Cytosine, Thymine and Uracil are similar with respect to

Options

A. C = O at 2nd position in pyrimidine ring
B. `NH_(2)` group at 4th carbon in pyrimidine ring
C. C = O at 4th position in pyrimidine ring.
D. absence of C = O group at 2nd position in pyrimide ring

Answer

To determine how cytosine, thymine, and uracil are similar, we can analyze their structures and functional groups.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Cytosine**: It has a six-membered ring with two nitrogen atoms and several functional groups, including an amino group (-NH2).
- **Thymine**: Similar to cytosine, it has a six-membered ring with two nitrogen atoms, but it also has a methyl group (-CH3) attached.
- **Uracil**: This compound also has a six-membered ring with two nitrogen atoms, but lacks the methyl group found in thymine.

2. **Analyze the Functional Groups**:
- All three compounds contain a six-membered ring structure with nitrogen atoms.
- Cytosine has an amino group (-NH2) that is not present in thymine and uracil.
- Thymine and uracil both have carbonyl groups (C=O) but differ in the presence of the methyl group in thymine.

3. **Compare the Positions of Functional Groups**:
- In cytosine, the carbonyl group (C=O) is present at the second position of the ring.
- In thymine, the carbonyl groups are present at the second and fourth positions.
- In uracil, the carbonyl groups are also at the second and fourth positions.

4. **Conclusion**:
- The similarity among cytosine, thymine, and uracil is that they all have a **carbonyl group (C=O) at the second position in the ring**. This is the defining characteristic that they share.

### Final Answer:
Cytosine, thymine, and uracil are similar with respect to having a **carbonyl group (C=O) at the second position in the six-membered ring**.

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