In the DNA molecule

Options

A. the total amount of purine nucleotides and pyrimidine nucleotides is not always equal
B. there are two strands which run parallel in the `5'rarr3'` direction
C. the proportion of adenine in relation to thymine varies with the organism
D. there are two strands which run antiparallel one in `5'rarr3'` direction and other in `3'rarr5'`

Explanation

To answer the question regarding the DNA molecule, we will analyze the statements given and determine which one is correct based on our understanding of DNA structure and the principles that govern it.

### Step-by-Step Solution:

1. **Understanding Purines and Pyrimidines**:
- In DNA, there are two types of nitrogenous bases: purines and pyrimidines.
- Purines include adenine (A) and guanine (G), while pyrimidines include cytosine (C) and thymine (T).
- Purines have a double-ring structure, making them larger than pyrimidines, which have a single-ring structure.

**Hint**: Remember the two categories of nitrogenous bases in DNA: purines (A, G) and pyrimidines (C, T).

2. **Base Pairing Rules**:
- According to Chargaff's rules, in a DNA molecule, adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C).
- This means that the number of adenine nucleotides is always equal to the number of thymine nucleotides (A = T), and the number of guanine nucleotides is always equal to the number of cytosine nucleotides (G = C).

**Hint**: Recall that A pairs with T and G pairs with C, leading to equal quantities of these bases.

3. **Directionality of DNA Strands**:
- DNA is composed of two strands that run in opposite directions, known as anti-parallel orientation.
- One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.

**Hint**: Focus on the concept of anti-parallel strands in DNA, which is a key characteristic of its structure.

4. **Evaluating the Statements**:
- The first statement claims that the total number of purine nucleotides and pyrimidine nucleotides is not always equal. This is incorrect because of the base pairing rules.
- The second statement suggests that both strands run parallel in the 5' to 3' direction, which is also incorrect due to the anti-parallel nature of DNA strands.
- The third statement indicates that the proportion of adenine to thymine varies with the organism, which contradicts Chargaff's rule that states A = T.
- The fourth statement is correct, as it aligns with the established understanding of DNA structure.

**Hint**: Carefully analyze each statement against the principles of DNA structure and Chargaff's rules.

5. **Conclusion**:
- Based on the analysis, the correct statement is that the two strands of DNA run in an anti-parallel direction, which is characteristic of the DNA molecule.

**Final Answer**: The correct option is the one that states the strands run in an anti-parallel direction.