If a double stranded DNA has 20 percent of cytosine, calculate the percent of thymine in the DNA.

Answer

To solve the problem of calculating the percent of thymine in a double-stranded DNA that has 20 percent of cytosine, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Composition of DNA**:
- DNA is made up of nucleotides, which consist of nitrogenous bases. The two types of bases are purines (adenine and guanine) and pyrimidines (cytosine and thymine).

2. **Apply Chargaff's Rule**:
- Chargaff's Rule states that in double-stranded DNA, the amount of adenine (A) is equal to thymine (T), and the amount of cytosine (C) is equal to guanine (G). This means that A = T and C = G.

3. **Identify Given Information**:
- The problem states that the percentage of cytosine (C) is 20%. Therefore, according to Chargaff's Rule, the percentage of guanine (G) will also be 20%.

4. **Calculate Total Percentage of C and G**:
- Since C = 20% and G = 20%, the total percentage of cytosine and guanine combined is:
\[ C + G = 20\% + 20\% = 40\% \]

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

6. **Apply Chargaff's Rule Again for A and T**:
- According to Chargaff's Rule, the percentage of adenine (A) is equal to the percentage of thymine (T). Therefore, we can set up the equation:
\[ A = T \]
- Since A + T = 60%, we can divide this equally:
\[ A = T = \frac{60\%}{2} = 30\% \]

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

### Final Answer:
The percent of thymine in the DNA is **30%**.

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