In E. coli DNA has 18% of bases of cytosine. What will be the fraction of adenine?

Options

A. 0.18
B. 0.32
C. 0.36
D. Data insufficient

Explanation

To solve the problem of finding the fraction of adenine in E. coli DNA, given that it has 18% cytosine, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Percentage of Cytosine (C)**:
- The problem states that the percentage of cytosine (C) is 18%.

2. **Determine the Percentage of Guanine (G)**:
- According to Chargaff's rules, the amount of cytosine is equal to the amount of guanine in DNA. Therefore, if C = 18%, then G = 18% as well.

3. **Calculate the Combined Percentage of Cytosine and Guanine**:
- Now, add the percentages of cytosine and guanine together:
\[ C + G = 18\% + 18\% = 36\% \]

4. **Determine the Total Percentage of Adenine (A) and Thymine (T)**:
- The total percentage of all four bases (A, T, G, C) in DNA must equal 100%. Thus, we can find the combined percentage of adenine and thymine:
\[ A + T = 100\% - (C + G) = 100\% - 36\% = 64\% \]

5. **Apply Chargaff's Rules to Find Adenine and Thymine**:
- According to Chargaff's rules, the amount of adenine is equal to the amount of thymine. Therefore, we can express this as:
\[ A = T \]
- Let A = T = x. Then:
\[ A + T = 2x = 64\% \]

6. **Solve for the Percentage of Adenine (A)**:
- To find the value of x (which represents both A and T), we can rearrange the equation:
\[ 2x = 64\% \]
\[ x = \frac{64\%}{2} = 32\% \]
- Therefore, the percentage of adenine (A) is 32%.

7. **Convert the Percentage of Adenine to Fraction**:
- To express the percentage as a fraction, we convert 32% to a decimal:
\[ A = 32\% = 0.32 \]

### Final Answer:
The fraction of adenine (A) in E. coli DNA is **0.32**.