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