Biological Systems MCAT Practice Exam

What occurs to one of the X chromosomes in female cells influenced by X inactivation?

1. It becomes a Barr body

2. It replicates excessively

3. It enhances gene expression

4. It degrades completely

The correct answer is: It becomes a Barr body

In female mammals, X inactivation is a mechanism that ensures dosage compensation between males (who have one X chromosome) and females (who have two X chromosomes). During early embryonic development, one of the two X chromosomes in each cell is randomly inactivated, which is a vital process to balance the expression of X-linked genes between the sexes. The inactivated X chromosome condenses into a structure known as a Barr body, which is largely transcriptionally inactive. This means that the genes on that X chromosome are not expressed, helping prevent an overexpression of genes that could lead to developmental issues. The presence of a Barr body is an essential feature of female cells and is a direct result of X inactivation. This process is not simply a degradation of the chromosome (which would imply complete loss of the genetic material) or an enhancement of gene expression. Additionally, while both X chromosomes initially replicate during the S phase of the cell cycle, one is then largely silenced and does not replicate excessively compared to the other. Thus, the formation of a Barr body from the inactivated X chromosome is the correct and defining characteristic of X inactivation.