In a double - stranded DNA , one of the nitrogen base is adenine, whose amount is found to be always equal to the amount of

Options

A. 5 - methyl uracil which is a purine
B. 5- methyl uracil which is a pyrimidine
C. Guanine which is a purine
D. Guanine which is a pyrimidine

Answer

To solve the question, we need to understand the relationship between the nitrogenous bases in double-stranded DNA, particularly focusing on adenine (A).

### Step-by-Step Solution:

1. **Identify the Nitrogen Bases**: In double-stranded DNA, there are two types of nitrogenous bases: purines and pyrimidines. The purines are adenine (A) and guanine (G), while the pyrimidines are cytosine (C) and thymine (T).

2. **Understand Chargaff's Rules**: According to Chargaff's rules, in double-stranded DNA:
- The amount of adenine (A) is always equal to the amount of thymine (T).
- The amount of guanine (G) is always equal to the amount of cytosine (C).

3. **Analyze the Question**: The question states that the amount of adenine is equal to the amount of a certain base. Based on Chargaff's rules, we know that adenine pairs with thymine in DNA.

4. **Evaluate the Options**:
- **5-methyl uracil**: This is a modified form of uracil, which is a pyrimidine. In DNA, uracil is not present; instead, thymine is present. Therefore, this option is not correct.
- **Guanine**: This is a purine and does not pair with adenine. Hence, this option is incorrect.
- **5-methyl uracil which is a pyrimidine**: As stated, 5-methyl uracil is a pyrimidine, but it does not pair with adenine in DNA.
- **Guanine which is a pyrimidine**: This is incorrect as guanine is a purine.

5. **Conclusion**: The correct answer is that the amount of adenine is always equal to the amount of thymine (not listed in the options). However, since the question asks for the closest relation, the only option that can be considered is the one that mentions pyrimidines. Therefore, the best choice among the given options would be **5-methyl uracil which is a pyrimidine**.

### Final Answer:
The amount of adenine is always equal to the amount of **5-methyl uracil which is a pyrimidine**.