(i) Denaturation of proteins
Step 1: Every functional protein has a fixed folded shape (its native state) that lets it do its job in the body. Denaturation is the disruption of this shape by outside agents such as heat, strong acid or base, alcohol, or salts of heavy metals.
Step 2: These agents snap the hydrogen bonds and other non-covalent links that maintain the helix (secondary structure) and the overall folding (tertiary structure). As a result the chain unwinds and clumps together, and the protein can no longer perform its biological function. Importantly, the covalent peptide backbone (primary structure) is not broken, so the amino-acid sequence survives.
Step 3: Everyday illustrations: an egg turning solid and white when cooked, and milk turning to curd, both show proteins denaturing.
(ii) Bases in nucleic acids
Step 4: Nucleic acids (DNA and RNA) carry heterocyclic nitrogenous bases attached to their sugar-phosphate backbone. They fall into two classes by ring type:
• Double-ring purines: Adenine and Guanine.
• Single-ring pyrimidines: Cytosine, Thymine and Uracil.
Step 5 (structures in words + formulae):
• Adenine, \(C_5H_5N_5\) — purine skeleton with an amino group.
• Guanine, \(C_5H_5N_5O\) — purine skeleton with one amino and one keto group.
• Cytosine, \(C_4H_5N_3O\) — pyrimidine ring with one amino and one keto group.
• Thymine, \(C_5H_6N_2O_2\) — pyrimidine ring with two keto groups and a methyl group.
• Uracil, \(C_4H_4N_2O_2\) — pyrimidine ring with two keto groups (no methyl).
Step 6: DNA uses A, T, G, C; RNA uses A, U, G, C (uracil in place of thymine).
\[\boxed{\text{DNA: A, T, G, C} \quad ; \quad \text{RNA: A, U, G, C}}\]