Transcriptional repression by methylation of DNA involves the methylation of which nucleotide?

• A. Adenine.
• B. Uracil.
• C. Cytosine.
• D. Thymine.
• E. Guanine.

Answer: (C)

Verified step by step guidance

1. Understand that DNA methylation is a process by which methyl groups are added to the DNA molecule, typically affecting gene expression.
2. Recognize that in eukaryotic cells, DNA methylation commonly occurs at the 5th carbon of the cytosine ring, forming 5-methylcytosine.
3. Recall that cytosine is one of the four main bases found in DNA, along with adenine, guanine, and thymine.
4. Note that methylation of cytosine is a key epigenetic mechanism that can lead to transcriptional repression, meaning it can silence gene expression.
5. Conclude that the nucleotide involved in transcriptional repression by methylation in DNA is cytosine.