frederick griffith experiment notes

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DNA Experiments (Griffith & Avery, McCarty, MacLeod & Hershey, Chase)

DNA, deoxyribonucleic acid, is the carrier of all genetic information. It codes genetic information passed on from one generation to another and determines individual attributes like eye color, facial features, etc. Although DNA was first isolated in 1869 by a Swiss scientist, Friedrich Miescher, from nuclei of pus-rich white blood cells (which he called nuclein ), its role in the inheritance of traits wasn’t realized until 1943. Miescher thought that the nuclein, which was slightly acidic and contained a high percentage of phosphorus, lacked the variability to account for its hereditary significance for diversity among organisms. Most of the scientists of his period were convinced by the idea that proteins could be promising candidates for heredity as they were abundant, diverse, and complex molecules, while DNA was supposed to be a boring, repetitive polymer. This notion was put forward as the scientists were aware that genetic information was contained within organic molecules.

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Griffith’s Transformation Experiment

In 1928, a young scientist Frederick Griffith discovered the transforming principle. In 1918, millions of people were killed by the terrible Spanish influenza epidemic, and pneumococcal infections were a common cause of death among influenza-infected patients. This triggered him to study the bacteria Streptococcus pneumoniae and work on designing a vaccine against it . It became evident that bacterial pneumonia was caused by multiple strains of S. pneumoniae, and patients developed antibodies against the particular strain with which they were infected. Hence, serum samples and bacterial isolates used in experiments helped to identify DNA as the hereditary material. 

He used two related strains of S. pneumoniae and mice and conducted a series of experiments using them. 

• When type II R-strain bacteria were grown on a culture plate, they produced rough colonies. They were non-virulent as they lacked an outer polysaccharide coat. Thus, when RII strain bacteria were injected into a mouse, they did not cause any disease and survived.
• When type I S-strain bacteria were grown on a culture plate, they produced smooth, glistening, and white colonies. The smooth appearance was apparent due to a polysaccharide coat around them that provided resistance to the host’s immune system. It was virulent and thus, when injected into a mouse, resulted in pneumonia and death. 
• In 1929, Griffith experimented by injecting mice with heat-killed SI strain (i.e., SI strain bacteria exposed to high temperature ensuing their death). But, this failed to harm the mice, and they survived.
• Surprisingly, when he mixed heat-treated SI cells with live RII cells and injected the mixture into the mice, the mice died because of pneumonia. Additionally, when he collected a blood sample from the dead mouse, he found that sample to contain live S-strain bacteria.

Conclusion of Griffith’s Transformation Experiment

Based on the above results, he inferred that something must have been transferred from the heat-treated S strain into non-virulent R strain bacteria that transformed them into smooth coated and virulent bacteria. Thus, the material was referred to as the transforming principle.

Following this, he continued with his research through the 1930s, although he couldn’t make much progress. In 1941, he was hit by a German bomb, and he died.

Avery, McCarty, and MacLeod Experiment

During World War II, in 1943, Oswald Avery, Maclyn McCarty, and Colin MacLeod working at Rockefeller University in New York, dedicated themselves to continuing the work of Griffith in order to determine the biochemical nature of Griffith’s transforming principle in an in vitro system. They used the phenotype of S. pneumoniae cells expressed on blood agar in order to figure out whether transformation had taken place or not, rather than working with mice. The transforming principle was partially purified from the cell extract (i.e., cell-free extract of heat-killed type III S cells) to determine which macromolecule of S cell transformed type II R-strain into the type III S-strain. They demonstrated DNA to be that particular transforming principle.

• Initially, type III S cells were heat-killed, and lipids and carbohydrates were removed from the solution.
• Secondly, they treated heat-killed S cells with digestive enzymes such as RNases and proteases to degrade RNA and proteins. Subsequently, they also treated it with DNases to digest DNA, each added separately in different tubes.
• Eventually, they introduced living type IIR cells mixed with heat-killed IIIS cells onto the culture medium containing antibodies for IIR cells. Antibodies for IIR cells were used to inactivate some IIR cells such that their number doesn’t exceed the count of IIIS cells. that help to provide the distinct phenotypic differences in culture media that contained transformed S strain bacteria.

