Final answer:
Frederick Griffith's transformation experiments in 1928 demonstrated that a 'transforming principle' allowed a non-pathogenic bacterial strain to acquire virulence from a heat-killed pathogenic strain, marking a breakthrough in the understanding of genetic material.
Step-by-step explanation:
British bacteriologist Frederick Griffith's experiments in 1928 marked a pivotal moment in understanding genetic material through his work with the bacteria Streptococcus pneumoniae, which causes pneumonia. He used two strains: the non-pathogenic R (rough) strain and the pathogenic S (smooth) strain. The R strain did not harm mice, while the S strain was lethal. Remarkably, when live R strain bacteria were mixed with heat-killed S strain bacteria and injected into mice, the mice died, demonstrating a transformation that allowed the non-pathogenic R strain to acquire the virulence of the S strain.
Griffith also observed that live bacteria isolated from these dead mice were all of the S strain, indicating that some component of the heat-killed S strain had transformed the R strain into a virulent form. This heralded the discovery of what he termed the "transforming principle," which was later determined by other researchers to be DNA, thus providing the first clue that DNA was the genetic material.