Dicentric chromosome

A dicentric chromosome is an abnormal chromosome with two centromeres. It is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments and the formation of dicentric fragments. The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation and paracentric inversion. Dicentric chromosomes have important roles in the mitotic stability of chromosomes and the formation of pseudodicentric chromosomes. Their existence has been linked to certain natural phenomena such as irradiation and have been documented to underlie certain clinical syndromes, notably Kabuki syndrome. The formation of dicentric chromosomes and their implications on centromere function are studied in certain clinical cytogenetics laboratories.

Formation

Most dicentric chromosomes are known to form through chromosomal inversions, which are rotations in regions of a chromosome due to chromosomal breakages or intra-chromosomal recombinations. Dicentric chromosomes were first detected in lymphocytes from blood smears of civil and military personnel who were assigned to deal with the aftermath of the 1986 Chernobyl nuclear disaster (liquidators). Radiation increases the probability that dicentric chromosomes form after every mitotic event, creating physical bridges between them in anaphase and telophase. This is considered "crisis", a type of cell cycle arrest, and most cells in this state suffer apoptosis. The resulting dicentric chromosomes are highly unstable, giving rise to chromosomal translocations, deletions and amplifications, such as the Robertsonian translocation. This results in broken, deleted gene products of dicentric fragments. == Applications ==

Applications

Dicentric chromosomes are studied in model organisms such as yeast (Saccharomyces cerevisiae). The use of S. cerevisiae as a classical genetic system dates back to the 1950s Strains of S. cerevisiae that tolerate aneuploidy can stabilize products of broken chromosomes during proliferation, which can be recovered and studied in a laboratory setting. FISH involves the use of fluorescent probes to detect and localize specific DNA sequences on chromosomes, and centromere-specific probes can be used to measure dicentric chromosome frequencies. This technique allows for the testing of human blood samples and tissue samples, which have been historically adopted by radiation laboratories. C-banding is a staining technique that can be used to visualize dicentric chromosomes. It specifically stains constitutive heterochromatin, which are regions of the chromosome at or near the centromere. == Consequences ==

Consequences

The existence of dicentric chromosomes has clinically relevant consequences for individuals, which may live with intellectual, neurological and physical disabilities. Tailed nuclei are signatures of radiation exposure in human tissue, microbiota, and aquatic invertebrates from the wake of recent nuclear disasters. such as Kabuki syndrome, a pediatric congenital disorder. a trisomy of chromosome 18, and Turner syndrome, a loss (or partial loss) of the X chromosome. Pseudodicentric chromosomes alone do not define these syndromes, because the contribution of other chromosomal abnormalities are also considered, such as the presence of isochromosomes in Turner patients. == See also ==

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