The science of genetics defines and analyzes heredity in the vast array of structural and physiologic functions that form the properties of organisms. The basic unit of heredity is the gene, a segment of deoxyribonucleic acid (DNA) that encodes in its nucleotide sequence information for a specific physiologic property. The traditional approach to genetics has been to identify genes on the basis of their contribution to phenotype, or the collective structural and physiologic properties of an organism. A phenotypic property, be it eye color in humans or resistance to antibiotics in a bacterium, is generally observed at the level of the organism. The chemical basis for variation in phenotype is change in genotype, or alteration in the DNA sequence, within a gene or within the organization of genes.
DNA as the fundamental element of heredity was suggested in the 1930s from a seminal experiment performed by Frederick Griffith. In this experiment (Figure 7-1), killed virulent Streptococcus pneumoniae type III-S (possessing a capsule), when injected into mice along with living but nonvirulent type II-R pneumococci (lacking a capsule), resulted in a lethal infection from which viable type III-S pneumococci were recovered. The implication was that some chemical entity transformed the live, nonvirulent strain to the virulent phenotype. A decade later, Avery, MacLeod, and McCarty discovered that DNA was the transforming agent. This formed the foundation for molecular biology as we understand it today. Subsequent investigations with bacteria revealed the presence of restriction enzymes, proteins that cleave DNA at specific sites, giving rise to DNA restriction fragments. Plasmids were identified as small genetic elements carrying genes and capable of independent replication in bacteria and yeasts. The introduction of a DNA restriction fragment into a plasmid allows the DNA fragment to be amplified many times. Amplification of specific regions of DNA also can be achieved with bacterial enzymes using polymerase chain reaction (PCR) or other enzyme-based method of nucleic acid amplification. DNA amplified by these sources and digested with appro-priate restriction enzymes can be inserted into plasmids. Genes can be placed under control of high-expression bacterial promoters that allow encoded proteins to be expressed at increased levels. Bacterial genetics have fostered the development of genetic engineering not only in prokaryotes but also in eukaryotes. This technology is responsible for the tremendous advances in the field of medicine realized today.
Griffiths' experiment demonstrating evidence for a transforming factor, later identified as DNA. In a series of experiments, mice were injected with live or killed encapsulated or nonencapsulated Streptococcus pneumoniae, as indicated in experiments A through D. The key experiment is D, showing that the killed encapsulated bacteria could supply a factor that allowed the nonencapsulated bacteria to kill mice. Besides providing key support for the importance of the capsule for pneumococcal virulence, experiment D also illustrates the principle of DNA as the fundamental ...