Summary

DNA as genetic material: Griffith's transformation experiment (1928) showed a "transforming principle" in heat-killed S. pneumoniae. Avery, MacLeod, and McCarty (1944) proved it was DNA. Hershey and Chase (1952) confirmed using sulfur-35 and phosphorus-32 labelled bacteriophages — only phosphorus-32 entered bacteria, proving DNA is the genetic material.

Structure of DNA: Watson and Crick (1953) proposed a double helix: two antiparallel polynucleotide chains held by hydrogen bonds between complementary bases (A pairs with T through two hydrogen bonds, G with C through three). The backbone is sugar-phosphate (3-prime–5-prime phosphodiester bonds). Chargaff's rule: the percentage of A equals the percentage of T, and the percentage of G equals the percentage of C.

DNA replication: Semiconservative — each daughter molecule has one parental and one newly synthesised strand. Meselson and Stahl (1958) proved it using nitrogen-15 density gradient centrifugation (confirmed in higher organisms by Taylor et al. using radioactive thymidine). DNA-dependent DNA polymerase adds nucleotides only in the 5-prime to 3-prime direction — the leading strand is made continuously, while the lagging strand is synthesised as short Okazaki fragments that are later joined together by DNA ligase.

Transcription: RNA synthesis from a DNA template. In prokaryotes, RNA polymerase binds the promoter; in eukaryotes, three RNA polymerases (I, II, III) exist. The primary transcript (hnRNA in eukaryotes) undergoes capping, tailing, and splicing (removal of introns) to form mature mRNA.

Genetic code: Triplet, degenerate, universal, non-overlapping, and commaless. 61 codons code for amino acids; 3 stop codons (UAA, UAG, UGA). AUG is both start codon and codes for methionine. Nirenberg and Khorana deciphered the code using cell-free systems.

Translation: mRNA binds to ribosome (rRNA + proteins). tRNA carries amino acids via anticodon–codon pairing. Peptide bond formation occurs at the peptidyl transferase centre of the ribosome. Process: initiation (AUG), elongation (aminoacyl-tRNA binding, peptide bond, translocation), termination (stop codon, release factor).

Regulation of gene expression: In prokaryotes, the lac operon (Jacob and Monod) — a cluster of genes under control of a promoter, operator, and repressor. In presence of lactose, allolactose binds repressor, allowing transcription. In eukaryotes, regulation is more complex: chromatin remodelling, transcription factors, enhancers, and post-transcriptional modifications.

DNA fingerprinting: Uses variable number tandem repeats (VNTRs) — short, repeating sequences unique to individuals. Process: restriction digestion, gel electrophoresis, Southern blotting, hybridisation with radioactive probes, autoradiography. Applications: forensics, paternity testing, genetic diversity studies. …