The Griffith Experiment: Videos & Practice Problems

Frederick Griffith's 1928 experiment revealed that bacteria can undergo transformation, a process where they uptake external DNA, leading to changes in genotype and phenotype. He demonstrated this by combining non-lethal rough bacteria (R strain) with heat-killed lethal smooth bacteria (S strain), resulting in the R strain transforming into a lethal form. This pivotal finding laid the groundwork for identifying DNA as the genetic material, although skepticism remained until further research confirmed this. Understanding transformation is crucial for grasping genetic inheritance and molecular biology.

The Griffith Experiment

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.

The bacteria that Griffith experimented with were termed "R" and "S" bacteria because:

• A. Of the way they grew on artificial media.
• B. The "S" bacteria formed smooth appearing colonies.
• C. The "R" bacteria formed rough appearing colonies.
• D. All are correct.

Answer: (D)

In his transformation experiments, what phenomenon did Griffith observe?

• A. Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain converts the living cells into the pathogenic form.
• B. Mixing heat-killed nonpathogenic bacteria with a living pathogenic strain makes the living strain nonpathogenic.
• C. Infecting mice with nonpathogenic strains of bacteria makes them resistant to pathogenic strains.
• D. Mice infected with a pathogenic strain of bacteria can spread the infection to other mice.

Answer: (A)

Here’s what students ask on this topic:

What was the main conclusion of the Griffith experiment?

The main conclusion of the Griffith experiment was that bacteria can undergo transformation, meaning they can uptake external DNA from their environment. This process leads to changes in their genotype and phenotype. Griffith demonstrated this by combining non-lethal rough bacteria (R strain) with heat-killed lethal smooth bacteria (S strain), resulting in the R strain transforming into a lethal form. This pivotal finding indicated that some genetic material, later identified as DNA, was responsible for this transformation.

How did Griffith's experiment demonstrate the concept of transformation in bacteria?

Griffith's experiment demonstrated the concept of transformation in bacteria by showing that non-lethal rough bacteria (R strain) could uptake genetic material from heat-killed lethal smooth bacteria (S strain) and become lethal. He injected mice with different combinations of these bacteria. When he combined the R strain with heat-killed S strain, the mice died, and he was able to isolate living S strain from the dead mice. This indicated that the R strain had transformed by uptaking genetic material from the heat-killed S strain.

What types of bacteria did Griffith use in his experiment?

Griffith used three types of bacteria in his experiment: the smooth bacteria (S strain), which is lethal due to its capsule; the rough bacteria (R strain), which is non-lethal due to its rough surface; and the heat-killed S strain, which is the lethal S strain that has been exposed to heat to kill it, rendering it non-lethal. These different strains were used to demonstrate the process of bacterial transformation.

Why was the discovery of DNA as the genetic material significant?

The discovery of DNA as the genetic material was significant because it fundamentally changed our understanding of genetics and heredity. Before this discovery, proteins were considered the likely candidates for genetic material due to their complexity. Griffith's experiment, followed by the work of Oswald Avery and others, provided strong evidence that DNA, not protein, is responsible for carrying genetic information. This laid the foundation for modern molecular biology and the study of DNA's role in inheritance, gene expression, and cellular function.

Who confirmed that DNA is the genetic material after Griffith's experiment?

After Griffith's experiment, the scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty confirmed that DNA is the genetic material. They conducted further experiments that identified the transforming substance in Griffith's experiment as DNA. Despite initial skepticism from the scientific community, their work provided crucial evidence that DNA, not proteins, is the genetic material responsible for heredity and transformation in bacteria.