: 230
Building blocks: Nucleotide = base + sugar + phosphate. Base types: purines (A, G — double ring) and pyrimidines (C, T/U — single ring). Watson-Crick model (1953): Two antiparallel polynucleotide strands wound into a right-handed double helix. Sugar-phosphate backbone on outside; bases pair inside. Base pairing rules: A=T (2 H-bonds) and G≡C (3 H-bonds). This complementarity is the basis for: (1) DNA replication (semiconservative — each daughter has one old + one new strand), (2) Transcription (DNA → mRNA by RNA polymerase), (3) Translation (mRNA → protein at ribosomes with tRNA adapters). Chargaff's rules for dsDNA: %A=%T, %G=%C, purines=pyrimidines=50%. The A+T/G+C ratio is species-specific. Higher GC → higher Tm (more H-bonds to break). DNA stability: no 2'-OH → resistant to alkaline hydrolysis → suitable for long-term genetic storage. RNA instability: 2'-OH → susceptible to base-catalyzed hydrolysis → shorter-lived (appropriate for temporary messengers). Central dogma: DNA → RNA → Protein. Exception: reverse transcriptase (retroviruses, HIV). For JEE calculations: given any one base %, calculate all others using Chargaff. H-bond count = 2(AT pairs) + 3(GC pairs). Total nucleotides in n bp = 2n.