Question

Describe the importance of Barr bodies; explain the stages and mechanisms of X inactivation in mammals including the importance of the Xic; identify the product the Xist gene and state its function(s) 4.

Answer 1

a) Barr body is named after Murray Barr who discovered it. It is the inactive X chromosome in a female somatic cell. X chromatin in its inactivated form is present as a mass against the nuclear membrane in female. These Barr bodies are present in 40% of females who are considered as chromatin positive and absent in males who are considered as chromatin negative. It is also called as sex chromatin as it indicates the presence of sex hormone.Ttheir importantce is in Barr body testing for determining the sex of an individual.

b) The X chromosome numerous genes that are essential for proper development and cell viability. However, females carry two copies of the X chromosome, resulting in a potentially toxic double dose of X-linked genes. To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation distinct from that used by organisms such as flies and worms. By way of the process called X-chromosome inactivation (XCI), female mammals transcriptionally silence one of their two Xs. The inactivated X chromosome then condenses into a compact structure of the Barr body, and it is stably maintained.

X inactivation (XCI) is done in two different ways: random and imprinted. Although both forms utilize the same RNAs and silencing enzymes, they differ in terms of their developmental timing and mechanism of action.

Imprinted X inactivation: Mechanism involves pre-silencing of the paternal X in the male germ line during a process whereby both sex chromosomes are inactivated during meiosis; here, the inactive state of the X could be transmitted to the next generation. Imprinted XCI may represent an ancestral form of X inactivation. Occurring before the appearance of XIST and TSIX, imprinted XCI led to the evolution of random XCI in placental mammals. Imprinted XCI occured in early mammals, such as marsupials.

Random X inactivation: Random X inactivation occurs in the early female embryo, where both the maternal and the paternal X chromosome have an equal chance of becoming inactivated. Each female cell has the task of trying to distinguish between two X chromosomes within the same nucleus, then designating one as an active X chromosome and the other as an inactive X. This process of silencing is accomplished independently in each cell, largely by XIST and TSIX. This type of XCI occurs in placental mammals.

Importance of XCI: As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome.

c) XIST: RNA plays an important role in X inactivation. Two non coding, complementary RNA's XIST and TSIX initiate and control the XCI process. This XIST RNA has four unique properties:

• The XIST gene does not encode a protein but rather produces a 17 kilobase (kb) functional RNA molecule. Hence, it is a noncoding RNA.
• XIST RNA is only expressed in cells containing at least two Xs and is not normally expressed in male cells.
• XIST RNA remains exclusively in the nucleus and is able to coat the chromosome from which it was produced.
• XIST RNA is expressed from an otherwise inactive X chromosome.

XIST RNA is both necessary and sufficient for inactivation, and it recruits various silencing protein complexes to label the future inactive X chromosome. Increased XIST expression represents a key initiation event in X inactivation, indicating the central role of this noncoding RNA.