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 solve the question regarding which statements about DNA are incorrect, we will analyze each statement one by one.

### Step 1: Analyze Statement I
**Statement I:** DNA has a double helix structure.
- **Analysis:** This statement is correct. DNA is indeed known for its double helix structure, which was first described by Watson and Crick.

### Step 2: Analyze Statement II
**Statement II:** DNA can be double-stranded or single-stranded in animals.
- **Analysis:** This statement is incorrect. In animals, DNA is typically found in a double-stranded form. Single-stranded DNA is primarily found in some viruses, not in animals.

### Step 3: Analyze Statement III
**Statement III:** The two chains have parallel polarity.
- **Analysis:** This statement is also incorrect. The two strands of DNA are anti-parallel, meaning one strand runs in the 5' to 3' direction while the other runs in the 3' to 5' direction.

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

### Conclusion
From the analysis:
- **Correct Statements:** I and IV
- **Incorrect Statements:** II and III

Thus, the incorrect statements regarding DNA are **II and III**.

### Final Answer
The incorrect statements regarding DNA are II and III.

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