chiasma assembly

Definition

Target type: biologicalprocess

The cell cycle process in which a connection between chromatids assembles, indicating where an exchange of homologous segments has taken place by the crossing-over of non-sister chromatids. [PMID:23396135]

Chiasma assembly is a critical process in meiosis, the specialized cell division that produces gametes (sperm and egg cells). It ensures the proper segregation of chromosomes during meiosis, preventing aneuploidy (abnormal chromosome numbers) in offspring. Here's a detailed description of the process:

1. **Synapsis:** During prophase I of meiosis, homologous chromosomes (those that carry the same genes but may have different versions of those genes) align and pair up along their entire length. This pairing is mediated by specialized proteins called synaptonemal complex proteins, which form a ladder-like structure that holds the chromosomes together.

2. **Recombination initiation:** While the chromosomes are synapsed, programmed double-strand breaks (DSBs) are introduced into the DNA. These breaks are not random; they occur at specific sites called "hotspots" that are enriched with certain DNA sequences.

3. **Strand exchange and Holliday junction formation:** Each DSB is processed by a complex machinery involving proteins like Spo11, Mre11, Rad50, and others. The ends of the broken DNA strands are then exchanged with their counterparts on the homologous chromosome, leading to the formation of a Holliday junction – a cross-shaped structure where four DNA strands are intertwined.

4. **Branch migration:** The Holliday junction can move along the DNA molecule, extending the region of recombination. This process is called branch migration.

5. **Resolution:** The Holliday junctions are resolved, cutting the exchanged strands and rejoining them. This resolution can occur in two ways, leading to either crossing over or non-crossing over events.

6. **Chiasma formation:** Crossing over events generate physical links between the two homologous chromosomes, known as chiasmata. These chiasmata are the visible manifestations of genetic recombination. They hold the homologous chromosomes together until anaphase I, ensuring their proper segregation to opposite poles of the cell.

7. **Significance of chiasma assembly:** Chiasma assembly is crucial for several reasons:

* **Genetic diversity:** Recombination generates new combinations of genes, increasing genetic diversity within a population.
* **Chromosomal segregation:** Chiasmata prevent chromosomes from segregating randomly during anaphase I, ensuring that each daughter cell receives a complete set of chromosomes.
* **Evolutionary advantage:** Genetic diversity provides raw material for natural selection, driving evolution and adaptation.

In summary, chiasma assembly is a tightly regulated and complex process that ensures proper chromosome segregation and contributes to genetic diversity. It is essential for the successful completion of meiosis and the production of healthy offspring.'
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Proteins (1)

Compounds (1)