Synapsis, Bivalents & Chiasmata — NEET Biology
📚 Practice ConceptDescribe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.
Arrange the following events of Prophase I of meiosis in the correct chronological order: 1. Formation of synaptonemal complex and initiation of synapsis. 2. Appearance of X-shaped structures called chiasmata. 3. Gradual visibility of chromosomes under light microscope. 4. Occurrence of crossing over between non-sister chromatids. 5. Full condensation of chromosomes and terminalisation of chiasmata. 6. Distinction of four chromatids per bivalent, appearing as tetrads.
View solution & NCERT explanation
Correct answer: A — 3, 1, 6, 4, 2, 5
Let's arrange the events based on the substages of Prophase I: Leptotene: Characterized by the compaction of chromosomes, making them gradually visible. (Event 3) Zygotene: Homologous chromosomes start pairing (synapsis) and the synaptonemal complex forms. Bivalents (or tetrads) are formed. (Event 1) Pachytene: The four chromatids of each bivalent become distinct (tetrads are clearly visible). Recombination nodules appear, and crossing over occurs. (Events 6, then 4) Diplotene: The synaptonemal complex dissolves, and recombined homologous chromosomes tend to separate, but remain linked at sites of crossovers, forming X-shaped chiasmata. (Event 2) Diakinesis: This is the final stage, marked by terminalisation of chiasmata and full condensation of chromosomes. (Event 5) Therefore, the correct chronological order is 3 → 1 → 6 → 4 → 2 → 5.
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