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 regarding the nitrogen base adenine in double-stranded DNA, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of DNA**:
- DNA is composed of two strands that form a double helix. Each strand is made up of nucleotides, which consist of a nitrogenous base, a sugar (deoxyribose in DNA), and a phosphate group.

**Hint**: Remember that DNA is a double-stranded molecule, and each strand is complementary to the other.

2. **Identify the Nitrogenous Bases**:
- The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine and guanine are purines, while thymine and cytosine are pyrimidines.

**Hint**: Classify the bases into purines and pyrimidines to understand their pairing.

3. **Base Pairing Rules**:
- According to the base pairing rules, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). This means that the amount of adenine in a DNA molecule will always be equal to the amount of thymine.

**Hint**: Recall that A pairs with T and G pairs with C in DNA.

4. **Chargaff's Rules**:
- Erwin Chargaff formulated rules that state the amount of adenine is equal to thymine (A = T) and the amount of guanine is equal to cytosine (G = C). This is a fundamental characteristic of double-stranded DNA.

**Hint**: Chargaff's rules are crucial for understanding the ratios of nitrogenous bases in DNA.

5. **Conclusion**:
- Since the question asks for the nitrogen base that is always equal to adenine, the answer is thymine. In the context of the options provided, thymine is also referred to as 5-methyl-uracil, which is another name for thymine.

**Hint**: Look for alternative names or representations of the bases in the options provided.

### Final Answer:
The amount of adenine in double-stranded DNA is always equal to the amount of thymine (5-methyl-uracil).