Q.Nucleic acids exhibit secondary structure, justify with example.

• A nucleic acid such as DNA or RNA is a polynucleotide, a chain built by joining nucleotides one after another through phosphodiester bonds, each nucleotide made of a nitrogenous base, a pentose sugar and a phosphate group.
• DNA is the clearest example of secondary structure in a nucleic acid: it exists as a double helix (the Watson-Crick model), in which two polynucleotide strands wind around a common central axis.
• The two strands run antiparallel to each other (the 5' end of one strand lies opposite the 3' end of the other), and they are held together by hydrogen bonds between complementary bases on the two strands — adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. …

A nucleic acid, whether DNA or RNA, is built as a polynucleotide — a long chain formed by linking nucleotides one after another through phosphodiester bonds. Each nucleotide is itself a combination of a nitrogenous base (a purine such as adenine or guanine, or a pyrimidine such as cytosine, thymine or uracil), a pentose sugar (deoxyribose for DNA, ribose for RNA), and a phosphate group. Taken purely as this linear sequence of bases, a nucleic acid has only a primary structure.

DNA, however, does not exist in the cell as a bare, unstructured single strand. Watson and Crick proposed — and it has since been confirmed — that DNA occurs as a double helix: two polynucleotide chains wound around a common central axis, like a twisted ladder. This coiled, two-stranded arrangement, over and above the plain linear sequence of nucleotides in either strand, is what constitutes the secondary structure of DNA.

Two further structural facts justify calling this a genuine, specific secondary structure rather than a vague coil:

• The strands are antiparallel. The two polynucleotide chains run in opposite chemical directions — the 5'-phosphate end of one strand is aligned against the 3'-hydroxyl end of the other. …

Method: Identifying the Concrete Example Behind an Abstract Claim

When a question says "X exhibits property Y, justify with example," the word "example" is doing real work — a correct-sounding general statement about the property, with no named, concrete instance behind it, does not actually answer the question. The right first move is to ask: what is the standard, textbook-named case that this general claim is describing? For nucleic acid secondary structure, that named case is the DNA double helix (the Watson-Crick model) — nothing else in the syllabus is offered as the example.

Once you have the concrete example in mind, build the justification around its specific, checkable features rather than a loose analogy. For DNA, that means stating: two polynucleotide strands, wound around a common axis (the helix itself), running antiparallel to each other, and held together by hydrogen bonds between complementary base pairs (A pairs with T via two hydrogen bonds, G pairs with C via three). Each of these details is independently verifiable and is exactly what an examiner is checking for — a vague description of "organising into a higher-order arrangement" without namin …