Chiasmata (singular: chiasma) is derived from the Greek word meaning “X-shaped cross.”
It represents the point of contact where two non-sister chromatids of homologous chromosomes are physically linked together.
This contact occurs specifically during the pachytene stage of Prophase I in meiosis I.
The chiasmata maintain the connection between homologous chromatids until Anaphase I of meiosis I, ensuring proper chromosome segregation.
The phenomenon of chiasmata was first discovered and described in 1909 by Frans Alfons Janssens, a Belgian cytologist.
Janssens introduced the term ‘chiasmatypie’ to describe this crossing-over behavior observed during meiosis.
Formation of Chiasmata
During the prophase of meiosis I, homologous chromosomes undergo pairing and recombination.
In this phase, crossing over facilitates regular exchanges between homologous chromosomes.
Prophase I is subdivided into five distinct phases:
Leptotene,
Zygotene,
Pachytene,
Diplotene,
Diakinesis.
Chiasmata form at specific sites where programmed DNA breaks, created by a specialized enzyme called Spo11, follow the complete recombination pathway to generate crossovers.
The precise mechanism of how crossing over translates into the physical formation of chiasmata, which represent exchanges of DNA sequence data, is still not fully understood.
The centromeres of sister chromatids are joined to form a single kinetochore, which attaches to microtubules during orientation of homologous chromosomes on the metaphase plate in meiosis I.
Homologous chromosomes move very close together and form a structure called the synaptonemal complex just before the onset of the pachytene stage.
Formation of the synaptonemal complex initiates chiasma formation and is essential for crossing-over and recombination to occur.
The synaptonemal complex is a protein-based lattice that forms between homologous chromosomes and begins at specific initiation sites, eventually spreading along the entire chromosome length.
This tight pairing of homologous chromosomes is known as synapsis.
Crossing over, the exchange of chromosomal segments between non-sister chromatids of homologous chromosomes, is facilitated by the presence of the synaptonemal complex.
Homologous chromosomes remain connected until anaphase of meiosis I, due to cohesion between chromosome arms distal to the chiasmata, which resists the pulling force exerted by spindle fibers.
At anaphase I, this distal cohesion is released, allowing chiasmata to separate, and the sister chromatids (with at least one crossover exchange) move together toward the same spindle pole.
Consequently, the two daughter cells produced in meiosis I are haploid, each containing chromosomes with two sister chromatids, drawn randomly from the two parents.
The number of chiasmata varies between species and depends on the length of the chromosome.
For accurate and effective homologous chromosome separation during meiosis I, there must be at least one chiasma per chromosome, though the number can reach up to 25 chiasmata per chromosome.
Structure of Chiasmata
The ultrastructure of chiasmata is still unknown, but it is believed that each chiasma consists of two unaltered sister chromatid arms, along with two recombinant arms that contain spliced DNA molecules and associated protein structures.
The distal sister chromatid arms, which remain cohesive between the chiasma and the telomeres, help stabilize the DNA complex on the chromosome.
A single chiasma is sufficient to link homologous chromosomes together during meiosis I.
In most species, the total number of chiasmata (across both males and females) is significantly greater than the total number of chromosomes.
Humans have 39 such chromosome arms on their 23 pairs of homologous chromosomes, excluding the five acrocentric short arms, which generally do not undergo crossovers.
Typically, only one chiasma is produced per arm in most cases.
In human males, the number of chiasmata usually ranges from 46 to 53.
In human females, a single chiasma can maintain stable homologous pairing for over 40 years, and it is released at the appropriate time as the oocyte matures into an egg.
Measurement of Chiasmata
The distances between neighboring chiasmata can be quantified in cytological units, such as microns, particularly in animal species with cytologically advantageous meiotic chromosomes.
It is possible to create a frequency distribution of these inter-chiasmata distances to analyze their spacing patterns.
Unlike the typical exponential distribution seen in linkage distance distributions, the distribution of inter-chiasmata distances is often found to be modal, indicating the presence of a most common spacing distance rather than a random exponential spread.
Significance of Chiasmata
During meiosis I, chiasmata are essential for ensuring that homologous chromosomes attach to opposing spindle poles and properly segregate to opposite ends of the cell.
At each chiasma, a chromosomal crossover occurs, involving the exchange of genetic material between the chromatids of homologous chromosomes.
The absence of chiasmata during meiosis often leads to improper chromosomal segregation, which can result in aneuploidy—an abnormal number of chromosomes in the daughter cells.
The presence of chiasmata marks the physical locations where genetic exchanges (crossovers) have been successfully completed.
Individuals with mutations in the cohesion subunit gene are typically infertile due to a lack of chiasmata, which prevents the completion of meiosis.
References
Andersen, S. L., & Sekelsky, J. (2010). Comparison of meiotic and mitotic recombination: Different pathways for double-strand break repair and their unique outcomes. BioEssays, 32(12), 1058–1066. https://doi.org/10.1002/bies.201000087
Fledel-Alon, A., Wilson, D. J., Broman, K., Wen, X., Ober, C., Coop, G., & Przeworski, M. (2009). Large-scale recombination patterns that support accurate chromosome separation in humans. PLoS Genetics, 5(9), e1000658. https://doi.org/10.1371/journal.pgen.1000658
Hirose, Y., Suzuki, R., Ohba, T., Hinohara, Y., Matsuhara, H., Yoshida, M., Itabashi, Y., Murakami, H., & Yamamoto, A. (2011). Role of chiasmata in monopolar attachment and co-segregation of sister chromatids during meiosis I. PLoS Genetics, 7(3), e1001329. https://doi.org/10.1371/journal.pgen.1001329
Passarge, E. (2001). Crossing-over during Prophase I. In Color Atlas of Genetics (p. 118). Thieme Medical Publishers, Stuttgart/New York.
Pollard, T. D., Earnshaw, W. C., Lippincott-Schwartz, J., & Johnson, G. T. (2017). Meiosis: Chromosome behavior and segregation. In Cell Biology (3rd ed., pp. 779–795). Elsevier. https://doi.org/10.1016/B978-0-323-34126-4.00045-1
Roger Williams University. (2022). Meiosis and Sexual Reproduction. Retrieved from https://rwu.pressbooks.pub/bio103/chapter/meiosis-and-sexual-reproduction/
Stahl, F. W. (2013). Crossover interference and its effects. In Brenner’s Encyclopedia of Genetics (2nd ed., pp. 226–228). Academic Press. https://doi.org/10.1016/B978-0-12-374984-0.00357-0
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