Question

DNA molecule having the length of 68Å, contains 10\% Adenine. How many number of hydrogen bonds are present between nitrogen bases totally in that DNA?

• A. 44
• B. 56
• C. 20
• D. 40

Hint:

Remember that Adenine pairs with Thymine, forming 2 hydrogen bonds, and Guanine pairs with Cytosine, forming 3 hydrogen bonds. Each type of pair spans two strands of the DNA helix, effectively doubling the hydrogen bonds accounted per base pair.

Answer 1

The Correct Option is B

In a DNA molecule, hydrogen bonds are formed between nitrogenous bases as follows: 
- Adenine (A) forms two hydrogen bonds with Thymine (T). 
- Guanine (G) forms three hydrogen bonds with Cytosine (C). Given that the DNA contains 10\% Adenine, and accordingly 10\% Thymine due to base-pairing rules, 20\% of the bases are A and T. The remaining 80\% are equally divided between Guanine and Cytosine. Calculating the total number of base pairs: \[ \text{Total base pairs} = \frac{68 \text{ Å}}{3.4 \text{ Å/base pair}} = 20 \text{ base pairs}. \] 

Hydrogen bonds calculations: - Number of A-T pairs: \( 10\% \times 20 \text{ base pairs} = 2 \text{ A-T pairs} \). - Hydrogen bonds from A-T pairs: \( 2 \text{ pairs} \times 2 \text{ bonds} = 4 \text{ bonds} \). - Number of G-C pairs: \( 40\% \times 20 \text{ base pairs} = 8 \text{ G-C pairs} \). - Hydrogen bonds from G-C pairs: \( 8 \text{ pairs} \times 3 \text{ bonds} = 24 \text{ bonds} \). 

Total hydrogen bonds: \( 4 + 24 = 28 \times 2 = 56 \). This reflects that each pair (A-T and G-C) accounts for two strands, hence the hydrogen bonds calculated per pair are doubled for the entire double helix structure, providing a total of 56 hydrogen bonds, aligning with option (2).