Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Two neutral solutions A and B contain double stranded DNA molecule with same length and same concentrations, solution A is heated to increase the temperature of solution and absorbance of both the solutions were measured at 260nm. The absorbance of the solution A was found more than that of solution B.
Reason (R): Denaturation of DNA increases the absorbance of UV rays at 260nm.
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 but (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:

Remember the Hyperchromic Shift:
- Double-stranded DNA: Lower UV absorbance (hypochromism).
- Single-stranded DNA: Higher UV absorbance (hyperchromism).
This change is used to monitor DNA melting and determine its melting temperature (\(\text{T}_m\)).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DNA absorbs UV light at 260 nm because of the conjugated double bonds in its purine and pyrimidine bases. The physical state of the DNA (single-stranded vs. double-stranded) affects its absorbance.

Step 2: Detailed Explanation:
- Assertion Analysis: Heating solution A causes the hydrogen bonds holding the two strands together to break, denaturing the double-stranded DNA into single strands. When absorbance is measured, solution A shows higher absorbance than the unheated, double-stranded solution B, validating the Assertion.
- Reason Analysis: Double-stranded DNA has a stacked base structure that shields the purines and pyrimidines, limiting their light absorption. When DNA denatures, the bases unstack and are fully exposed to the light path, increasing UV absorbance at 260 nm by approximately \(30-40\%\). This phenomenon is called the hyperchromic effect, validating the Reason.
- Since the hyperchromic effect resulting from thermal denaturation explains why the heated solution A shows a higher absorbance than the unheated solution B, (R) is the correct explanation of (A).

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