Which statement is not true for the base ratio of DNA ?

Options

A. Base ratio is constant for a given species.
B. Base ratio is variable for different species.
C. Base ratio does not depend on age of organism .
D. Base ratio is different for different tissues of organism.

Answer

To determine which statement is not true for the base ratio of DNA, we can follow these steps:

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

### Step 2: Analyze the Base Ratios
According to Chargaff's findings:
- The ratio of A + G (total purines) to C + T (total pyrimidines) is constant for a given species.
- This ratio can vary between different species but remains constant within a single species.

### Step 3: Evaluate the Statements
Now, let's evaluate the statements provided in the question:
1. **Base ratio is constant for a given species** - This is true.
2. **Base ratio is variable for different species** - This is also true.
3. **Base ratio does not depend on age of organism** - This statement is true as well; the base ratio remains constant regardless of the organism's age.
4. **Base ratio is different for different tissues of organism** - This statement is false. The base ratio remains constant across different tissues of the same organism.

### Conclusion
The statement that is not true for the base ratio of DNA is:
**Base ratio is different for different tissues of organism.**