Which are purines ?

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Bases**:
- Nitrogen bases are the building blocks of nucleic acids (DNA and RNA). They are classified into two categories: purines and pyrimidines.

2. **Identifying Purines**:
- Purines are one of the two types of nitrogen bases. They are characterized by a two-ring structure.

3. **Listing Purines**:
- The two main purines found in nucleic acids are:
- **Adenine (A)**
- **Guanine (G)**

4. **Comparison with Pyrimidines**:
- For context, pyrimidines are the other type of nitrogen bases, which include:
- **Thymine (T)** - found in DNA
- **Cytosine (C)** - found in both DNA and RNA
- **Uracil (U)** - found in RNA (replaces thymine)

5. **Base Pairing**:
- In DNA, adenine pairs with thymine through double hydrogen bonds, while guanine pairs with cytosine through triple hydrogen bonds. In RNA, adenine pairs with uracil instead of thymine.

### Final Answer:
The purines are **Adenine (A)** and **Guanine (G)**.

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