Question
What is the product of deamination of methylated cytosine?
| A. |
Uracil |
| B. |
Cytosine |
| C. |
Guanine |
| D. |
Thymine |
Show Answer
|
Correct Answer » D Explanation |
 |
Cytosine can undergo methylation to form 5-methylcytosine, an important epigenetic modification in DNA.
When 5-methylcytosine undergoes spontaneous deamination:
- The amino group is removed
- It gets converted into thymine
Reaction:
5-Methylcytosine → Thymine
Explanation of the Correct Answer
D. Thymine
- Normal cytosine deamination produces uracil.
- However, methylated cytosine (5-methylcytosine) deaminates to thymine.
- This is an important source of point mutations in DNA.
Clinical importance:
- CG dinucleotides are mutation hotspots because:
- Cytosine methylation is common
- Deamination produces thymine
- DNA repair mechanisms may fail to recognize this mismatch
Why Other Options Are Incorrect
A. Uracil
- Uracil is formed by deamination of unmethylated cytosine.
- Reaction:
- Cytosine → Uracil
- Not the product of methylated cytosine.
B. Cytosine
- Cytosine is the substrate, not the deamination product.
C. Guanine
- Guanine is unrelated to deamination of methylated cytosine.