Question

The average energy of each hydrogen bond in $A—T$ pair is $x$ kcal mol${}^{-1}$ and that in $G—C$ pair is $y$ kcal mol${}^{-1}$. Assuming that no other interaction exists between the nucleotides, the approxim ate energy required in kcal mol${}^{-1}$ to split the following double stranded DNA into two single strands is

[Each dashed line may represent more than one hydrogen bond between the base pairs]

• A. 10 x + 9y
• B. 5x + 3y
• C. 15x + 6y
• D. 5x + 4.5y

Answer 1

Answer: (A)

Number of $H$-bonds in each $A—T$ pair $=2$.
Number of $H$-bonds in $G—C$ pair $=3$.
The given double stranded DNA has $5$
$A—T$ and $3G—C$ pairs.
$\therefore$ Total number of $H$-bonds in $5A—T$ pair
$=5\times 2=10$
Total number of H-bonds in $3G—C$ pair
$=3\times 3=9$
$\Rightarrow$ Average energy of $10H$-bonds in $A—T$ pair $=10x$ .
Average energy of $9H$ -bonds in $G—C$ pair $=9y$.
Thus, the approximate energy required to split the double stranded DNA in two single strands $=10x+9y$.