The percentage of adenine in DNA isolated from human liver is observed to be 30.7% . What is the expected percentage of thymine, guanine, and cytosine ?

Options

A. T = 19.3 %, G = 19.3%, C = 30.7%
B. T = 19.3 %, G = 30.7%, C = 19.3%
C. T = 30.7 %, G = 19.3%, C = 30.7%
D. T = 30.7 %, G = 19.3%, C = 19.3%

Answer

To solve the problem, we need to determine the expected percentages of thymine, guanine, and cytosine in the DNA, given that the percentage of adenine is 30.7%.

### Step-by-Step Solution:

1. **Understand the Base Pairing Rule**:
In DNA, adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C). According to Chargaff's rules, the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine.

2. **Set Up the Known Values**:
We know that the percentage of adenine (A) is 30.7%. Therefore, the percentage of thymine (T) will also be 30.7% because A = T.

3. **Calculate the Total Percentage of A and T**:
Since A and T are equal, we can add their percentages:
\[ A + T = 30.7\% + 30.7\% = 61.4\% \]

4. **Determine the Remaining Percentage for G and C**:
The total percentage of all four bases (A, T, G, and C) must equal 100%. Therefore, we can find the combined percentage of guanine (G) and cytosine (C) by subtracting the percentage of A and T from 100%:
\[ G + C = 100\% - 61.4\% = 38.6\% \]

5. **Divide the Remaining Percentage Between G and C**:
Since G and C are also equal (G = C), we can divide the remaining percentage by 2 to find the individual percentages:
\[ G = C = \frac{38.6\%}{2} = 19.3\% \]

6. **Summarize the Results**:
- Percentage of Adenine (A) = 30.7%
- Percentage of Thymine (T) = 30.7%
- Percentage of Guanine (G) = 19.3%
- Percentage of Cytosine (C) = 19.3%

### Final Answer:
- Adenine (A): 30.7%
- Thymine (T): 30.7%
- Guanine (G): 19.3%
- Cytosine (C): 19.3%