Summary

This chapter traced the complete molecular story of heredity, from the search for the identity of the genetic material to its structure, its copying, its expression, its regulation, and its most consequential real-world applications. Three landmark experiments jointly established that DNA, not protein, is the genetic material: Griffith's 1928 bacterial transformation experiment first showed that a heritable "transforming principle" could pass between bacterial cells; Avery, MacLeod and McCarty's 1944 biochemical work identified that transforming principle as DNA specifically, by showing DNase (and only DNase) destroyed its activity; and Hershey and Chase's 1952 radioactive-labelling experiment on T2 bacteriophage gave the final, independent confirmation, showing that phage DNA (not phage protein) enters the bacterial host and directs new phage production.

Building on the established fact that DNA is the genetic material, Watson and Crick's 1953 double-helical model — resting on Rosalind Franklin's X-ray data and Chargaff's A=T, G=C rule — explained DNA's structure: two antiparallel polynucleotide strands, sugar-phosphate backbones outside, held together by specific, complementary base pairing (A-T via two hydrogen bonds, G-C via three). RNA differs chemically (ribose sugar, uracil, generally single-stranded) and specialises functionally into mRNA (carries the code), rRNA (builds the ribosome) and tRNA (adapts codon to amino acid). Roughly two metres of DNA is packaged, first around histone octamers as nucleosomes, then through further coiling, to fit inside a microscopic nucleus, with heterochromatin (dense, inactive) and euchromatin (loose, active) reflecting this packaging's link to gene activity.

DNA replicates semiconservatively — proved directly by Meselson and Stahl's 1958 density-gradient experiment on E. coli — through the coordinated action of helicase, topoisomerase, SSBs, primase and DNA polymerase at the replication fork, producing a continuous leading strand and a discontinuous lagging strand of Okazaki fragments later sealed by DNA ligase. The Central Dogma (DNA to RNA to protein) frames the two processes of gene expression that follow: transcription, in which RNA polymerase copies one DNA strand (the template strand) of a transcription unit (promoter-gene-terminator) into RNA, with eukaryotic hnRNA further processed by splicing (removing introns), 5' capping and 3' poly-A tailing before export as mature mRNA; and translation, in which the ribosome reads mRNA codons three bases at a time, using the genetic code's triplet, degenerate, unambiguous, (near-)universal and commaless properties, with tRNA anticodons delivering matching amino acids to build a polypeptide from the AUG start codon to a UAA/UAG/UGA stop codon. …