Question

What is a Barr body, and where is it found in a cell?

Short answer

Answer: A Barr body is an inactivated X chromosome present in the nucleus of most female mammalian cells, serving as a mechanism to balance the expression levels of X-linked genes between males and females. It is formed during early development and can be found within the nucleus, typically at the nuclear periphery.

Step-by-step solution

Definition of a Barr Body

A Barr body, also known as a sex chromatin, is an inactivated X chromosome present in the nucleus of most female mammalian cells. It represents a mechanism called X-chromosome inactivation used by female organisms to balance the expression levels of X-linked genes between males and females.

Formation of a Barr Body

In female mammals, one of the two X chromosomes is inactivated during early development to prevent the overexpression of X-linked genes. This inactivated X chromosome then condenses and becomes a transcriptionally silent structure called a Barr body. The process of X-chromosome inactivation is random, and each cell inactivates either the maternal or paternal X chromosome. As a result, females are considered "mosaic" because their cells may express genes from either the maternal or paternal X chromosome.

Location of a Barr Body in a Cell

In a cell, the Barr body is found within the nucleus, typically at the nuclear periphery. Being a condensed structure, it is tightly associated with the nuclear lamina and heterochromatin regions. This inactivated X chromosome remains in the Barr body state throughout the cell's life, and it is also replicated and passed to daughter cells during cell division.