If a double - stranded DNA has 30% of guanine , what will be the percentage of adenine and thymine base pairs ?

Options

A. `20%`
B. `30%`
C. `40%`
D. `60%`

Answer

To solve the question regarding the percentage of adenine and thymine base pairs in a double-stranded DNA that has 30% guanine, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Chargaff's Rule**:
Chargaff's rule states that in a double-stranded DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of cytosine (C) is equal to the amount of guanine (G). Therefore, A = T and C = G.

2. **Identify the Given Information**:
The problem states that the double-stranded DNA has 30% guanine (G). According to Chargaff's rule, this means that cytosine (C) will also be 30%.

3. **Calculate the Total Percentage of G and C**:
Since G = 30% and C = 30%, we can calculate the total percentage of guanine and cytosine combined:
\[ G + C = 30\% + 30\% = 60\% \]

4. **Determine the Percentage of A and T**:
The total percentage of all four bases (A, T, G, C) in DNA must equal 100%. Therefore, we can find the percentage of adenine (A) and thymine (T) combined:
\[ A + T = 100\% - (G + C) = 100\% - 60\% = 40\% \]

5. **Divide A and T Equally**:
Since A = T according to Chargaff's rule, we can divide the percentage of A + T equally:
\[ A = T = \frac{40\%}{2} = 20\% \]

6. **Final Answer**:
The question asks for the percentage of adenine and thymine base pairs combined:
\[ A + T = 20\% + 20\% = 40\% \]

Thus, the percentage of adenine and thymine base pairs in the DNA is **40%**.