Gene Regulation and Epigenetics · USMLE Step 1

In X-chromosome inactivation in females, which epigenetic mechanism plays a central role in silencing one X chromosome?

  1. Widespread histone acetylation across the inactive X chromosome
  2. Active DNA demethylation of genes along the inactive X chromosome
  3. DNA methylation and heterochromatin formation on the inactive X
  4. Point mutations inactivating individual genes on the X chromosome
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Correct answer: DNA methylation and heterochromatin formation on the inactive X

X-chromosome inactivation involves widespread DNA methylation and chromatin condensation (heterochromatin formation) of one X chromosome, silencing most of its genes, a classic epigenetic mechanism.

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