Epigenetic mark: At which short sequence context is DNA most commonly methylated in many organisms (for example, CpG islands in vertebrates)?

Introduction / Context:
DNA methylation is a key epigenetic modification. In vertebrates, methylation occurs predominantly at cytosines in CpG dinucleotides and is associated with gene regulation, genomic imprinting, and transposon silencing.

Given Data / Assumptions:

• Cytosine methylation at CpG sites is the canonical mark in mammals.
• CpG islands are CpG-rich regions often found near gene promoters.
• Other promoter motifs (TATA, CAAT) are not methylation consensus sites.

Concept / Approach:
Identify the sequence context most commonly methylated: CpG. While some species show non-CpG methylation, the best-known and most frequent context in vertebrates is CpG.

Step-by-Step Solution:
Recall methyltransferase enzymes target cytosines at CpG dinucleotides. Promoter motifs (TATA/CAAT) are binding sites, not methylation consensus sequences. Select “CG dinucleotide sequences (CpG sites)”.

Verification / Alternative check:
Bisulfite sequencing patterns across genomes show dense CpG methylation outside of CpG islands and relative protection within unmethylated islands at active promoters.

Why Other Options Are Wrong:
AC, TATA, and CAAT motifs are not the canonical methylation targets; methylation is not random.

Common Pitfalls:
Assuming methylation occurs uniformly; ignoring species- and context-specific variation.

Final Answer:
CG dinucleotide sequences (CpG sites).