The Griffith experiment, conducted by Frederick Griffith in 1928, was pivotal in identifying the role of genetic material in organisms. At that time, the specific genetic factor was unknown, but today we recognize it as DNA. Griffith's work demonstrated that bacteria could undergo transformation, which refers to their ability to uptake 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 non-lethal rough strain (R strain), and heat-killed S strain. The S strain is characterized by a smooth surface due to a protective capsule, making it deadly. In contrast, the R strain has a rough surface and is non-lethal. 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 non-lethal 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 any live S strain initially.
From these observations, Griffith concluded that the living R strain had transformed by taking up genetic material from the heat-killed S strain. This transformation allowed the R strain to acquire the lethal characteristics of the S strain, demonstrating the concept of genetic transformation. Although Griffith did not identify the genetic material at the time, subsequent research by scientists such as Oswald Avery, Maclyn McCarty, and Colin MacLeod later confirmed that DNA was the transforming substance.
Despite this groundbreaking discovery, skepticism remained among scientists regarding DNA as the genetic material, primarily because proteins were better understood at the time. This skepticism led to further experiments to validate the role of DNA in heredity. The Griffith experiment laid the foundation for future research that ultimately established DNA as the primary genetic material in living organisms.