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Zygotene

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Second stage of prophase I in Meiosis

Zygotene (from greek "paired threads") is the second stage of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes. It follows the Leptotene stage and is followed by Pachytene stage.

Synapsis completion

The key event during zygotene is the completion of synapsis between homologous chromosomes. Synapsis began during the previous leptotene stage, with the homologous chromosomes starting to pair together and associate lengthwise, facilitated by the synaptonemal complex protein structure.

In zygotene, the synaptonemal complex forms more extensively between the paired chromosomes. It zips the homologs together along their entire length, with the lateral elements of the complex associated with each chromosome and the central region holding them together. This allows intimate pairing and genetic recombination events.

Chromosome condensation

The chromosomes continue condensing during zygotene into distinct threadlike structures. Each chromosome now appears thicker as the sister chromatids are closely aligned.

Recombination nodules

As synapsis completes, proteinaceous recombination nodules begin to appear along the synaptonemal complex between the homologous chromosomes. These represent sites of genetic crossover events, where exchange of chromosomal segments occurs between the non-sister chromatids.

Key recombination proteins like MLH1/3 and MSH4/5 mark the sites of crossover formation. The number and positioning of these crossovers is regulated to ensure at least one crossover per chromosome arm for proper segregation in later meiotic stages.

Transition to pachytene

Once synapsis and crossing over are complete, the cell transitions to the pachytene stage of prophase I. Pachytene features fully condensed and paired chromosomes along their length, with distinctly visible recombination nodules.

Importance

The zygotene stage is crucial for genetic recombination and proper chromosome segregation in meiosis. Defects in synapsis, recombination, or crossover regulation can lead to aneuploidy and chromosomal abnormalities in gametes.

References

  1. ^ "Stages of Meiosis and Sexual Reproduction | Learn Science at Scitable". Nature.com. Retrieved 2024-05-08.
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  5. Zickler, D.; Kleckner, N. (1998-01-01). "The leptotene-zygotene transition of meiosis". Annual Review of Genetics. 32: 619–626. doi:10.1146/annurev.genet.32.1.619. PMID 9928494.
  6. Zhang, Fengguo; Liu, Mengfei; Gao, Jinmin (2022). "Alterations in synaptonemal complex coding genes and human infertility". International Journal of Biological Sciences. 18 (5): 1933–1943. doi:10.7150/ijbs.67843. ISSN 1449-2288. PMC 8935243. PMID 35342360.
  7. Naranjo, Tomás (2012). "Finding the Correct Partner: The Meiotic Courtship". Scientifica. 2012: 509073. doi:10.6064/2012/509073. ISSN 2090-908X. PMC 3820632. PMID 24278707.
  8. Zickler, Denise; Kleckner, Nancy (2015). "Recombination, Pairing, and Synapsis of Homologs during Meiosis". Cold Spring Harbor Perspectives in Biology. 7 (6): a016626. doi:10.1101/cshperspect.a016626. ISSN 1943-0264. PMC 4448610. PMID 25986558.
  9. Azumi, Y. (2002-06-17). "Homolog interaction during meiotic prophase I in Arabidopsis requires the SOLO DANCERS gene encoding a novel cyclin-like protein". The EMBO Journal. 21 (12): 3081–3095. doi:10.1093/emboj/cdf285. PMC 126045. PMID 12065421.
  10. "Pachytene - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2024-02-15.
  11. Saito, Takamune T.; Colaiácovo, Monica P. (2017). "Regulation of crossover frequency and distribution during meiotic recombination". Cold Spring Harbor Symposia on Quantitative Biology. 82: 223–234. doi:10.1101/sqb.2017.82.034132. ISSN 0091-7451. PMC 6542265. PMID 29222342.
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