A DNA molecule having `A+G//C+T=0.71` shows that the molecule is:

Options

A. Linear
B. Circular
C. Single-stranded
D. Double-stranded

Answer

To solve the question regarding the DNA molecule with the ratio \( \frac{A+G}{C+T} = 0.71 \), we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Components of DNA**:
- DNA is made up of nitrogenous bases, which are categorized into two types: purines (adenine (A) and guanine (G)) and pyrimidines (cytosine (C) and thymine (T)).

2. **Recall Chargaff's Rule**:
- Chargaff's rule states that in a double-stranded DNA molecule, the amount of purines is equal to the amount of pyrimidines. This can be expressed as:
\[ A + G = C + T \]
- From this, we can derive the ratio:
\[ \frac{A + G}{C + T} = 1 \]
- This rule applies specifically to double-stranded DNA.

3. **Analyze the Given Ratio**:
- The question provides the ratio \( \frac{A + G}{C + T} = 0.71 \).
- Since this ratio is not equal to 1, it indicates that the DNA does not follow Chargaff's rule.

4. **Determine the Type of DNA**:
- If the ratio \( \frac{A + G}{C + T} \) is not equal to 1, it suggests that the DNA is likely single-stranded. This is because single-stranded DNA does not have the complementary base pairing that is characteristic of double-stranded DNA.

5. **Evaluate the Options**:
- The options provided are:
1. Linear
2. Circular
3. Single-stranded
4. Double-stranded
- Since both linear and circular DNA can be double-stranded, and the ratio indicates that the DNA is not double-stranded, the only viable option left is single-stranded DNA.

### Conclusion:
The DNA molecule with the ratio \( \frac{A + G}{C + T} = 0.71 \) is **single-stranded DNA**.

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