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 problem, we need to determine the percentage of adenine (A) and thymine (T) in a double-stranded DNA molecule given that it contains 30% guanine (G).

### Step-by-step Solution:

1. **Understand Base Pairing**: In DNA, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). This means the amount of adenine will equal the amount of thymine, and the amount of guanine will equal the amount of cytosine.

2. **Given Information**: We know that the percentage of guanine (G) is 30%. Therefore, the percentage of cytosine (C) must also be 30% because G pairs with C.

\[ \text{Percentage of G} = 30\% \]
\[ \text{Percentage of C} = 30\% \]

3. **Calculate Total Percentage of G and C**: Since G and C together account for 60% of the total bases in the DNA, we can calculate the remaining percentage for A and T.

\[ \text{Total percentage of G and C} = 30\% + 30\% = 60\% \]

4. **Determine Remaining Percentage for A and T**: The total percentage of all bases (A, T, G, C) must equal 100%. Therefore, the percentage of adenine (A) and thymine (T) combined is:

\[ \text{Percentage of A + T} = 100\% - 60\% = 40\% \]

5. **Calculate Percentage of A and T**: Since A pairs with T, the percentage of adenine (A) will equal the percentage of thymine (T). Therefore, we can divide the remaining percentage equally:

\[ \text{Percentage of A} = \text{Percentage of T} = \frac{40\%}{2} = 20\% \]

6. **Final Answer**: The total percentage of adenine and thymine base pairs combined is:

\[ \text{Percentage of A + T} = 20\% + 20\% = 40\% \]

Thus, the final answer is **C: 40%**.