Barr Bodies | Dosage Compensation - Lyonisation (Heterochromatinisation) | Genetics| Rajesh Biology

Описание: In mammals, males are heterogametic (XY) and females, homogametic
(XX). As the female has two copies of the X chromosome, Dosage
Compensation is achieved by the random inactivation of one of the two ‘X’
chromosomes. The extra X-chromosome undergoes heterochromatinisation
and becomes inactive during early embryonic development. The descendant
cells inherit the same inactivated X chromosome. The heterochromatinized
X-chromosome appears as a darkly-staining body attached to the nuclear
membrane. This phenomenon was first described by Murray L. Barr and
the heterochromatinised X chromosomes are now called Barr Bodies.
Barr and Bertram observed chromatin bodies in the nerve cells of female
cats that were not present in the cells of the male. In humans, this body
can be easily demonstrated in the female cells derived from the buccal
mucosa (cheek cells) or in fibroblasts (undifferentiated connective tissue
cells), but not in similar male cells. A Barr body appears as a small
drumstick-like projection (drumstick appendage) on one of the lobes of
some neutrophils in females . With this technique, the sex of human embryos
can be distinguished at early stages of development. This cellular
characteristic generally seems to apply to all mammals.
X-inactivation (also called Lyonisation, proposed by Mary Lyon and Liane Russell) is a process by which one of the two copies of the X-chromosome
present in the body cells of female mammals is inactivated. The inactive
X-chromosome is ‘silenced’ by making it a transcriptionally inactive
structure called heterochromatic body. As female mammals have two Xchromosomes in each cell, X-inactivation in their cells causes them not to
have twice as many products of the X-chromosomal genes as those of the
males which possess a single copy of the X-chromosome in each cell
(Dosage Compensation).
NOTE : Regardless of how many X-chromosomes a somatic cell possesses,
all but one of them appears to be inactivated and can be seen as Barr
bodies. For example, no Barr body is seen in the somatic cells of Turner
females 45, XO; one is seen in Klinefelter males 47,XXY; two in 47,XXX
females; three in 48,XXXX females; and so on. Therefore, the number
of Barr bodies follows an N-1 rule (N minus one rule), where N is the
total number of X-chromosomes present.

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