When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material
•The key factor in determining the genetic material was choosing appropriate experimental organisms
•The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them
Evidence That DNA Can Transform Bacteria
•The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928
•Griffith worked with two strains of a bacterium, one pathogenic and one harmless
•When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic
•He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA
•In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA
•Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria
•Many biologists remained skeptical, mainly because little was known about DNA
Evidence That Viral DNA Can Program Cells
•More evidence for DNA as the genetic material came from studies of viruses that infect bacteria
•Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research
•In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2
•To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection
•They concluded that the injected DNA of the phage provides the genetic information.
Additional Evidence That DNA Is the Genetic Material
•Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases
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