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

Answer

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

### Step-by-Step Solution:

1. **Understand Chargaff's Rules**: According to Chargaff's rules, in DNA:
- The amount of adenine (A) is equal to the amount of thymine (T).
- The amount of guanine (G) is equal to the amount of cytosine (C).

2. **Identify Given Information**: We know that:
- Cytosine (C) = 18%

3. **Calculate Guanine (G)**: Since G = C, we can conclude:
- Guanine (G) = 18%

4. **Calculate Total Percentage of G and C**:
- Total (G + C) = G + C = 18% + 18% = 36%

5. **Calculate Total Percentage of A and T**:
- Since A + T = 100% - (G + C), we find:
- A + T = 100% - 36% = 64%

6. **Determine the Fraction of Adenine (A)**:
- Since A = T, we can express A as:
- A + A = 64% (because A = T)
- 2A = 64%
- A = 64% / 2 = 32%

7. **Convert to Fraction**:
- To express adenine as a fraction of the total bases, we convert the percentage to a fraction:
- Fraction of adenine = 32% = 32/100 = 0.32

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

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