number of H-bonds between guanine and cytosine are-

Options

A. one
B. two
C. three
D. Four

Answer

To determine the number of hydrogen bonds between guanine and cytosine, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Nitrogen Bases**:
- In DNA, the four nitrogenous bases are adenine (A), thymine (T), guanine (G), and cytosine (C).
- We are specifically interested in guanine (G) and cytosine (C).

2. **Understand Base Pairing**:
- Guanine pairs with cytosine through hydrogen bonds, while adenine pairs with thymine.
- The hydrogen bonds are crucial for maintaining the structure of the DNA double helix.

3. **Determine the Number of Hydrogen Bonds**:
- Guanine and cytosine form three hydrogen bonds between them.
- In contrast, adenine and thymine form two hydrogen bonds.

4. **Analyze the Structures**:
- Guanine has a double-ring structure (purine), while cytosine has a single-ring structure (pyrimidine).
- The specific groups in these bases facilitate the formation of hydrogen bonds.

5. **Identify the Bonding Sites**:
- In cytosine, the amino group (NH2) acts as a hydrogen bond donor, while the carbonyl group (C=O) and the nitrogen atom (N) serve as hydrogen bond acceptors.
- In guanine, the carbonyl group (C=O) at position 6 acts as a hydrogen bond acceptor, while the amino group and nitrogen at position 1 act as donors.

6. **Conclusion**:
- The specific interactions between the functional groups of guanine and cytosine lead to the formation of three hydrogen bonds.
- Therefore, the final answer is that the number of hydrogen bonds between guanine and cytosine is **three**.

### Final Answer:
The number of hydrogen bonds between guanine and cytosine is **three**.