sister chromatids. Thus, B matches 1 and iii. NCERT states: "This stage [pachytene] is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids... Crossing over leads to recombination of genetic material...". C. Diplotene: Recognized by the dissolution of the synaptonemal complex and the appearance of X-shaped structures called chiasmata at the sites of crossovers. Thus, C matches 3 and i. NCERT states: "The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata." D. Leptotene: The chromosomes become gradually visible and compaction continues throughout this stage. Thus, D matches 2 and iv. NCERT states: "During leptotene stage the chromosomes become gradually visible... The compaction of chromosomes continues throughout leptotene." Based on this analysis, the correct match is A-4-ii, B-1-iii, C-3-i, D-2-iv.