Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : If a sloution of DNA is heated and DNA molecules in the solution are completely denatured it increases the absorption of UV light at 260nm
Reason (R) : Denaturation causes seperation of DNA strands and bases in the DNA strands are exposed.
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 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:

The increase in UV absorption upon DNA denaturation is called the hyperchromic shift.
Monitoring this absorbance change at 260 nm as temperature increases is how DNA melting curves are generated.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Double-stranded DNA absorbs UV light at 260 nm.
When DNA is denatured, its physical structure changes, altering its light absorption properties.
This phenomenon is known as the hyperchromic effect.

Step 2: Detailed Explanation:
- Assertion (A) is true:
When a solution of double-stranded DNA (dsDNA) is heated, the hydrogen bonds holding the two strands together break.
This causes the dsDNA to melt or denature into single-stranded DNA (ssDNA).
This denaturation is accompanied by a significant increase (typically about \(30-40\%\)) in its absorbance of UV light at 260 nm.
- Reason (R) is true:
In native dsDNA, the hydrophobic purine and pyrimidine bases are stacked closely inside the double helix.
This stacking restricts their electronic transitions, reducing their ability to absorb UV light (hypochromicity).
When the DNA is denatured, the two strands separate, disrupting this base stacking.
This exposes the nitrogenous bases to the solvent, allowing them to freely absorb UV light.
Thus, the separation of DNA strands and exposure of bases (R) directly explains why the absorbance increases (A).

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