Which of the following statements about DNA is not correct ?

Options

A. It has a double helix structure
B. It undergoes replication
C. The two strands in a DNA molecule are exactly similar
D. It contains the pentose sugar, 2-deoxyribose

Answer

To determine which statement about DNA is not correct, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Analyze the First Statement: "It has a double helix structure."**
- DNA is known for its double helix structure, which was famously described by Watson and Crick. This statement is correct.

2. **Analyze the Second Statement: "It undergoes replication."**
- DNA replication is a fundamental process where DNA makes a copy of itself. This is essential for cell division and is a well-established fact. This statement is also correct.

3. **Analyze the Third Statement: "Two strands in a DNA molecule are exactly similar."**
- This statement is incorrect. The two strands of DNA are not exactly similar; they are complementary. This means that the sequence of bases on one strand determines the sequence of bases on the other strand (A pairs with T and G pairs with C). Therefore, this statement is the incorrect one.

4. **Analyze the Fourth Statement: "It contains pentose sugar and two deoxyribose."**
- DNA contains a pentose sugar called deoxyribose. The phrase "two deoxyribose" may be misleading, but it refers to the fact that each strand of DNA is made up of nucleotides that contain deoxyribose. However, the statement itself is somewhat ambiguous but can be interpreted as correct in the context of DNA structure.

### Conclusion:
The statement that is not correct about DNA is: **"Two strands in a DNA molecule are exactly similar."**