In DNA molecule, which of the following base pair is present

Options

A. Cytosine and adenine
B. Adenine and thymine
C. Adenine and guanine
D. Cytosine and thymine

Answer

To solve the question regarding which base pair is present in a DNA molecule, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of DNA**:
- DNA (deoxyribonucleic acid) is a double helical structure composed of nucleotides. Each nucleotide consists of a nitrogenous base, a sugar (deoxyribose), and a phosphate group.

2. **Identify the Nitrogenous Bases**:
- The nitrogenous bases in DNA are divided into two categories: purines and pyrimidines.
- **Purines**: Adenine (A) and Guanine (G)
- **Pyrimidines**: Cytosine (C) and Thymine (T)

3. **Know the Base Pairing Rules**:
- In DNA, specific base pairing occurs:
- Adenine (A) pairs with Thymine (T) through **two hydrogen bonds**.
- Guanine (G) pairs with Cytosine (C) through **three hydrogen bonds**.

4. **Evaluate the Options**:
- The options provided are:
- A) Cytosine and Adenine
- B) Adenine and Thymine
- C) Adenine and Guanine
- D) Cytosine and Thymine

5. **Analyze Each Option**:
- **Option A (Cytosine and Adenine)**: Incorrect. Adenine pairs with Thymine, not Cytosine.
- **Option B (Adenine and Thymine)**: Correct. This is a valid base pair in DNA.
- **Option C (Adenine and Guanine)**: Incorrect. Adenine pairs with Thymine, and Guanine pairs with Cytosine.
- **Option D (Cytosine and Thymine)**: Incorrect. Cytosine pairs with Guanine, and Thymine pairs with Adenine.

6. **Conclusion**:
- The correct base pair present in a DNA molecule is **Adenine and Thymine**.

### Final Answer:
**B) Adenine and Thymine**