The Griffith Experiment: Videos & Practice Problems

Frederick Griffith's 1928 experiment revealed that bacteria can undergo transformation, where they uptake external DNA, leading to genotypic and phenotypic changes. He demonstrated this by injecting mice with lethal smooth (S strain) and nonlethal rough (R strain) bacteria. Surprisingly, when R strain was combined with heat-killed S strain, the mice died, and living S strain was recovered. This indicated that the R strain had transformed by acquiring genetic material from the dead S strain, later identified as DNA, establishing DNA as the genetic material, despite initial skepticism favoring proteins.

The Griffith Experiment

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.

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 genetic material from their environment. This was demonstrated when nonlethal rough (R strain) bacteria were combined with heat-killed lethal smooth (S strain) bacteria, resulting in the death of mice and the recovery of living S strain bacteria. This indicated that the R strain had acquired genetic material from the dead S strain, leading to a genotypic and phenotypic change. This experiment laid the groundwork for identifying DNA as the genetic material.

How did Griffith's experiment demonstrate the process of bacterial transformation?

Griffith's experiment demonstrated bacterial transformation by showing that nonlethal rough (R strain) bacteria could acquire genetic material from heat-killed lethal smooth (S strain) bacteria. When he injected mice with a combination of R strain and heat-killed S strain, the mice died, and living S strain bacteria were recovered from the dead mice. This indicated that the R strain had taken up genetic material from the dead S strain, transforming into the lethal S strain. This process of transformation involved the uptake of external DNA, leading to a change in the bacteria's genotype and phenotype.

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

The discovery of DNA as the genetic material was significant because it identified the molecule responsible for heredity and the transmission of genetic information. Before this discovery, proteins were considered the primary candidates for genetic material due to their complexity and diversity. Griffith's experiment, followed by the work of Avery, McCarty, and MacLeod, provided strong evidence that DNA, not proteins, carried genetic information. This breakthrough laid the foundation for modern genetics and molecular biology, leading to a deeper understanding of how genetic information is stored, replicated, and expressed in living organisms.

What were the key components used in Griffith's experiment?

The key components used in Griffith's experiment were two types of bacteria: the lethal smooth (S strain) and the nonlethal rough (R strain). Additionally, he used heat-killed S strain bacteria. The S strain had a smooth surface due to a protective capsule, making it lethal, while the R strain had a rough surface and was nonlethal. By injecting these bacteria into mice in various combinations, Griffith demonstrated that the R strain could transform by acquiring genetic material from the heat-killed S strain, leading to the death of the mice and the recovery of living S strain bacteria.

How did Griffith's experiment influence subsequent research in genetics?

Griffith's experiment significantly influenced subsequent research in genetics by providing the first evidence of bacterial transformation and suggesting that genetic material could be transferred between organisms. This finding prompted further investigations into the nature of the genetic material. The work of Avery, McCarty, and MacLeod built on Griffith's findings, identifying DNA as the transforming substance. This discovery shifted the focus of genetic research towards understanding DNA's structure and function, ultimately leading to the elucidation of the double helix structure by Watson and Crick and the development of molecular biology as a field.