Griffith Experiment: Search of Genetic Material

The Griffith experiment was conducted in the year 1928 by an English bacteriologist named Frederick Griffith.

• Frederick Griffith conducted this experiment to confirm if the transformation of genetic information is possible through bacteria.
• In 1928, Frederick Griffith conducted a series of experiments with Streptococcus pneumonia bacteria and mice.
• He concluded that the R-strain bacteria had acquired a "transforming principle" from the heat-killed S bacteria.
• This allows them to "transform" into smooth-coated bacteria, which are virulent.

Key Terms: Griffith experiment, Frederick Griffith, Biomolecule, Transformation, Strains, S and R strain, Genetic material, Bacteria,  Pneumonia, DNA

---

Who was Frederick Griffith?

[Click Here for Sample Questions]

The "Griffith's Experiment," conducted by English bacteriologist Frederick Griffith in 1928, revealed the transformation of a non-pathogenic pneumococcal bacteria into a virulent strain.

• Griffith combined live non-virulent bacteria with a heat-inactivated virulent strain in this experiment.
• He was the first person to discover the "transforming principle," which led to the identification of DNA as a carrier of genetic information.
• He proposed that bacteria can convey genetic information through a process known as transformation.
• Griffith's purpose was not to find the genetic material but to develop a vaccination for pneumonia.
• Griffith conducted experiments with two related strains of bacteria known as R and S.

Also Read:

---

R Strain And S Strain Bacteria

[Click Here for Sample Questions]

Streptococcus pneumonia has different types or strains. Griffith selected two distinct strains for his experiment. 

• Smooth strain or S strain: It is a type of bacteria having smooth surfaces.
• Rough strain or R strain: It is a type of bacteria having rough surfaces.

Bacteria of the S strain have smooth surfaces because they develop a polysaccharide protective coating on the outermost layer.

• Apart from the physical changes, Griffith observed another important difference between the S and R strains of bacteria.
• The S strain is the "virulent" strain capable of causing death in mice.
• Whereas the R strain is the "nonvirulent" variant that does not cause death in mice.
• Griffith found that when he injected the bacteria into mice, those infected with the virulent S strain died of pneumonia.
• But those infected with the nonvirulent R strain survived.

---

Griffith’s Transformation Experiment

[Click Here for Sample Questions]

[Click Here for Sample Questions]

Griffith was investigating the possibilities of producing a pneumonia vaccine. He used two types of pneumococcus (Streptococcus pneumonia) bacteria that infect mice.

• A virulent (producing disease) Smooth strain (S strain)
• A non-virulent Rough strain (R strain).

Experiment

The S strain created a polysaccharide capsule that shielded itself from the immune system of the host, causing the host to die, but the R strain lacked that protective capsule and was destroyed by the immune system of the host.

• As a part of the research, Griffith attempted to inject mice with heat-killed S bacteria.
• S bacteria that were heated to high temperatures, causing the cells to die, are heat-killed bacteria.
• The heat-killed S bacteria, as expected, didn't cause disease in the mouse.
• When harmless R bacteria were mixed with harmless heat-killed S bacteria and put into a mouse, the experiment took an unexpected turn. 
• Not only did the mouse get pneumonia and die, but Griffith found live S bacteria in a blood sample from the dead mouse.

He determined that a factor or biomolecule from the heat-killed S bacteria had entered the live R bacteria, causing them to synthesize a polysaccharide coating and become virulent. As a result, this factor "transformed" R bacteria into S bacteria. 

• Griffith referred to this factor as the "transforming principle".
• He concluded that it transported genetic material from the S bacteria to the R bacteria. 
• The process is currently known as bacterial transformation, and it is used in a wide range of important genetic engineering applications.

Griffith Experiment

---

Impact of The Griffith Experiment

[Click Here for Sample Questions]

One of the properties of hereditary material is that it changes phenotype. Griffith described the phenotypic-changing factor as the transforming principle.

• His work on the transforming principle got the most attention.
• However only after a group of Canadian and American scientists began investigating the chemical nature of the principle in Oswald Avery's laboratory. 
• Following extensive research, Avery's group decided that deoxyribonucleic acid was the molecule identified by Griffith as the transforming principle.
• Since scientists considered protein molecules to be genetic material, this discovery is far-fetched.
• It is now recognized that deoxyribonucleic acid, or DNA, encodes all cell functions and transmits genetic information between parents and offspring.
• However, DNA was viewed as a less suitable option for genetic material in the 1940s.
• Research on Griffith's work by Avery and others offered the first conclusive proof that DNA could be the genetic material.

---

Conclusion of Griffith Experiment

[Click Here for Sample Questions]

Griffith's main objective was to find a pneumonia cure. Griffith injected several types of pneumococci into mice in order to observe if the bacteria would spread and finally kill the mice.

• Griffith came to the conclusion that heat-killed virulent bacteria transformed living, non-virulent bacteria into virulent bacteria.
• The two strains of Streptococcus pneumonia that he used for his experiment are distinct from one another.
• Because they have a polysaccharide coat on them. 
• The Journal of Hygiene published Griffith's research.
• It was his experiments with mice that led to his major discovery of bacterial transformation in 1928.
• Through Griffith's experiment, it was discovered that bacteria can transfer genetic information.

---

DNA as Genetic Material

[Click Here for Sample Questions]

The Griffith experiment was a significant advancement in the search for genetic material. However, it was unable to clarify how genetic material's biochemistry works.

• To find out the biochemical nature of hereditary material, a group of scientists, Oswald Avery, Colin MacLeod, and Maclyn McCarty, continued the Griffith experiment.
• After their discovery, the concept that proteins are genetic material was changed to DNA as genetic material.
• Avery and his team purified proteins, DNA, RNA, and other biomolecules from the heat-killed S-strain bacteria.
• Apart from this, they discovered that DNA is the genetic material and it is alone responsible for the transformation of the R-strain bacteria.
• They also observed that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) didn’t inhibit transformation but DNase did.
• Although it was not accepted by all, they concluded DNA as genetic material.

---

Things to Remember

• Frederick Griffith who was an English bacteriologist by profession conducted the Griffith experiment in the year 1928.
• He conducted the Griffith experiment on two strains of bacterium named Diplococcus or Streptococcus pneumonia or Pneumococcus.
• The two strains on which he conducted the Griffith experiment are Smooth(S) or Capsulated type and Rough (R) or Non-capsulated type strain.
• The Griffith experiment was performed on mice.
• Frederick Griffith was not able to identify the biochemical nature of the genetic material from his experiment.
• Later, a group of scientists named Oswald Avery, Colin McLeod, and Maclyn McCarty repeated the Griffith experiment.
• They concluded that the biochemical nature of the genetic material is DNA.

Also Read: