Which of the following is not a salient features of the Double-helix structure of DNA

Options

A. two chains have parallel polarity
B. a purine comes opposite to a pyrimidine
C. approximately uniform distance between the two strands of the helix
D. The two chains are coiled in a right-handed fashion

Answer

To determine which of the following is not a salient feature of the double-helix structure of DNA, we need to analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA is composed of two strands that form a double helix. Each strand has a directionality, indicated by the 5' and 3' ends.

2. **Evaluating the First Statement**:
- The first statement claims that "two chains have parallel polarity."
- This is incorrect because the two strands of DNA are anti-parallel. This means if one strand runs in the 5' to 3' direction, the other runs in the 3' to 5' direction.
- **Conclusion**: This statement is not a salient feature of DNA.

3. **Evaluating the Second Statement**:
- The second statement says "a purine comes opposite to a pyrimidine."
- This is correct. In DNA, adenine (a purine) pairs with thymine (a pyrimidine), and guanine (a purine) pairs with cytosine (a pyrimidine).
- **Conclusion**: This statement is a salient feature of DNA.

4. **Evaluating the Third Statement**:
- The third statement mentions "approximately uniform distance between the two strands of the helix."
- This is also correct. The distance between the two strands remains consistent throughout the length of the DNA molecule.
- **Conclusion**: This statement is a salient feature of DNA.

5. **Evaluating the Fourth Statement**:
- The fourth statement states "the two chains are coiled in a right-handed fashion."
- This is correct as well. The double helix of DNA is indeed right-handed.
- **Conclusion**: This statement is a salient feature of DNA.

6. **Final Conclusion**:
- The only statement that is not a salient feature of the double-helix structure of DNA is the first one: "two chains have parallel polarity." Therefore, the correct answer is option A.

### Final Answer:
The statement that is not a salient feature of the double-helix structure of DNA is: **"two chains have parallel polarity."**

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