Chapter 4: DNA, RNA & Flow of Genetic Information

DNA, RNA & Flow of Genetic Information details the covalent structure of DNA and RNA as linear polymers of nucleotides, each composed of a sugar (deoxyribose or ribose), a phosphate, and a nitrogenous base, joined by phosphodiester bridges that form a directional backbone. The summary explains the Watson-Crick double-helix model, emphasizing how antiparallel strands are stabilized by hydrogen bonding between complementary base pairs (adenine with thymine, guanine with cytosine) and van der Waals stacking forces. It explores structural variations such as the standard B-DNA, the wider A-DNA, and the left-handed Z-DNA, alongside the supercoiling observed in circular prokaryotic chromosomes and the stem-loop structures found in single-stranded nucleic acids. The mechanism of heredity is defined through semiconservative replication, a hypothesis confirmed by the Meselson-Stahl experiment using density-gradient centrifugation. Key enzymatic processes are highlighted, including the action of DNA polymerases that synthesize DNA in the 5'-to-3' direction requiring a primer, and the unique reverse transcriptase found in retroviruses like HIV which converts RNA genomes into DNA. The text further elucidates gene expression through transcription by RNA polymerase, which initiates at specific promoter sequences like the prokaryotic Pribnow box or the eukaryotic TATA box, to produce messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Finally, the chapter covers the universal, degenerate genetic code where triplet codons specify amino acids, and concludes with the complex organization of eukaryotic genes, which are mosaics of coding exons and non-coding introns that require processing by spliceosomes, allowing for evolutionary exon shuffling and alternative splicing.