At the diplotene stage of meiosis, homologous chromosomes are held together by which one of the following structures? (1 mark)

Question

BlackTom AI · Accepted Answer

Chiasmata are the visible X-shaped points where homologous chromosomes remain physically connected during diplotene of prophase I.

1. Relevant knowledge points and concepts
- Prophase I of meiosis is divided into leptotene, zygotene, pachytene, diplotene and diakinesis.
- During pachytene homologous chromosomes undergo crossing over; DNA exchange creates physical links.
- The synaptonemal complex (which held homologs tightly paired) disassembles by diplotene, but homologs remain attached at sites of crossing over called chiasmata (singular: chiasma).
- Chiasmata are the actual structures that hold homologues together until anaphase I so they segregate properly.

2. Step-by-step reasoning process
- In zygotene homologous chromosomes synapse; in pachytene crossing over occurs and recombination nodules form.
- After recombination, the synaptonemal complex breaks down in diplotene.
- Despite synaptonemal complex loss, the homologous chromosomes stay associated at recombination sites; these are observed as chiasmata.
- Therefore, at diplotene the persistent connections are chiasmata.

3. Intermediate “steps” (conceptual sequence)
- Leptotene: condensation begins.
- Zygotene: pairing (synapsis) begins.
- Pachytene: crossing over (formation of chiasmata).
- Diplotene: synaptonemal complex disassembles; chiasmata remain visible.
- Diakinesis: chromosomes condense further; chiasmata terminalize before metaphase I.

4. Why the correct answer is correct
- Chiasmata are the remnant physical links produced by crossing over and are specifically the structures that hold homologous chromosomes together at diplotene.

5. Why the other options are incorrect
- Kinetochores: protein complexes at centromeres that attach chromosomes to spindle microtubules, not the connectors between homologues at diplotene.
- The spindle: a microtubule apparatus for chromosome movement, which forms later (metaphase/anaphase), not the tether at diplotene.
- Centromeres: regions that hold sister chromatids together and serve as kinetochore sites; they do not explain homologous pairing via crossing over at diplotene.

6. Conclusion
- The correct choice is b. Chiasmata, which matches the provided answer.

At the diplotene stage of meiosis, homologous chromosomes are held together by which one of the following structures? (1 mark)单项选择题

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