Double hydrogen bond occurs in DNA between

Options

A. Adenine and thymine
B. uracil and thymine
C. Adenine and guanine
D. Thymine and cytosine.

Answer

To answer the question "Double hydrogen bond occurs in DNA between," we will analyze the options provided and apply our knowledge of DNA structure and base pairing.

### Step-by-Step Solution:

1. **Understand DNA Structure**: DNA is composed of two strands that form a double helix. Each strand consists of a sugar-phosphate backbone and nitrogenous bases attached to the sugar.

2. **Identify the Nitrogenous Bases**: The nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).

3. **Know the Base Pairing Rules**: According to Chargaff's rules, adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G).

4. **Determine the Type of Hydrogen Bonds**:
- Adenine (A) and Thymine (T) form **two hydrogen bonds**.
- Cytosine (C) and Guanine (G) form **three hydrogen bonds**.

5. **Evaluate the Options**:
- **Adenine and Thymine**: Correct, they form two hydrogen bonds.
- **Uracil and Thymine**: Incorrect in DNA; uracil is found in RNA, not DNA.
- **Adenine and Guanine**: Incorrect; they do not pair with each other.
- **Thymine and Cytosine**: Incorrect; both are pyrimidines and do not pair with each other.

6. **Conclusion**: The correct answer is that double hydrogen bonds occur in DNA between adenine and thymine.

### Final Answer:
**Double hydrogen bonds occur in DNA between adenine and thymine.**

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