In DNA percentage of thymine is 20, then what is the percentage of guanine ?

Options

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

Explanation

To solve the question, "In DNA, if the percentage of thymine is 20, then what is the percentage of guanine?", we can follow these steps:

### Step-by-Step Solution:

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

2. **Identify Given Information**: The problem states that the percentage of thymine (T) is 20%.

3. **Calculate the Percentage of Adenine (A)**: Since T = A according to Chargaff's rule, if T = 20%, then A = 20% as well.

4. **Set Up the Equation for Total Percentage**: The total percentage of all four bases (A, T, G, C) in DNA must equal 100%. Therefore, we can write the equation:
\[ A + T + G + C = 100\% \]

5. **Substitute Known Values**: We know that:
- A = 20%
- T = 20%
- Let G = X (the percentage of guanine)
- Since G = C (by Chargaff's rule), we can also say C = X.

Substituting these into the equation gives:
\[ 20\% + 20\% + X + X = 100\% \]

6. **Simplify the Equation**: This simplifies to:
\[ 40\% + 2X = 100\% \]

7. **Solve for X**: To find X, subtract 40% from both sides:
\[ 2X = 100\% - 40\% \]
\[ 2X = 60\% \]
Now, divide both sides by 2:
\[ X = 30\% \]

8. **Conclusion**: The percentage of guanine (G) is therefore 30%.

### Final Answer:
The percentage of guanine in the DNA is **30%**.

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