Question

Barr body in mammals represents
                   (A) All the heterochromatin in female cells
                   (B) One of the two X chromosomes in somatic cells of females.
                   (C) All the heterochromatin in male and female cells
                   (D) The Y chromosome in somatic cells of male

Answer 1

Hint:-In eukaryotic cells, the DNA and proteins (histones) together form a complex structure called chromatin. The formation of this structure protects the DNA and helps in the replication of DNA. The chromatin has less dense parts called euchromatin. Euchromatin is more accessible, therefore, easily transcribed. The more dense parts of chromatin are called heterochromatin. Heterochromatin consists of constitutive heterochromatin that is usually not transcribed and remains condensed. Facultative heterochromatin, like Barr bodies, is formed when certain genes are silenced.

Complete step-by-step solution:-
Medical researcher M.L. Barr and his student E.G. Bertram noticed this condensed body in the nerve cells of female cats. They found no such bodies in male cats. The Barr body or the sex chromatin body is basically one of the two X chromosomes in the somatic cells of mammalian females. This particular X chromosome is rendered inactive. The process by which the X chromosome is inactivated is known as lyonization.
The Barr body is a part of the heterochromatin, therefore it appears condensed when viewed under a microscope. The X chromosome that will remain active is found in the euchromatin. The X chromosome that becomes a Barr body is randomly selected and remains inactive permanently.
In mammals, the X inactivation center is present in the X chromosome and it codes for twelve genes. Out of these twelve genes, two genes play a major role in this inactivation process. XIST gene codes for XIST RNA that is expressed in cells that have two or more X chromosomes. It inactivates the chromosome. TSIX functions in a manner exactly opposite to that of XIST. Cells that express more of TSIX usually do not inactivate their X chromosome.
The correct answer is Option (B). Barr bodies in mammals represent one of the two X chromosomes in female somatic cells.

Note:-
Barr bodies are always less than the number of X chromosomes (N-1, N being the total number of X chromosomes.)
In marsupials such as kangaroos, the X chromosome that is inactivated comes from the male. All the placental mammals undergo random inactivation of the X chromosome. A hypothesis explains the random inactivation is that the entirety of gene expression does not fall on the mother's X chromosome.