(a)(i) Describe the synthetic theory of evolution. (10 marks) (a)(ii) Describe common types of mimicry in insects with suitable…

(a)(i) Synthetic theory. The synthetic theory, or modern synthesis, is the Neo-Darwinian framework uniting Darwinian natural selection with Mendelian inheritance and population genetics. It treats evolution as change in allele frequencies in populations. Hardy–Weinberg equilibrium is the null model: without mutation, migration, selection, drift or non-random mating, allele and genotype frequencies remain constant, so departures reveal evolutionary forces. Dobzhansky linked genetic variation in natural populations to selection; Mayr clarified the biological species concept and reproductive isolation; Simpson integrated fossil sequences with gradual change. Indian scientist P. Maheshwari is not a founder of the synthesis; his work was plant embryology. The synthesis explains adaptation, microevolution and speciation through selection on heritable variation, and remains the central framework for evolutionary biology.

(a)(ii) Mimicry in insects. Mimicry is resemblance to another organism or object that benefits the mimic. Batesian mimicry: a palatable mimic resembles an unpalatable model; female Papilio polytes mimics unpalatable Pachliopta aristolochiae in India. Müllerian mimicry: unpalatable species converge on shared warning signals, strengthening predator learning; Heliconius erato and H. melpomene are classic co-mimics. Aggressive mimicry: a predator or parasite imitates a harmless signal to approach prey; female Photuris fireflies mimic the flash signals of Photinus fireflies to lure and eat them. Automimicry: self-mimicry within the same species, such as eyespots on butterfly wings resembling vertebrate eyes and deflecting attacks from vital parts.

(b) Chain elongation in prokaryotes. In prokaryotes, translation begins when the 30S subunit, initiation factors and formylmethionyl-tRNA bind the 5′ UTR and AUG; the 50S subunit joins to form the 70S initiation complex. Elongation is cyclic. A charged aminoacyl-tRNA matching the codon enters the A site, guided by codon–anticodon pairing and EF-Tu·GTP; GTP hydrolysis releases EF-Tu. The growing peptide is on the P-site tRNA. The 23S rRNA peptidyl-transferase centre catalyses peptide bond formation, transferring the nascent peptide to the A-site amino acid and leaving the P-site tRNA deacylated. EF-G·GTP then binds, and GTP hydrolysis drives translocation: the ribosome moves one codon, the peptidyl-tRNA shifts from A to P, and the deacylated tRNA moves to E and exits. The A site is empty for the next charged tRNA. This ordered cycle ensures codon fidelity and processive chain growth, adding amino acids from N to C terminus. Termination occurs when a stop codon enters the A site; release factors RF2/RF3 promote hydrolysis of the peptide–tRNA bond and release the protein. Prokaryotic 70S ribosomes differ from eukaryotic 80S ribosomes in subunit size, initiation factors, formylmethionine start, and coupling of transcription and translation.

(c) Ribozyme action and applications. Ribozymes are RNA molecules that catalyse reactions, mainly by acid–base catalysis, where conserved nucleobases and metal ions donate/accept protons and stabilise transition states. Group I introns self-splice by a 2′-OH of a conserved internal adenosine attacking the 5′ splice site, followed by exon ligation. Group II introns form a lariat intermediate and catalyse their own excision and splicing. RNase P is an RNA enzyme that processes tRNA 5′ ends; hammerhead ribozymes have a compact pseudoknot with a catalytic core that cleaves a specific phosphodiester bond. Technologically, engineered hammerhead ribozymes can be designed as sequence-specific RNA-cleaving agents for antisense strategies; this is direct catalytic cleavage, not RNA interference. RNAi therapeutics are a separate RNA-based field using small RNAs and RISC. Synthetic biology uses ribozymes as genetic switches, self-cleaving elements and RNA-editing tools. In COVID-19 mRNA vaccines, stability and translation depend on the cap, modified nucleotides, UTRs and poly(A) tail; ribozymes are not the stabilizing feature. Ribozymes support the RNA-world hypothesis, showing RNA can bridge genotype and phenotype by storing information and catalysing chemistry; later molecular novelty may involve gene duplication and neo-functionalization, but that is an extension beyond the original modern synthesis.