The Structure Of

Example 4.1

Gift from the Dead

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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