Molecular Basis of Inheritance

The quest to identify the genetic material began with Frederick Griffith's 1928 transformation experiment, where he demonstrated that a "transforming principle" from heat-killed virulent (S-strain) pneumococcus could convert non-virulent (R-strain) bacteria into virulent forms. In 1944, Avery, MacLeod, and McCarty biochemically proved this transforming principle was DNA by showing that only DNase destroyed the transforming ability while protease and RNase had no effect. The definitive proof came from Alfred Hershey and Martha Chase in 1952, who used bacteriophage T₂ labelled with radioactive ³²P (incorporated into DNA) and ³⁵S (incorporated into protein). After infection and centrifugation, ³²P appeared in the bacterial pellet with the infected cells while ³⁵S remained in the supernatant, conclusively establishing DNA as the hereditary material.

The DNA molecule, as described by James Watson and Francis Crick in 1953, is a right-handed double helix with two antiparallel polynucleotide strands held together by hydrogen bonds between complementary bases: adenine pairs with thymine through two hydrogen bonds (A=T), and guanine pairs with cytosine through three hydrogen bonds (G triple-bond C). This arrangement satisfies Chargaff's rules, which state that in any DNA molecule, the amount of adenine equals thymine (A=T) and guanine equals cytosine (G=C). The sugar-phosphate backbone runs in opposite directions (5' to 3' and 3' to 5'), and the helix has a major and minor groove where proteins interact with the bases.

DNA packaging in eukaryotes involves winding approximately 200 base pairs of DNA around a histone octamer (two copies each of H₂A, H₂B, H₃, and H₄) to form a nucleosome, the fundamental unit of chromatin. Nucleosomes compact further into the 30 nm fibre, then into loops, and ultimately into the highly condensed metaphase chromosome.

DNA replication is semiconservative, as proven by Meselson and Stahl using ¹⁵N-labelled E. coli DNA and CsCl density gradient centrifugation. At the replication fork, helicase unwinds the double helix, primase synthesizes a short RNA primer, DNA polymerase III extends the new strand in the 5' to 3' direction (reading the template 3' to 5'), and ligase seals Okazaki fragments on the lagging strand.

The central dogma describes the flow of genetic information: DNA is transcribed into mRNA by RNA polymerase (which, unlike DNA polymerase, does not require a primer). The template strand is read 3' to 5' while mRNA is synthesized 5' to 3'. In eukaryotes, the primary transcript undergoes post-transcriptional processing: 5' capping, 3' polyadenylation, and intron splicing. The genetic code consists of 64 triplet codons: 61 sense codons (including AUG as the universal start codon encoding methionine) and 3 stop codons (UAA, UAG, UGA). The code is degenerate (multiple codons for one amino acid), universal, non-overlapping, and non-ambiguous.

Translation occurs on ribosomes with three functional sites: the A (aminoacyl) site where charged tRNA enters, the P (peptidyl) site where peptide bonds form, and the E (exit) site from which discharged tRNA departs. Initiation begins when the small ribosomal subunit binds mRNA at the 5' cap and scans to the first AUG, where initiator tRNA (carrying methionine) pairs with the start codon.

Gene regulation in prokaryotes is exemplified by the Lac operon, comprising structural genes (lacZ, lacY, lacA), a promoter, an operator, and a regulatory gene (lacI) encoding a repressor protein. In the absence of lactose, the repressor binds the operator and blocks transcription. When lactose (the inducer, converted to allolactose) is present, it binds the repressor, changing its conformation so it can no longer bind the operator, allowing RNA polymerase to transcribe the structural genes. The Human Genome Project revealed approximately 20,000-25,000 genes across 3.2 billion base pairs, and DNA fingerprinting uses Variable Number Tandem Repeats (VNTRs) for individual identification through Southern blotting. The key testable concept is understanding that DNA polymerase synthesizes in the 5' to 3' direction (reading template 3' to 5'), that RNA polymerase does not need a primer, and that lactose acts as the inducer (not the substrate) for Lac operon transcription.