Which statement/s is/are incorrect regarding DNA ?
I. DNA has a double helix structure.
II. DNA can be double-stranded or single-stranded in animals
III. The two chains have parallel polarity.
IV. The pitch of the helix is 3.4 nm.

Options

A. I, II, III, IV
B. II, III, IV
C. II, IV
D. II, III

Answer

To determine which statements regarding DNA are incorrect, let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I: DNA has a double helix structure.**
- **Analysis:** DNA is well-known for its double helix structure, which consists of two polynucleotide strands coiled around each other, resembling a twisted ladder.
- **Conclusion:** This statement is **correct**.

2. **Statement II: DNA can be double-stranded or single-stranded in animals.**
- **Analysis:** While DNA can be double-stranded, single-stranded DNA is typically found in certain viruses (like some DNA viruses) but not in animals. In animals, DNA is predominantly double-stranded.
- **Conclusion:** This statement is **incorrect**.

3. **Statement III: The two chains have parallel polarity.**
- **Analysis:** In the double helix structure of DNA, the two strands run in opposite directions, meaning they have anti-parallel polarity. One strand runs from the 5' to 3' direction while the other runs from 3' to 5'.
- **Conclusion:** This statement is **incorrect**.

4. **Statement IV: The pitch of the helix is 3.4 nm.**
- **Analysis:** The pitch of the DNA double helix, which refers to the distance for one complete turn of the helix, is indeed approximately 3.4 nanometers.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
The incorrect statements regarding DNA are **Statement II** and **Statement III**.

### Answer:
The answer is that **Statements II and III are incorrect**.

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