Heterochromatin is tightly coiled chromosomal material that carries genes and is largely inert. It exists in two forms and represses gene transcription through position effect variegation. It is found in all eukaryotes except bacteria and can contribute to aging and degenerative conditions.
Heterochromatin is a form of tightly coiled chromosomal material that carries genes and is considered genetically largely inert. It exists in two forms: constitutive and facultative heterochromatin. Constitutive heterochromatin is considered to be fixed in both form and function and is found on chromosomes 1, 9, 16, and Y, or typically in locations such as telomeres at the ends of chromosomes. Facultative heterochromatin can develop into a normal euchromatin condition, which comprises most of the genetically active material in the human body and is found on inactive X chromosomes.
Where heterochromatin formation is present, it usually represses the transcription of genetic information in itself or nearby chromatin regions, and this effect on gene expression is known as position effect variegation. The reason that heterochromatin formation prevents gene expression has been narrowed down in research to one of three causes. The protein heterochromatin can affect regions of nearby normal chromatin by repressing their ability to express genes, or it can reside in regions where transcription is normally excluded, such as in the chromocenter. The third method of repression does not apply to human genetic coding and is instead a limiting factor on gene expression in some insect species that have been studied for the effect, such as fruit flies.
Position effect variegation has now been established by science as a condition in all species of eukaryotes, including yeasts, and every other life form except bacteria, some blue-green algae, and other primitive organisms. In mammalian organisms, heterochromatin is concentrated in the centromere, or central structure of a chromosome, where the two chromatids are held together to form an X shape. It is also found at the telomeres, or ends of chromosome segments, of deoxyribonucleic acid (DNA).
At the centromere location, heterochromatin is a form of satellite DNA, which is a string of short, repetitive nucleotide sequences. Normally, satellite DNA is not involved in transcription as it is not in this case. It is also known to be a highly mutable form of DNA prone to carcinogenic effects, but, in this case, centromeric heterochromatin is a stable and protective feature of the chromosome. The location appears to play a primarily structural or architectural role by serving as a binding medium for kinectors, which are the locations on either side of a chromosomal centromere where spindle fibers are attached during the cell division process.
The presence of heterochromatin in the body can have a direct correlation with the aging process. It was once believed that the positions of this chromosomal material were established in an organism’s early development and maintained throughout its life cycle. Research from the Geron Corporation now indicates that concentrations decline as the body ages, and this has been observed in both yeast and mouse studies in which heterochromatin’s ability to suppress gene expression gradually fades over time. As gene expression increases in these regions, it can contribute to mutations and degenerative conditions associated with aging.
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