What is a Barr body?

• A. A protein complex involved in DNA replication
• B. A type of RNA molecule involved in gene expression
• C. A structure formed by active X chromosomes in male cells
• D. An inactivated X chromosome in a female cell

Answer: (D)

Verified step by step guidance

1. Understand that a Barr body is related to the X chromosome in female cells. Females have two X chromosomes, while males have one X and one Y chromosome.
2. Recognize that in female cells, one of the X chromosomes becomes inactivated to prevent an overdose of gene expression from having two active X chromosomes. This inactivated X chromosome is known as a Barr body.
3. Learn that the process of X chromosome inactivation is random, meaning either the maternal or paternal X chromosome can become a Barr body in different cells.
4. Note that the Barr body is visible under a microscope as a dense, dark-staining structure in the nucleus of the cell.
5. Understand that the presence of a Barr body is a normal phenomenon in female cells and is crucial for dosage compensation, ensuring that females, like males, have one functional copy of the X chromosome.