Which of the following is an incorrect statement?

Options

A. Chemically, adenine is 6 amino purine
B. Cytosine and adenine are purine bases
C. Two DNA strands are antiparallel due to hydrogen bonds holding them together
D. Pitch of dsDNA is 3.4 nm

Answer

To determine which of the following statements is incorrect, we will analyze each option step by step.

### Step 1: Analyze Option 1
**Statement:** Chemically, adenine is a 6-amino purine.

**Analysis:**
- Adenine is indeed a purine base, and it has an amino group (-NH2) attached to the 6th carbon of its structure.
- This statement is **correct**.

### Step 2: Analyze Option 2
**Statement:** Cytosine and adenine are purine bases.

**Analysis:**
- Purine bases include adenine and guanine. Cytosine, on the other hand, is a pyrimidine base, along with thymine and uracil.
- Therefore, this statement is **incorrect**.

### Step 3: Analyze Option 3
**Statement:** The two DNA strands are anti-parallel due to hydrogen bonds holding them together.

**Analysis:**
- DNA strands run in opposite directions (5' to 3' and 3' to 5'), which is referred to as anti-parallel.
- Hydrogen bonds do hold the two strands together, but the anti-parallel nature is primarily due to the orientation of the strands.
- This statement is **correct**.

### Step 4: Analyze Option 4
**Statement:** Each turn of dsDNA is 3.4 nm.

**Analysis:**
- The double helix structure of DNA does indeed turn every 10 base pairs, and this corresponds to a distance of approximately 3.4 nm.
- This statement is **correct**.

### Conclusion
After analyzing all the options, we find that the **incorrect statement** is **Option 2: Cytosine and adenine are purine bases**.

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