Adenine is

Options

A. Purine
B. Pyrimidine
C. Nucleoside
D. Nucleotide.

Answer

### Step-by-Step Solution:

1. **Identify the Structure of Adenine**:
- Adenine is one of the two types of nitrogenous bases known as purines. The other purine is guanine.

2. **Classify Nitrogenous Bases**:
- Nitrogenous bases are classified into two categories: purines and pyrimidines.
- Purines include adenine (A) and guanine (G).
- Pyrimidines include cytosine (C), thymine (T), and uracil (U).

3. **Understand the Role of Adenine in Nucleic Acids**:
- Adenine is found in both DNA and RNA.
- In DNA, adenine pairs with thymine (T), while in RNA, adenine pairs with uracil (U).

4. **Define Nucleotides**:
- A nucleotide is the basic building block of nucleic acids.
- It consists of three components: a nitrogenous base (which can be adenine, guanine, cytosine, thymine, or uracil), a sugar (deoxyribose in DNA and ribose in RNA), and a phosphate group.

5. **Formation of Nucleotides**:
- When a purine or pyrimidine base combines with a sugar, it forms a nucleoside.
- When a nucleoside further combines with a phosphate group, it forms a complete nucleotide.

### Final Answer:
Adenine is a purine nitrogenous base that plays a crucial role in the structure of nucleotides, which are the building blocks of DNA and RNA.

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