If the total amount of adenine and thymine in a double-stranded DNA is 60%, the amount of guanine in this DNA will be

Options

A. `15%`
B. `20%`
C. `30%`
D. `40%`

Answer

To solve the question regarding the percentage of guanine in a double-stranded DNA where the total amount of adenine and thymine is 60%, we can follow these steps:

### Step-by-Step Solution:

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

2. **Calculate the Percentage of Adenine and Thymine**: The problem states that the total percentage of adenine and thymine is 60%. Therefore:
- A + T = 60%

3. **Determine Individual Percentages of Adenine and Thymine**: Since adenine equals thymine, we can express this as:
- A = T
- If we let A = x, then T = x as well. Thus:
- x + x = 60%
- 2x = 60%
- x = 30%
- Therefore, A = 30% and T = 30%.

4. **Calculate the Remaining Percentage for Guanine and Cytosine**: Since the total percentage of all four bases (A, T, G, C) must equal 100%, we can find the combined percentage of guanine and cytosine:
- A + T + G + C = 100%
- 30% + 30% + G + C = 100%
- 60% + G + C = 100%
- G + C = 40%

5. **Determine Individual Percentages of Guanine and Cytosine**: Since guanine equals cytosine (G = C), we can express this as:
- Let G = y, then C = y as well. Thus:
- y + y = 40%
- 2y = 40%
- y = 20%
- Therefore, G = 20% and C = 20%.

6. **Conclusion**: The amount of guanine in the DNA is 20%.

### Final Answer:
The amount of guanine in this DNA will be **20%**.

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