Which statement is true regarding the inactive X chromosome in female mammals?

• A. It does not undergo replication.
• B. It possesses a less condensed chromatin structure compared to the active X chromosome.
• C. It's among the last chromosomes to replicate.
• D. It is actively transcribed.

Answer: (C)

The correct statement regarding the inactive X chromosome in female mammals is that it is among the last chromosomes to replicate. In female mammals, one of the two X chromosomes is randomly inactivated during early embryonic development, a process known as X-inactivation. This inactivated X chromosome undergoes extensive modifications that lead to its transformation into a Barr body, rendering it largely transcriptionally silent.

This inactivation has several consequences for the replication timing of the chromosome. The inactive X chromosome is generally held in a more condensed state, which makes it less accessible for replication at the same time as its active counterpart. As a result, it replicates later in the S phase of the cell cycle compared to the active X chromosome, typically falling among the last regions of the genome to be replicated.

Understanding this concept highlights the regulatory mechanisms in gene expression and replication associated with X-inactivation, showcasing the differences in cellular processes regarding active and inactive chromatin.