Experimental demonstration that DNA is the genetic material. Two closely related strains of the bacterium Streptococcus pneumonia differ from each other both in their appearance under the microscope and in their pathogenicity. One strain appears smooth (S) and causes death when injected into mice, and the other appears rough (R) and is nonlethal. The experiment in (A) shows that a substance present in the S strain can change (or transform) the R strain into the S strain and that this change is inherited by subsequent generations of bacteria. The experiment in (B) identifies the substance as DNA.