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Difficulty: MediumCell Biology, Genetics, and Evolution

With reference to meiotic cell division in sexually reproducing organisms, arrange the following events occurring during Prophase I in their correct chronological sequence from earliest to latest:

  1. 1Pairing of homologous chromosomes (synapsis) and assembly of the synaptonemal complex
  2. 2Exchange of genetic material through crossing over at recombination nodules between non-sister chromatids
  3. 3Dissolution of the synaptonemal complex leaving homologous chromosomes attached only at X-shaped chiasmata
  4. 4Complete terminalization of chiasmata accompanied by the disintegration of the nucleolus and nuclear envelope

Answer

Pairing of homologous chromosomes (synapsis) → Exchange of genetic material (crossing over) → Dissolution of synaptonemal complex with visible chiasmata → Terminalization of chiasmata and nuclear envelope breakdown
Prophase I of meiosis comprises five distinct chronological sub-stages: Leptotene (compaction of chromosomes), Zygotene (synapsis and formation of synaptonemal complex), Pachytene (crossing over between non-sister chromatids), Diplotene (dissolution of the synaptonemal complex and visibility of chiasmata), and Diakinesis (terminalization of chiasmata and breakdown of the nuclear envelope). Therefore, synapsis precedes crossing over, followed by chiasmata emergence and subsequent terminalization.

Step-by-Step Solution

1
Identify the stage associated with chromosome pairing and synapsis
Synapsis occurs during Zygotene, where homologous chromosomes align and form the synaptonemal complex.
Physical pairing of homologues must precede any coordinated reciprocal genetic exchange.
2
Identify the stage associated with genetic recombination
Crossing over occurs during Pachytene, facilitated by the recombinase enzyme complex at recombination nodules.
Recombination takes place between aligned non-sister chromatids within the established bivalent.
3
Identify the stage characterized by synaptonemal complex dissolution and chiasmata formation
During Diplotene, the synaptonemal complex dissolves, and homologous chromosomes separate except at crossover sites (chiasmata).
The structural dissolution reveals the physical crossing points between non-sister chromatids.
4
Identify the final sub-stage of Prophase I
During Diakinesis, chiasmata shift to chromosome ends (terminalization), the spindle apparatus forms, and the nuclear envelope and nucleolus disappear.
This completes Prophase I and prepares the cell for Metaphase I alignment.

Key Concept

Sequential sub-stages of Prophase I in Meiosis: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis
Estimated Time:1m 15s
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Topics

Cell Biology, Genetics, and Evolution

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