Methylation Typically Occurs on C (Cytosine) Nucleotides
The correct answer is C (cytosine). DNA methylation involves the addition of a methyl group to cytosine nucleotides, making this the fundamental nucleotide target for methylation in biological systems 1.
Why Cytosine is the Answer
Methylation specifically targets cytosine bases in DNA, most commonly in the CpG dinucleotide context (where a cytosine is followed by a guanine). The evidence consistently demonstrates that:
- In vertebrates and mammals, methylation predominantly occurs at cytosines within CpG sites 1
- The methylation process converts cytosine to 5-methylcytosine (5mC), which is the most common chemically modified base in eukaryotic genomes 2
- Methylation calling algorithms specifically count reads covering "every single C, both methylated and unmethylated" and report these based on context (CpG, CHG, and CHH) 1
Context-Specific Methylation Patterns
While cytosine is the target nucleotide, the specific context matters:
- CpG context: The most prevalent form in differentiated mammalian cells 3
- CHG and CHH contexts: Also occur (where H represents non-G nucleotides), particularly in plants 4
- DNA methylation between strands is highly correlated at CpG sites in vertebrates 1
Why Not the Other Options
- U (Uracil): Not found in DNA; only in RNA
- G (Guanine): Not methylated itself, but adjacent to cytosine in CpG dinucleotides
- T (Thymine): Not a methylation target; unmethylated cytosines actually convert to thymine during bisulfite sequencing treatment 1
Answer: C