14.3 Basics of DNA ReplicationÄuring cell division, each daughter cell receives a copy of each molecule of DNA by a process known as DNA replication. In general, eukaryotic chromosomes contain a linear DNA molecule packaged into nucleosomes, and have two distinct regions that can be distinguished by staining, reflecting different states of packaging and compaction. Most prokaryotes contain a single, circular chromosome. Prokaryotes are much simpler than eukaryotes in many of their features. A purine always pairs with a pyrimidine A pairs with T, and G pairs with C. The diameter of the double helix, 2 nm, is uniform throughout. Alternating deoxyribose sugars and phosphates form the backbone of the structure, and the nitrogenous bases are stacked like rungs inside. Some of the salient features are that the two strands that make up the double helix have complementary base sequences and anti-parallel orientations. The currently accepted model of the double-helix structure of DNA was proposed by Watson and Crick. Chargaff found that the ratio of A = T and C = G, and that the percentage content of A, T, G, and C is different for different species. Later experiments by Hershey and Chase using bacteriophage T2 proved that DNA is the genetic material. Avery, MacLeod, and McCarty showed that DNA is required for the transformation of bacteria.
Frederick Griffith's experiments with strains of Streptococcus pneumoniae provided the first hint that DNA may be the transforming principle. 14.1 Historical Basis of Modern UnderstandingÄNA was first isolated from white blood cells by Friedrich Miescher, who called it nuclein because it was isolated from nuclei.