Chapter 12: Structure of the Nucleic Acids

DNA structure is presented as a double-stranded antiparallel helix stabilized by complementary base pairing between adenine-thymine and guanine-cytosine, along with hydrophobic interactions from base stacking, while RNA typically exists as a single strand that forms secondary structures through intramolecular base pairing. The chapter integrates clinical examples demonstrating therapeutic applications and disease mechanisms: antiretroviral nucleoside analogs such as zidovudine and lamivudine that inhibit HIV reverse transcriptase; chemotherapeutic pyrimidine analogs like 5-fluorouracil that disrupt thymidylate synthase and DNA synthesis in cancer cells; and antibiotics such as azithromycin that selectively target bacterial ribosomes without affecting eukaryotic protein synthesis machinery. DNA packaging involves wrapping around histone octamers to form nucleosomes, which subsequently coil into higher-order chromatin structures essential for genome compaction and transcriptional regulation. The chapter differentiates eukaryotic linear chromosomes from prokaryotic circular genomes and discusses the distinction between haploid and diploid chromosome numbers. RNA function is detailed across three major types: messenger RNA carrying genetic instructions from DNA to ribosomes with its characteristic 5-prime cap and 3-prime poly-adenylation; ribosomal RNA composing the catalytic and structural components of ribosomes with distinct sedimentation coefficients across cellular compartments; and transfer RNA delivering amino acids during translation through anticodon-codon interactions and specialized modified nucleotides. The chapter also addresses DNA denaturation and renaturation processes, hybridization techniques, and molecular mechanisms of DNA-targeting drugs including intercalating agents that disrupt replication and transcription. Throughout, the central dogma framework unites these concepts, demonstrating how information flows from DNA through RNA to proteins, while illustrating how disruptions in nucleic acid structure contribute to infection, malignancy, and genetic pathology.