If a double stranded DNA has 20% of cytosine, what will be the percentage of adenine in it ?

Options

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

Explanation

To solve the problem of determining the percentage of adenine in a double-stranded DNA that has 20% cytosine, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Chargaff's Rule**: According to Chargaff's rule, 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 can be summarized as:
- A = T
- G = C

2. **Identify Given Information**: We are given that the percentage of cytosine (C) is 20%. Therefore, according to Chargaff's rule, the percentage of guanine (G) will also be 20%.
- C = 20%
- G = 20%

3. **Calculate Total Percentage of C and G**: Since C and G are both 20%, we can calculate the total percentage of these two bases:
- Total (C + G) = 20% + 20% = 40%

4. **Determine Remaining Percentage for A and T**: The total percentage of all four bases (A, T, G, C) must equal 100%. Therefore, we can find the combined percentage of adenine (A) and thymine (T):
- A + T = 100% - (C + G)
- A + T = 100% - 40% = 60%

5. **Apply Chargaff's Rule for A and T**: Since A = T, we can express the percentages of A and T as:
- A + A = 60%
- 2A = 60%
- A = 60% / 2 = 30%

6. **Final Answer**: Therefore, the percentage of adenine (A) in the DNA is 30%.

### Conclusion:
The percentage of adenine in the double-stranded DNA that has 20% cytosine is **30%**.