The percentage of guanine in a double stranded DNA la 20%, what will be the percentage of thymine is the DNA?

Options

A. 0.2
B. 0.3
C. 0.4
D. 0.6

Answer

To find the percentage of thymine in a double-stranded DNA where the percentage of guanine is given as 20%, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Chargaff's Rules**: According to Chargaff's rules, in double-stranded DNA:
- The percentage of adenine (A) is equal to the percentage of thymine (T).
- The percentage of guanine (G) is equal to the percentage of cytosine (C).

2. **Identify Given Information**: We know that the percentage of guanine (G) is 20%.

3. **Calculate Cytosine Percentage**: Since G = C, the percentage of cytosine (C) will also be 20%.

4. **Calculate Total Percentage of G and C**:
- G + C = 20% + 20% = 40%.

5. **Calculate Remaining Percentage for A and T**:
- The total percentage of all four bases (A, T, G, C) in DNA must equal 100%.
- Therefore, the percentage of adenine (A) and thymine (T) combined is:
- 100% - (G + C) = 100% - 40% = 60%.

6. **Divide Remaining Percentage Between A and T**:
- Since A = T, we can divide the remaining percentage equally:
- A + T = 60%
- A = T = 60% / 2 = 30%.

7. **Final Result for Thymine**:
- Therefore, the percentage of thymine (T) is 30%.

### Conclusion:
The percentage of thymine in the DNA is **30%** or **0.3** when expressed as a decimal.