Which of the following will DNA melt at the lowest temperature?

Options

A. 5'– AATAAAGC – 3'
3'– TTATTTGCG – 5'
B. 5'– AATGCTGC – 3'
3'– TTACGACG– 5'
C. 5'– ATGCTGAT – 3'
3'– TACGACTA – 5'
D. 5' – GCATAGUT– 3'
3' – CGTATCGA – 5'

Answer

To determine which DNA will melt at the lowest temperature, we need to understand the relationship between the base pairs in DNA and the melting temperature (Tm). The melting temperature is influenced by the number of hydrogen bonds between the base pairs:

1. **Identify the Base Pairing**:
- Adenine (A) pairs with Thymine (T) through **2 hydrogen bonds**.
- Guanine (G) pairs with Cytosine (C) through **3 hydrogen bonds**.

2. **Understand the Impact of Base Composition**:
- The more GC pairs present in a DNA strand, the higher the melting temperature because GC pairs require more energy (higher temperature) to break due to the presence of 3 hydrogen bonds.
- Conversely, more AT pairs will result in a lower melting temperature since they only require 2 hydrogen bonds to break.

3. **Analyze the Given Options**:
- We need to assess the number of A-T and G-C pairs in each option. The option with the highest A-T content will have the lowest melting temperature.

4. **Count the Base Pairs**:
- For each option, count the number of A-T pairs (which contribute 2 hydrogen bonds each) and G-C pairs (which contribute 3 hydrogen bonds each).
- The option with the highest number of A-T pairs will melt at the lowest temperature.

5. **Conclusion**:
- After counting the A-T and G-C pairs in each option, we find that the option with the highest A-T content will be the one that melts at the lowest temperature.

Based on the analysis, the answer is **Option A**, which has the highest A-T content.