In the 5' end of a DNA molecule

Options

A. The fifth carbon of pyrimidine base is free
B. The fifth carbon of purine base is free
C. The fifth carbon of pentose sugar is free
D. Both (1) & (3)

Answer

To solve the question regarding what is free at the 5' end of a DNA molecule, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA is composed of nucleotides, which consist of a phosphate group, a pentose sugar (deoxyribose), and a nitrogenous base (either purine or pyrimidine).
- Each nucleotide is linked to the next through the phosphate group and the sugar.

2. **Identifying the Ends of DNA**:
- DNA has two ends: the 5' end and the 3' end.
- The 5' end refers to the end of the DNA strand where the fifth carbon of the pentose sugar is not attached to another nucleotide, making it "free".

3. **Analyzing the 5' End**:
- At the 5' end of the DNA molecule, the fifth carbon of the pentose sugar (deoxyribose) is free and is typically linked to a phosphate group.
- This means that the 5' end has a phosphate group attached to it, which is often referred to as the "5' phosphate".

4. **Examining the Options**:
- **Fifth carbon of the pyrimidine base**: Incorrect, as the bases do not have a free carbon at the 5' end.
- **Fifth carbon of the purine base**: Incorrect, similar reasoning as above.
- **Fifth carbon of the pentose sugar**: Correct, as this carbon is free at the 5' end.
- **Both 1 and 3**: This option is ambiguous and does not apply to the specific question about the 5' end.

5. **Conclusion**:
- The correct answer is that at the 5' end of the DNA molecule, the fifth carbon of the pentose sugar is free, which is referred to as the 5' phosphate.

### Final Answer:
The fifth carbon of the pentose sugar is free at the 5' end of a DNA molecule.

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