Correct option is D
Explanation-
The question is about hyperchromicity (increase in absorbance at 260 nm - A260) during heating of nucleic acids, which is a classic indicator of nucleic acid denaturation/melting (Tm - melting temperature). When double-stranded nucleic acids (DNA/DNA, RNA/RNA, DNA/RNA hybrids) are heated, they undergo strand separation (melting), leading to an increase in A260 absorbance (hyperchromic effect). However, single-stranded nucleic acids, especially if they don't have strong secondary structures, won’t show a sharp melting transition like double-stranded molecules do.
Option d: Single stranded DNA with imperfect secondary structures
Single-stranded DNA with random or imperfect secondary structures (like loops or hairpins) doesn't show a single sharp melting transition. The absorption change would be gradual and broad over a temperature range, not a sharp, typical melting curve like that seen for perfect double-stranded helices. The reason is lack of extensive, uniform double-stranded regions. This is the correct answer: This sample is NOT expected to generate a typical sharp melting curve.
Incorrect Options-
Option a: Double stranded DNA
Clear melting transition with sharp increase in A260 at Tm. The reason is breakage of hydrogen bonds between complementary strands. So, expected to show typical melting profile (sharp hyperchromic shift).
Option b: Double stranded RNA
This shows a sharp Tm with a characteristic hyperchromic effect. It also has extensive base pairing (though it's usually A-form helix rather than B-form like DNA). So, expected to show melting profile.
Option c: DNA:RNA hybrid
DNA-RNA hybrids also have defined base pairing and exhibit a melting curve with a sharp Tm. The reason is stable double-stranded hybrid. So, expected to show melting profile.
Final Answer - Option d - Single stranded DNA with imperfect secondary structures
