Which of the following is correct about the thermal ends of a linear DNA chain?

Options

A. 5' phosphate end 3' phosphate end
B. 5' phosphate end and 3' OH end
C. 5' OH end 3' phosphate end
D. 5' OH end and 5' OH end

Answer

To solve the question regarding the thermal ends of a linear DNA chain, we need to understand the structure of DNA and the significance of its ends.

### Step-by-Step Solution:

1. **Understanding DNA Structure**: DNA is composed of two strands that form a double helix. Each strand has a directionality, which is indicated by the 5' (five prime) and 3' (three prime) ends.

2. **Identifying the Ends**:
- The **5' end** of a DNA strand has a phosphate group attached to the fifth carbon of the sugar molecule (deoxyribose).
- The **3' end** has a hydroxyl group (-OH) attached to the third carbon of the sugar.

3. **Analyzing the Options**:
- **Option 1**: 5'-phosphate and 3'-phosphate. This is incorrect because the 3' end cannot have a phosphate group; it must have a hydroxyl group.
- **Option 2**: 5'-phosphate and 3'-OH. This is correct because the 5' end has a phosphate group, and the 3' end has a hydroxyl group.
- **Option 3**: 5'-OH and 3'-phosphate. This is also incorrect for the same reason as option 1.
- **Option 4**: 5'-OH and 5'-OH. This is incorrect because both ends cannot be the same; one must be a phosphate and the other a hydroxyl.

4. **Conclusion**: The correct statement about the thermal ends of a linear DNA chain is that the 5' end has a phosphate group, and the 3' end has a hydroxyl group.

### Final Answer:
The correct option is: **5'-phosphate and 3'-OH**.

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