A double stranded DNA has 20 % cytosine. What will be the percentage of thiamine in it?

Options

A. 0.2
B. 0.4
C. 0.3
D. 0.1

Answer

To solve the problem of determining the percentage of thymine in a double-stranded DNA that has 20% cytosine, 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 thymine (T), and the amount of guanine (G) is equal to cytosine (C). This means:
- A = T
- G = C

2. **Identify the Percentage of Cytosine**: The problem states that the percentage of cytosine (C) is 20%. Therefore:
- C = 20%

3. **Determine the Percentage of Guanine**: Since G is equal to C, we can conclude:
- G = 20%

4. **Calculate Total Percentage of Pyrimidines**: The total percentage of pyrimidines (C + T) must equal the total percentage of purines (A + G). Since we have already established that G = 20% and C = 20%, we can express this as:
- Total Pyrimidines = C + T = 20% + T
- Total Purines = A + G = A + 20%

5. **Set Up the Equation**: According to Chargaff's rules, the total percentage of purines equals the total percentage of pyrimidines:
- A + 20% = 20% + T

6. **Calculate the Total Percentage of DNA**: The total percentage of DNA must equal 100%. Therefore, we can express this as:
- A + T + G + C = 100%
- A + T + 20% + 20% = 100%
- A + T + 40% = 100%
- A + T = 60%

7. **Substituting into the Equation**: We can now substitute A + T = 60% into the earlier equation:
- A + 20% = 20% + T
- (60% - T) + 20% = 20% + T
- 60% - T + 20% = 20% + T
- 80% - T = 20% + T
- 80% = 20% + 2T
- 60% = 2T
- T = 30%

8. **Conclusion**: The percentage of thymine (T) in the DNA is 30%.

### Final Answer:
The percentage of thymine in the double-stranded DNA is **30%**.

---