DNA was extracted from E.coli. The proportion of cytosine was found to be 30%, then what will be the amount of adenine?

Options

A. 0.6
B. 0.7
C. 0.2
D. 0.4

Answer

To solve the problem of determining the amount of adenine when the proportion of cytosine in E. coli DNA is 30%, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Proportion of Cytosine**:
The problem states that the proportion of cytosine (C) is 30%.

2. **Calculate the Proportion of Guanine**:
According to Chargaff's rules, the amount of cytosine is equal to the amount of guanine (G) in DNA. Therefore, if C = 30%, then G = 30% as well.

3. **Calculate the Total Proportion of Cytosine and Guanine**:
Now we add the proportions of cytosine and guanine together:
\[ C + G = 30\% + 30\% = 60\% \]

4. **Determine the Remaining Proportion for Adenine and Thymine**:
Since the total percentage of nucleotides must equal 100%, we can find the remaining percentage for adenine (A) and thymine (T) by subtracting the total of C and G from 100%:
\[ 100\% - 60\% = 40\% \]

5. **Divide the Remaining Proportion Between Adenine and Thymine**:
Adenine and thymine are also paired in equal amounts. Therefore, we divide the remaining 40% equally between adenine and thymine:
\[ A = T = \frac{40\%}{2} = 20\% \]

6. **Conclusion**:
The amount of adenine (A) is therefore 20%.

### Final Answer:
The amount of adenine in the DNA extracted from E. coli is **20%**.

---