Question

Why the absorption of UV light by double stranded DNA increases (hyperchromic effect) when the DNA is denatured?

• A. Due to loss of base stacking \
• B. Due to increase in base stacking \
• C. Due to renaturation \
• D. Due to inhibition of light

Hint:

Stacked bases hide from light $\rightarrow$ lower absorption. Unstacked bases (denatured single strands) are fully exposed $\rightarrow$ higher absorption (Hyperchromic shift).

Answer 1

The Correct Option is A

Step 1: Concept The nitrogenous bases in nucleic acids strongly absorb ultraviolet (UV) light, with an absorption peak near 260 nm. The physical arrangement of these bases significantly changes how much light they can absorb.

Step 2: Meaning The hyperchromic effect refers to a notable increase in UV light absorption that occurs when double-stranded DNA denatures into single strands.

Step 3: Analysis * In intact double-helical DNA, the purine and pyrimidine bases are tightly stacked on top of each other parallel to the helical axis. This base-stacking interaction restricts the electronic transitions of the aromatic rings, shielding them and limiting their UV absorption. * When DNA is denatured (e.g., by heating), the hydrogen bonds break and the two strands separate. This unstacking of the bases exposes the aromatic rings, allowing them to freely absorb more UV light energy. * Therefore, the loss of base stacking directly causes the hyperchromic shift.

Step 4: Conclusion Thus, option A correctly identifies the loss of base stacking as the cause of this effect. Final Answer: (A)