Thirty percent of the bases in a sample of DNA extracted from eukaryotic cells is adenine. What percentage of cytosine is present in this DNA?

Options

A. `10%`
B. `20%`
C. `30%`
D. `40%`

Answer

To solve the problem, we will use Chargaff's rules, which state that in a double-stranded DNA molecule, the amount of adenine (A) is equal to thymine (T), and the amount of cytosine (C) is equal to guanine (G).

### Step-by-Step Solution:

1. **Identify the percentage of adenine (A)**:
- We are given that 30% of the bases in the DNA sample is adenine (A).

2. **Apply Chargaff's rule for thymine (T)**:
- According to Chargaff's rule, the percentage of thymine (T) is equal to the percentage of adenine (A). Therefore, if A = 30%, then T = 30% as well.

3. **Calculate the total percentage of adenine and thymine**:
- Total percentage of A and T = A + T = 30% + 30% = 60%.

4. **Determine the remaining percentage for cytosine (C) and guanine (G)**:
- Since the total percentage of all bases must equal 100%, we can find the combined percentage of cytosine (C) and guanine (G):
- C + G = 100% - (A + T) = 100% - 60% = 40%.

5. **Apply Chargaff's rule for cytosine (C) and guanine (G)**:
- According to Chargaff's rule, the percentage of cytosine (C) is equal to the percentage of guanine (G). Therefore, we can express this as:
- C = G.

6. **Set up the equation for C and G**:
- Since C + G = 40% and C = G, we can substitute G with C:
- C + C = 40%,
- 2C = 40%.

7. **Solve for cytosine (C)**:
- Dividing both sides by 2 gives:
- C = 40% / 2 = 20%.

8. **Conclusion**:
- Therefore, the percentage of cytosine (C) present in the DNA is 20%.

### Final Answer:
The percentage of cytosine in the DNA is **20%**.