For a double strand DNA, one strand is given below :

The amount of energy required to split the double strand DNA into two single strands is _____ kcal mol⁻¹ .

[Given: Average energy per H-bond for A-T base pair = 1.0 kcal mol⁻¹ , G-C base pair = 1.5 kcal mol⁻¹ , and A-U base pair = 1.25 kcal mol⁻¹ . Ignore electrostatic repulsion between the phosphate groups.]

The correct answer is 41.

To determine the total amount of energy required to split the double-stranded DNA into two single strands, we need to analyze the base sequence of the given strand and calculate the energy contribution of all the hydrogen bonds present between the complementary strands.

Step 1: Identify the sequence and base composition
From the given image, the sequence of one strand of the DNA is:
5' - A - G - T - C - A - C - G - T - A - A - G - T - C - 3'

By counting each nucleotide in this sequence, we find:

• Adenine (A) = 4
• Thymine (T) = 3
• Guanine (G) = 3
• Cytosine (C) = 3

In a double-stranded DNA molecule:

• Each A on one strand pairs with T on the complementary strand, and vice versa. Thus, the total number of A-T base pairs is equal to the sum of A and T on the single strand:
  Number of A-T base pairs = 4 + 3 = 7
• Each G on one strand pairs with C on the complementary strand, and vice versa. Thus, the total number of G-C base pairs is equal to the sum of G and C on the single strand:
  Number of G-C base pairs = 3 + 3 = 6

Step 2: Determine the hydrogen bonds per base pair
Recall the structure of Watson-Crick base pairing:

• An A-T base pair is held together by 2 hydrogen bonds.
• A G-C base pair is held together by 3 hydrogen bonds.

Step 3: Calculate the energy required to break the hydrogen bonds
Given data:

• Average energy per H-bond for A-T base pair = 1.0 kcal mol^-1
• Average energy per H-bond for G-C base pair = 1.5 kcal mol^-1

The energy required to break all A-T base pairs is:
$\text{Energy for A-T pairs}=7\times 2\text{ H-bonds}\times 1.0{\text{ kcal mol}}^{-1}=14.0{\text{ kcal mol}}^{-1}$

The energy required to break all G-C base pairs is:
$\text{Energy for G-C pairs}=6\times 3\text{ H-bonds}\times 1.5{\text{ kcal mol}}^{-1}=27.0{\text{ kcal mol}}^{-1}$

Step 4: Calculate the total energy
Summing up the energies:
$\text{Total Energy}=14.0+27.0=41.0{\text{ kcal mol}}^{-1}$

Thus, the total amount of energy required to split the double-stranded DNA into two single strands is 41 kcal mol^-1.