If the total amount of adenine and thymine in a ds DNA is 55%, the amount of guanine in this DNA will be

Options

A. `45%`
B. `27.5%`
C. `25%`
D. `22.5%`

Answer

To solve the problem, we need to apply Chargaff's rules, which state 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). Here’s a step-by-step solution:

### Step-by-Step Solution:

1. **Identify the Given Information**:
- The total amount of adenine (A) and thymine (T) in the DNA is 55%.

2. **Calculate the Remaining Percentage for Cytosine and Guanine**:
- Since A + T = 55%, the remaining percentage for cytosine (C) and guanine (G) is:
\[ C + G = 100\% - (A + T) = 100\% - 55\% = 45\% \]

3. **Apply Chargaff's Rule**:
- According to Chargaff's rule, the amount of cytosine (C) is equal to the amount of guanine (G):
\[ C = G \]

4. **Set Up the Equation**:
- Since C + G = 45% and C = G, we can substitute G for C:
\[ C + C = 45\% \]
\[ 2C = 45\% \]

5. **Solve for C and G**:
- Divide both sides by 2 to find the value of C (and G):
\[ C = \frac{45\%}{2} = 22.5\% \]
\[ G = 22.5\% \]

6. **Conclusion**:
- The amount of guanine (G) in the DNA is 22.5%.

### Final Answer:
The amount of guanine in this DNA will be **22.5%**.

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