Which of the following is a purine ?

Options

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

Answer

To determine which of the following is a purine, we need to understand the classification of nitrogenous bases. Here’s a step-by-step solution:

### Step 1: Understand the Types of Nitrogenous Bases
Nitrogenous bases are classified into two categories: purines and pyrimidines.

### Step 2: Identify Purines
Purines are larger, double-ring structures. The two main purines found in nucleic acids are:
- Adenine (A)
- Guanine (G)

### Step 3: Identify Pyrimidines
Pyrimidines are smaller, single-ring structures. The pyrimidines include:
- Cytosine (C)
- Thymine (T)
- Uracil (U)

### Step 4: Analyze the Options
If the question provides a list of nitrogenous bases, we need to identify which of those bases are purines.

### Step 5: Conclusion
From the information provided, the purines are adenine and guanine. Therefore, if guanine is one of the options, it is the correct answer.

### Final Answer
**Guanine is a purine.**

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