![Q.41 Determine the correctness or otherwise of the following Assertion [a] and the Reason [r] Assertion: The association constant in water for the G-C base pair is three times lower than that for the A-T base pair. Reason: There are three hydrogen bonds in the G-C base pair and two in the A-T base pair. (A) Both [a] and [r] are true and [r] is the correct reason for [a] (B) [a] is false but [r] is true (C) Both [a] and [r] are false (D) Both [a] and [r] are true and [r] is not the correct reason for [a]](https://www.letstalkacademy.com/wp-content/uploads/2026/01/slide_41-12.png)
Q.41 Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]
Assertion: The association constant in water for the G-C base pair is three times lower than
that for the A-T base pair.
Reason: There are three hydrogen bonds in the G-C base pair and two in the A-T base pair.
(A) Both [a] and [r] are true and [r] is the correct reason for [a]
(B) [a] is false but [r] is true
(C) Both [a] and [r] are false
(D) Both [a] and [r] are true and [r] is not the correct reason for [a]
G-C base pairs form three hydrogen bonds, making them more stable than A-T base pairs with two hydrogen bonds, so the assertion claiming lower association for G-C is false. The reason stating the hydrogen bond counts is true but does not explain the assertion. Correct answer is (B).
Assertion-Reason Breakdown
The assertion claims the G-C base pair’s association constant in water is three times lower than A-T’s, implying weaker binding. This is false because G-C pairs are stronger due to three hydrogen bonds versus two in A-T, yielding higher stability and association constants around 10^5 M^{-1} for G-C compared to 10^4 M^{-1} for A-T. Water solvation equalizes some differences but does not reverse the order.
The reason correctly states G-C has three hydrogen bonds and A-T has two, a fundamental DNA structure fact. However, more bonds enhance stability, contradicting the assertion’s direction.
Option Analysis
| Option | Correctness | Explanation |
|---|---|---|
| (A) Both true, [r] explains [a] | False | Assertion false; reason true but would predict opposite stability . |
| (B) [a] false, [r] true | True | Matches facts: G-C stronger, hydrogen bond count accurate . |
| (C) Both false | False | Reason true as standard textbook knowledge . |
| (D) Both true, [r] not explanation | False | Assertion false despite true reason . |
Stability Factors
G-C pairs dominate duplex stability despite water’s hydration effects compensating somewhat for A-T’s fewer bonds. Simulations show free energies near -6.4 kcal/mol for both in water, but G-C remains stronger overall. For exams, recall G-C > A-T stability drives higher melting temperatures in GC-rich DNA.
