In a DNA molecule

Options

A. There are two strands which run antiparallelone in 5'`rarr`5'
B. The total amount of purine nucleotides and pyrimidine nucleotides is not always equal
C. Three are two strands which run parallel in the 5'`rarr`3' direction
D. Guanine, Adenine - Purines

Answer

**Step-by-Step Solution:**

1. **Understanding DNA Structure**: DNA 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. **Strand Orientation**: 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. **Chargaff's Rule**: According to Chargaff's rule, in a DNA molecule, the amount of purines (adenine and guanine) is equal to the amount of pyrimidines (cytosine and thymine). This means that the number of adenine (A) equals the number of thymine (T), and the number of guanine (G) equals the number of cytosine (C).

4. **Identifying Purines and Pyrimidines**: In DNA, the purines are adenine (A) and guanine (G), while the pyrimidines are cytosine (C) and thymine (T).

5. **Evaluating Statements**: Based on the information provided:
- The statement that the total amount of purine nucleotides and pyrimidine nucleotides is not always equal is incorrect. This contradicts Chargaff's rule.
- The statement about the strands running parallel is also incorrect; they run anti-parallel.
- The statement identifying guanine and adenine as purines is correct.

6. **Conclusion**: The only correct statement regarding the DNA molecule is that guanine and adenine are purines.

**Final Answer**: The correct statement is that guanine and adenine are purines.

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