A condensed and inactivated X chromosome which is present in female cells is called a Barr body. Barr body meaning relates to the inaccessibility of proteins causing gene transcription. It regulates the transcription of X-linked gene product.
The functional X chromosome is covered by euchromatin, whereas Barr body is packed in heterochromatin. The active X chromosome has a substantially larger volume and remains more spread out.
Formation of Barr Bodies
Inactivation of any chromosome is random and such chromosome remains inactivate throughout the life of that cell. Hence, there is only one active X chromosome in a particular cell.
The number of Barr body always remains less by one unit compared to the total number of X chromosomes.
When attempting to understand what is a Barr body, it is important to take note of mutation. Even when an additional X chromosome is added, such as Klinefelter syndrome in males, Barr body will still be less than X chromosomes.
The Barr body diagram indicated below.
Barr Body Function
Significance of Barr body is understood with its utility in detection of physiological abnormality. In female cells, Barr body may be present in lobes of neutrophils. Such detection is important for reporting any abnormality.
Lyonisation Barr body is the process of making an X chromosome compact. It completely shuts down to form the Barr body. The inactivation of a gene is required so that it does not undergo transcription. To understand what is a Barr body, one needs to be familiar with the Lyon Hypothesis.
The hypothesis is –
At the point of early development, inactivation takes place in a random manner.
Once inactivation is initiated, the subsequent cells or progeny cells retain inactivation of the same X-chromosome.
The process allows human females to retain the regular number of genes as present in human males. Female gene has two XX chromosomes, and males have one X and one Y chromosome.
The chromosome inactivation centre is located near X chromosomes centromere. The gene, X-inactive specific transcript (Xist), leads the inactivation. Hence, the gene is found to be covered with Xist RNA.
When the inactivation process is initiated, the production of Xist gene is enhanced, which silences the gene by coating the particular chromosome.
The calico cat exemplifies X-inactivation mechanism. If a female calico cat is heterozygous for tan and black coat colour gene present in X, the other two X chromosomes will be inactivated. Such inactivation leads to two alleles of the gene of coat colour. However, the inactivation of the two X chromosomes randomly takes place during cell development.
Barr body experiment is further required in the areas of counting the number of X chromosomes by cells, role of non-coding RNAs and also the model of X inactivation in general.
i. Barr bodies are typically found in _______________
(a) Male and female somatic cells
(b) Female somatic cells
(c) Male somatic cells
(d) Male gametes
ii. If a female's cells have three chromosomes, then what is the number of Barr bodies in each cell?
[To check your answer, see the solution mentioned at the end of the article]
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1. What is the definition of Barr body?
Barr body definition is an inactive sex chromosome present in the nucleus of female somatic cells which is between divisions.
2. Why do males do not possess Barr bodies?
Males have only one active chromosome. Hence, they do not possess any Barr bodies.
3. Why is Barr body formed?
Barr body is formed when an X chromosome is compacted into a dense and small body where the genes remain inactivated.
i. (b) Female somatic cells
ii. (b) Two