In the DNA molecule

Options

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

Answer

**Step-by-Step Solution:**

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

2. **Identifying the Orientation of DNA Strands:**
- The two strands of DNA run in opposite directions, which is referred to as "anti-parallel." One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.

3. **Evaluating the Options:**
- **Option 1:** "There are two strands which run anti-parallel (5' to 3' and 3' to 5')."
- This statement is correct.
- **Option 2:** "The total amount of purine nucleotides is not equal to the total amount of pyrimidine nucleotides."
- This statement is incorrect. According to Chargaff's rules, the amount of purines (A + G) is equal to the amount of pyrimidines (T + C).
- **Option 3:** "There are two strands which run parallel in the 5' to 3' direction."
- This statement is incorrect. The strands run anti-parallel, not parallel.
- **Option 4:** "The proportion of adenine in relation to thymine varies among organisms."
- This statement is incorrect. Chargaff's rules state that the amount of adenine (A) is equal to thymine (T) in any given DNA molecule.

4. **Conclusion:**
- The only correct statement regarding DNA from the given options is that the two strands run anti-parallel (5' to 3' and 3' to 5').

**Final Answer:**
- The correct statement is: "There are two strands which run anti-parallel (5' to 3' and 3' to 5')."

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