Chapter 10: DNA Structure & Analysis

DNA Structure & Analysis comprehensively details the foundational experimental evidence that established deoxyribonucleic acid (DNA) as the universal genetic material in almost all living organisms, a molecule required to perform the essential functions of replication, information storage, expression, and permitting variation through mutation. Before this realization, proteins were generally favored as the hereditary substance due to their structural complexity, contrasting with the initially flawed tetranucleotide hypothesis proposed by Levene for DNA. The shift in understanding began with Frederick Griffith's 1927 discovery of bacterial transformation, demonstrating that a heritable agent, the "transforming principle," derived from heat-killed virulent Diplococcus pneumoniae (IIIS) could convert live avirulent cells (IIR) into infectious IIIS strains. The chemical identity of this agent was pinpointed in 1944 by Avery, MacLeod, and McCarty, whose rigorous testing proved that only the enzyme DNase destroyed the transformative capacity, confirming DNA as the genetic factor responsible for heredity. Further decisive confirmation came from the Hershey-Chase experiment in 1952, which utilized radioisotopes to selectively label the protein coat (35 S) and the DNA core (32 P) of T2 bacteriophage; they proved that only the 32 P-labeled DNA entered the host bacterial cell to direct viral reproduction. In eukaryotes, DNA's role was supported by indirect observations, such as its correlation with chromosomal location and the stoichiometric relationship between cell ploidy and DNA content, and later directly confirmed by modern recombinant DNA technology. Although DNA is paramount, RNA acts as the genetic material in certain viruses, including retroviruses, which rely on the enzyme reverse transcriptase to synthesize a DNA intermediate. Chemically, nucleic acids are constructed from nucleotides, composed of a phosphate, a pentose sugar, and a nitrogenous base (purines A/G or pyrimidines C/T/U). DNA is distinguished by using deoxyribose and thymine, while RNA uses ribose and uracil. Drawing upon Chargaff’s rules (A=T, G=C) and critical X-ray data from Rosalind Franklin, James Watson and Francis Crick modeled the definitive structure of DNA as a right-handed double helix. This structure features two antiparallel polynucleotide strands linked by specific complementary base pairing: Adenine forms two hydrogen bonds with Thymine, and Guanine forms three hydrogen bonds with Cytosine. While B-DNA is the biologically relevant form, alternative conformations like the dehydrated A-DNA and the left-handed Z-DNA also exist. RNA typically exists as a single strand and functions in genetic expression through molecules like mRNA, tRNA, and rRNA. Finally, key analytical methods include the hyperchromic shift, where increased UV absorption upon heating indicates DNA denaturation and is used to determine the melting temperature (T m ), a value related to G-C content, alongside techniques like molecular hybridization (e.g., FISH) and electrophoresis for separating molecules by size.