Week 12/ Draft 3

An initial step in identifying DNA as the source of genetic information came with the discovery of a phenomenon called transformation. This phenomenon was first observed in 1928 by Fred Griffith, an English physician whose special interest was the bacterium that causes pneumonia: Streptococcus pneumoniae. Griffith had succeeded in isolating several different strains of S. pneumoniae (type I, II, III, and so forth). In the virulent (disease-causing) forms of a strain, each bacterium is surrounded by a polysaccharide coat, which makes the bacterial colony appear smooth (S) when grown on an agar plate. Griffith found that these virulent forms occasionally mutated to nonvirulent forms, which lack a polysaccharide coat and produce a rough-appearing colony (R).