We are retracing, generally, the order of events that led to an understanding of the molecular details of information transfer in biological systems. Mendel's predictions, originally considered unlikely, were supported by an understanding of the segregation of chromosomes during meiosis. By the early twentieth century, it was accepted that Mendelian inheritance of genes physically located on chromosomes was an appropriate description of heredity. Note, though, that we have said nothing about the molecular nature of chromosomes. Today, we will think about some of the experiments that led to the identification of DNA as the genetic material. Then, we can discuss some of the details of DNA structure and replication.
Chromosomes, the physical location of genes in the cell are composed of DNA and protein. Proteins, as we know, are composed of 20 amino acid monomers in a linear polypeptide chain. DNA is also a linear polymer, but it is composed of only four monomers. Proteins seemed at first to be the best candidates for the genetic material, as the diversity of possible combinations of monomers suggested that information could be stored at high density.
Experiments in molecular genetics often involve altering the genotype of an organism and then assessing the results of the change on the phenotype. In one such experiment (see Figure 15.1) the genetic material of one type of bacterium (strain S) can be used to change the genotype (and therefore the phenotype) of another (strain R becomes S). This is called a genetic transformation. Later, Avery andMacLeod pruified DNA as the transforming material.
Viral particles (composed of DNA and protein) also genetically change their host cells. They inject genetic material that directs the cell to make more copies of the virus. The definitive experiment showing that virus DNA (but not protein) injected into cells is shown in figure 15.2. DNA, then must be the genetic material.
Using an X-ray diffraction pattern of DNA produced by Rosalind Franklin, Watson and Crick proposed a double-helical structure of DNA. This structure has several important features. First, the repeating unit, or backbone of the helix is a sugar - phosphate polymer with nitrogenous base (A, T, C, G) side groups that project into the center of the helical cylinder. (See figure 15.5) Each base one strand of the helix is paired with a complementary base on the opposite, antiparallel strand. The base pairing of purines with pyramidines ensures a constant diameter for the helix.
This structure has important implications for the function of the genetic material. First, most of the reactive groups on the DNA molecule are sequestered on the inside of the helix, and are not subject to environmental modification. Second, the presence of two strands allows replication to occur simultaneously on both strands, with each strand serving as a template for a new antiparallel strand. The experiment in figure 15.8 represents the conclusive evidence for this semiconservative mode of DNA replication.