Question

Given below are two statements:
Statement (I) : If two DNA double strands of the same length D1 and D2 are heated in a solution of the same composition and found that D1 and D2 are completely denatured at 60$^o$C and 65$^o$C temperature respectively.
Statement (II) : Melting temperature of a DNA molecule depends on the ratio of Guanine and Adenine bases present in their structure, D2 contains more guanine base percentage compared to that of D1
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement I and Statement II are correct
• B. Both Statement I and Statement II are incorrect
• C. Statement I is correct but Statement II is incorrect
• D. Statement I is incorrect but Statement II is correct

Hint:

Remember:
- G-C pairs = 3 hydrogen bonds (Higher melting temp).
- A-T pairs = 2 hydrogen bonds (Lower melting temp).
Higher G-C content always shifts the DNA melting curve to the right.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The melting temperature (\(T_m\)) of DNA is the temperature at which half of the DNA molecules in a solution are denatured into single strands.
The \(T_m\) is determined by the nucleotide base composition of the DNA molecule.

Step 2: Detailed Explanation:
- Statement (I) is correct:
It describes a valid experimental scenario where DNA strand D1 denatures at a lower temperature (\(60^\circ\text{C}\)) than DNA strand D2 (\(65^\circ\text{C}\)).
This indicates that D2 has a higher thermal stability than D1.
- Statement (II) is correct:
The melting temperature (\(T_m\)) of a DNA molecule is directly proportional to its Guanine-Cytosine (G-C) content.
G-C base pairs are held together by three hydrogen bonds, whereas Adenine-Thymine (A-T) base pairs have only two.
More energy (heat) is required to break three hydrogen bonds than two.
Thus, a higher percentage of G-C pairs (and hence a higher ratio of Guanine to Adenine) increases the \(T_m\) of DNA.
Since D2 has a higher denaturation temperature (\(65^\circ\text{C}\)) than D1 (\(60^\circ\text{C}\)), D2 must contain a higher percentage of G-C pairs, and therefore more guanine, compared to D1.
Thus, both statements are correct.

Step 3: Final Answer:
Both Statement I and Statement II are correct.