Which is not consistent with double helical structure of DNA?

Options

A. `A=T, CequivG`
B. Density of DNA decrease on heating
C. `A + T//C + G` is not constant
D. Both (a) and (b)

Explanation

To determine which statement is not consistent with the double helical structure of DNA, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA is composed of two strands that form a double helix. The strands are held together by base pairing between nucleotides: adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds.

2. **Analyzing the Options**:
- **Option 1**: "A = T and C = G"
- This statement is consistent with the double helical structure of DNA. Adenine pairs with thymine and cytosine pairs with guanine.

- **Option 2**: "Density of DNA decreases on heating"
- This statement is also consistent. When DNA is heated, the hydrogen bonds between the bases break, leading to denaturation, which decreases the density of the DNA.

- **Option 3**: "A + T upon C + G is not constant"
- This statement is inconsistent with the double helical structure of DNA. In a given species, the ratio of (A + T) to (C + G) is generally constant. However, this ratio can vary between different species.

- **Option 4**: "Both A and B"
- Since both options A and B are consistent with the double helical structure, this option is not correct.

3. **Conclusion**:
- The statement that is not consistent with the double helical structure of DNA is **Option 3**: "A + T upon C + G is not constant".

### Final Answer:
**Option 3: A + T upon C + G is not constant.**