Which DNA molecule has lower melting temperature?

Options

A.
B.
C.
D.

Answer

To determine which DNA molecule has the lower melting temperature, we need to analyze the composition of the DNA strands provided in the options. The melting temperature (Tm) of DNA is influenced by the ratio of adenine-thymine (A-T) base pairs to guanine-cytosine (G-C) base pairs. Here’s a step-by-step solution:

### Step-by-Step Solution:

1. **Understand DNA Structure**: DNA is a double-stranded molecule composed of nucleotides, which include a sugar, a phosphate group, and a nitrogenous base (adenine, thymine, guanine, or cytosine). The strands run in opposite directions (5' to 3' and 3' to 5').

2. **Base Pairing**: A-T pairs are connected by two hydrogen bonds, while G-C pairs are connected by three hydrogen bonds. This means that G-C pairs are more stable and contribute to a higher melting temperature compared to A-T pairs.

3. **Identify A-T and G-C Content**: For each DNA molecule, count the number of A-T pairs and G-C pairs. The more A-T pairs a DNA molecule has, the lower its melting temperature will be.

4. **Analyze Each Option**:
- **Option 1**: Count A-T and G-C pairs.
- **Option 2**: Count A-T and G-C pairs.
- **Option 3**: Count A-T and G-C pairs.
- **Option 4**: Count A-T and G-C pairs.

5. **Compare A-T Content**: After counting the A-T and G-C pairs in all options, compare the counts. The DNA strand with the highest number of A-T pairs will have the lowest melting temperature.

6. **Conclusion**: Based on the counts, identify which option has the maximum A-T pairs. This option represents the DNA molecule with the lower melting temperature.

### Example Calculation:
- For **Option 1**: 2 A-T pairs, 2 G-C pairs.
- For **Option 2**: 5 A-T pairs, 2 G-C pairs.
- For **Option 3**: 2 A-T pairs, 5 G-C pairs.
- For **Option 4**: 1 A-T pair, 4 G-C pairs.

From the example counts, **Option 2** has the highest A-T content (5 A-T pairs), indicating it has the lowest melting temperature.

### Final Answer:
The DNA molecule with the lower melting temperature is **Option 2**.

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