Question

Which of the following is/are posttranslational modification(s) involved in epigenetic control of gene expression?

• A. Arginine methylation
• B. Lysine acetylation
• C. Cytosine methylation
• D. Cytosine deamination

Hint:

Ask which options modify a protein after translation versus which ones modify the DNA base directly.

Answer 1

The Correct Option is A, B

Step 1: Understand what the question is asking.
Epigenetic control means regulating gene expression without changing the underlying DNA sequence, mainly by changing how tightly DNA is packaged around histone proteins and by chemically marking the DNA itself.
The question specifically asks for posttranslational modifications, that is, chemical changes made to a protein after it has been translated, not changes made directly to DNA bases.

Step 2: Check option (A), arginine methylation.
Arginine residues on the tails of histone proteins (such as histone H3 and H4) get methylated by protein arginine methyltransferases after the histone protein is made.
This is a modification of an amino acid side chain on an existing protein, so it is a posttranslational modification, and it changes how histones interact with DNA and other regulatory proteins, altering chromatin structure and gene expression. So (A) is correct.

Step 3: Check option (B), lysine acetylation.
Lysine residues on histone tails are acetylated by histone acetyltransferases (HATs) after translation, neutralizing the positive charge on lysine and loosening histone-DNA contacts, which generally opens up chromatin and promotes transcription.
Since this modifies an existing histone protein after it is made, it is also a posttranslational modification, and it is one of the best known epigenetic marks. So (B) is correct.

Step 4: Check option (C), cytosine methylation.
Cytosine methylation adds a methyl group directly to a cytosine base in DNA (usually at CpG sites), forming 5-methylcytosine. This is a chemical modification of the DNA molecule itself, not of a protein.
Cytosine methylation is a major epigenetic mechanism, but it is not a posttranslational modification because no protein is being modified after translation, it is a direct DNA modification. So (C) is not a valid answer to this specific question.

Step 5: Check option (D), cytosine deamination.
Cytosine deamination is the loss of an amino group from cytosine, converting it to uracil (or converting 5-methylcytosine to thymine). This is a chemical change to DNA that is usually a source of mutation and DNA damage, not a regulated epigenetic mark, and it is not a protein modification either. So (D) is wrong on both counts.

Final Answer:
Only arginine methylation and lysine acetylation are chemical modifications made to proteins (histones) after translation that also control gene expression epigenetically; the cytosine changes act directly on DNA, not on a protein.
\[ \boxed{\text{(A) Arginine methylation and (B) Lysine acetylation}} \]