In DNA, adenine always pairs with

Options

A. Adenine
B. Thymine
C. Cytosine
D. Guanine

Answer

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA stands for deoxyribonucleic acid and is the genetic material in living organisms. It consists of two strands that form a double helix.

2. **Identifying Nitrogenous Bases**:
- DNA contains four types of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
- These bases are categorized into two groups: purines (adenine and guanine) and pyrimidines (cytosine and thymine).

3. **Base Pairing Rules**:
- According to Chargaff's rules, adenine pairs specifically with thymine (A-T) through two hydrogen bonds.
- Cytosine pairs with guanine (C-G) through three hydrogen bonds.

4. **Evaluating the Options**:
- The question asks what adenine pairs with in DNA.
- Option 1: Adenine pairs with adenine – **Incorrect**.
- Option 2: Adenine pairs with thymine – **Correct**.
- Option 3: Adenine pairs with cytosine – **Incorrect**.
- Option 4: Adenine pairs with guanine – **Incorrect**.

5. **Conclusion**:
- Therefore, the correct answer is that adenine always pairs with thymine in DNA.

### Final Answer:
Adenine always pairs with thymine in DNA.