Chapter 9: DNA Structure and Analysis

Early experiments resolved the decades-long debate between protein and nucleic acid as the hereditary substance through a series of landmark studies. Griffith's transformation experiments in Streptococcus pneumoniae demonstrated that an unknown chemical principle could alter bacterial phenotypes, while Avery, MacLeod, and McCarty systematically identified this principle as deoxyribonucleic acid through enzymatic degradation studies. The Hershey-Chase experiment provided decisive confirmation using radioactive isotope labeling, showing that DNA, not protein, serves as the genetic material during viral infection. Additional evidence from transfection studies, examination of DNA distribution patterns across species, ultraviolet mutagenesis at characteristic wavelengths, and advances in recombinant technology further solidified DNA's universal role in heredity across prokaryotes and eukaryotes alike. The molecular architecture section explores the Watson-Crick double helix model, a right-handed antiparallel structure stabilized by complementary base pairing between adenine-thymine and guanine-cytosine. This elegant model synthesized Chargaff's ratio principles and X-ray crystallographic evidence from Franklin's groundbreaking work. The chapter examines structural variants including A-form and Z-form DNA, highlighting how structural diversity relates to functional variation. RNA is presented as structurally analogous yet functionally distinct, containing ribose instead of deoxyribose and uracil in place of thymine, with diverse roles encompassing messenger RNA, transfer RNA, ribosomal RNA, and regulatory forms such as microRNA and small interfering RNA. The final section surveys analytical methodologies essential for molecular investigation. Molecular hybridization exploits base complementarity for sequence identification through fluorescent in situ hybridization and related techniques. Gel electrophoresis separates nucleic acids by molecular weight and charge, enabling fragment isolation and characterization. Physicochemical approaches including ultraviolet absorption spectroscopy, hyperchromic effects, and thermal melting temperature analysis reveal compositional and stability information. The chapter concludes by introducing computational biology tools like sequence alignment algorithms, demonstrating how modern genetics integrates experimental biochemistry with bioinformatics.