If the total amount of adenine and thymine in a double-stranded DNA is 60%, then the amount of guanine in this DNA will be

Options

A. 0.15
B. 0.2
C. 0.3
D. 0.4

Answer

To solve the problem, we will follow these steps:

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

### Step 2: Set Up the Given Information
We are given that the total amount of adenine and thymine (A + T) is 60%. We can express this mathematically as:
- A + T = 60%

### Step 3: Use Chargaff's Rule to Find A and T
Since A = T, we can substitute T with A in the equation:
- A + A = 60%
- 2A = 60%
- A = 30%

Since A = T, we also have:
- T = 30%

### Step 4: Calculate G + C
We know that the total percentage of all four nucleotides (A + T + G + C) must equal 100%. Therefore:
- A + T + G + C = 100%
Substituting the known values:
- 30% + 30% + G + C = 100%
- 60% + G + C = 100%
- G + C = 100% - 60%
- G + C = 40%

### Step 5: Use Chargaff's Rule to Find G and C
Since G = C, we can substitute C with G in the equation:
- G + G = 40%
- 2G = 40%
- G = 20%

### Step 6: Conclusion
Thus, the amount of guanine (G) in the DNA is 20%, which can also be expressed as 0.2 when converted to a decimal.

### Final Answer:
The amount of guanine in this DNA will be **0.2**.

---