The Griffith experiment, conducted by Frederick Griffith in 1928, was pivotal in identifying the role of genetic material in controlling the traits of organisms. At the time, the specific genetic factor was unknown, but today we recognize it as DNA. Griffith's work demonstrated that bacteria possess the ability to undergo transformation, which refers to the uptake of external DNA from their environment, leading to changes in both genotype and phenotype.
In his experiment, Griffith utilized three types of bacteria: the lethal smooth strain (S strain), the nonlethal rough strain (R strain), and heat-killed S strain. The S strain is lethal due to its smooth surface and protective capsule, while the R strain has a rough surface and is nonlethal. The heat-killed S strain, as the name suggests, is the S strain that has been killed by heat, rendering it incapable of reproduction.
Griffith's experimental procedure involved several key steps. In the first experiment, he injected the lethal S strain into mice, resulting in their death. In the second experiment, he injected the nonlethal R strain, and the mice survived. The third experiment involved the heat-killed S strain, which also did not kill the mice. The critical moment came in the fourth experiment, where Griffith combined the R strain and the heat-killed S strain. Contrary to his expectations, this combination resulted in the death of the mice. Remarkably, he was able to isolate living S strain bacteria from the deceased mice, despite not injecting them with live S strain.
This unexpected outcome led Griffith to conclude that the living R strain had transformed by taking up genetic material from the heat-killed S strain. The R strain effectively absorbed the DNA released from the dead S strain, resulting in its transformation into a lethal S strain. Although Griffith did not identify the genetic material at the time, his findings laid the groundwork for future research.
Later, scientists Oswald Avery, Maclyn McCarty, and Colin MacLeod identified DNA as the transforming substance in Griffith's experiment. Despite this discovery, skepticism remained among scientists regarding DNA as the genetic material, as proteins were more widely understood at the time. This skepticism prompted further experiments to confirm that DNA, rather than proteins, is the true genetic material. The Griffith experiment remains a foundational study in genetics, illustrating the concept of transformation and the role of DNA in heredity.
