Gene Regulation and Epigenetics · USMLE Step 1

Which chemical modification of DNA is most commonly involved in gene silencing when present in promoter CpG islands?

  1. Phosphorylation of cytosine
  2. Uracil incorporation
  3. Methylation of cytosine at CpG sites
  4. Acetylation of guanine
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Correct answer: Methylation of cytosine at CpG sites

Methylation of cytosines at CpG dinucleotides (forming 5-methylcytosine) in promoter regions is frequently associated with transcriptional repression of the associated gene.

Difficulty: Medium Question 2 of 20

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