Deoxyribonucleic acid was known to contain genetic information several years before its structure was finally determined. As early as 1928 Fred Griffith carried out the now famous pneumococ-cus transformation experiment. There are several strains of the bacterium Diplococcuspneumoniae. Some strains cause pneumonia and are said to be virulent; others do not and are nonvirulent. The virulent bacteria possess a polysaccharide coat and, when grown on an agar plate, create colonies with a smooth appearance—these are S bacteria. Nonvirulent bacteria do not have this polysaccharide coat, and their colonies are rough in appearance—these are R bacteria. Mice injected with S bacteria developed pneumonia and died, whereas those injected with R bacteria were unaffected. When S bacteria were killed by heat treatment before injection, the mice remained healthy. However, those mice injected with a mixture of live R bacteria and heat-killed S bacteria died of pneumonia. This observation meant that something in the heat-killed S bacteria carried the information that enabled bacteria to make the polysaccharide coat and therefore to change—transform—the R bacteria into a virulent strain. Later experiments in the mid-1940s by Oswald Avery, Maclyn McCarty, and Colin MacLeod clearly demonstrated that the transforming factor was DNA. They made extracts of S bacteria and treated them with enzymes that destroy either DNA, RNA, or protein. These extracts were then mixed with R bacteria. Only the extract rich in DNA was able to transform R bacteria into S bacteria.
mouse injected with S bacteria dies mouse injected with S bacteria dies mouse injected with R bacteria stays healthy
mouse injected with heat-killed S bacteria stays healthy
mouse injected with live R bacteria + heat-killed S bacteria dies
Figure 4.1. 2'-deoxyadenosine-5'-triphosphate.
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