Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): GC rich dsDNA has higher melting point than the AT rich dsDNA
Reason (R): Hydrogen bonding energy of a guanine-cytosine (GC) pair is higher than a adenine-thymine (AT) pair
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true and (R) is not the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

G-C base pairs share three hydrogen bonds, while A-T pairs share only two.
This additional hydrogen bond increases the total bonding energy of G-C pairs, resulting in a higher melting point for GC-rich DNA.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The melting temperature (\(T_m\)) of double-stranded DNA is the temperature at which half of the DNA molecules dissociate into single strands.
This value is determined by the nucleotide base composition and the thermal stability of the base pairs.

Step 2: Detailed Explanation:
Let us analyze the relationship between base-pairing and DNA melting temperature:
Assertion (A) states that GC-rich double-stranded DNA has a higher melting point than AT-rich DNA.
This is a correct observation because GC-rich DNA requires more thermal energy to disrupt the interactions holding the complementary strands together, resulting in a higher \(T_m\).
Thus, Assertion (A) is true.
Reason (R) explains the molecular basis of this observation.
Guanine-Cytosine (G-C) base pairs are held together by three hydrogen bonds, whereas Adenine-Thymine (A-T) base pairs are held together by only two hydrogen bonds.
Because of this difference, the total hydrogen-bonding energy of a G-C pair is greater than that of an A-T pair, contributing to the higher thermal stability of GC-rich DNA sequences.
Thus, Reason (R) is true and directly provides the physical explanation for Assertion (A).

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A). This corresponds to option (A).