Observation of Avery, McCarty, and MacLeod Experiment

The culture treated with DNase did not yield transformed type III S strain bacteria which indicated that DNA was the hereditary material responsible for transformation. 

Conclusion of Avery, McCarty, and MacLeod Experiment

DNA was found to be the genetic material that was being transferred between cells, not proteins.

Hershey and Chase Experiment

Although Avery and his fellows found that DNA was the hereditary material, the scientists were reluctant to accept the finding. But, not that long afterward, eight years after in 1952, Alfred Hershey and Martha Chase concluded that DNA is the genetic material. Their experimental tool was bacteriophages-viruses that attack bacteria which specifically involved the infection of Escherichia coli with T2 bacteriophage.

T2 virus depends on the host body for its reproduction process. When they find bacteria as a host cell, they adhere to its surface and inject its genetic material into the bacteria. The injected hereditary material hijacks the host’s machinery such that a large number of viral particles are released from them. T2 phage consists of only proteins (on the outer protein coat) and DNA (core) that could be potential genetic material to instruct E. coli to develop its progeny. They experimented to determine whether protein or DNA from the virus entered into the bacteria.

• Bacteriophage was allowed to grow on two of the medium: one containing a radioactive isotope of phosphorus( 32 P) and the other containing a radioactive isotope of sulfur ( 35 S).
• Phages grown on radioactive phosphorus( 32 P) contained radioactive P labeled DNA (not radioactive protein) as DNA contains phosphorus but not sulfur.
• Similarly, the viruses grown in the medium containing radioactive sulfur ( 35 S) contained radioactive 35 S labeled protein (but not radioactive DNA) because sulfur is found in many proteins but is absent from DNA.
• E. coli were introduced to be infected by the radioactive phages.
• After the progression of infection, the blender was used to remove the remains of phage and phage parts from the outside of the bacteria, followed by centrifugation in order to separate the bacteria from the phage debris.
• Centrifugation results in the settling down of heavier particles like bacteria in the form of pellet while those light particles such as medium, phage, and phage parts, etc., float near the top of the tube, called supernatant.

Observation of Hershey and Chase Experiment

On measuring radioactivity in the pellet and supernatant in both media, 32 P was found in large amount in the pellet while 35 S in the supernatant that is pellet contained radioactively P labeled infected bacterial cells and supernatant was enriched with radioactively S labeled phage and phage parts.

Conclusion of Hershey and Chase Experiment

Hershey and Chase deduced that it was DNA, not protein which got injected into host cells, and thus, DNA is the hereditary material that is passed from virus to bacteria.

• Fry, M. (2016). Landmark Experiments in Molecular Biology. Academic Press.
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• https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-discovery-and-structure/a/classic-experiments-dna-as-the-genetic-material

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Discovery of DNA as the Hereditary Material using Streptococcus pneumoniae

No one could have predicted that experiments designed to understand bacterial pneumonia would lead to the discovery of DNA as the hereditary material. In the early part of the twentieth century, before the advent of antibiotics, pneumococcal infections claimed many more lives than they do today. Researchers on both sides of the Atlantic were thus actively engaged in studying Streptococcus pneumoniae , the bacterium responsible for clinical infections. Early on, it became apparent that multiple strains of S. pneumoniae were responsible for causing bacterial pnuemonia. Researchers also noted that patients developed antibodies to the particular strain, or serotype, with which they were infected, but these antisera were not universally reactive against pneumococcal strains. However, the bacterial isolates and serum samples from these clinical studies provided the critical reagents for the experiments that ultimately led to the identification of DNA as the hereditary material.

Pneumococcal Research Provides Critical Tools in DNA Research

Although numerous scientists engaged in pneumococcal research during the first half of the twentieth century, two of these researchers played an especially important role in the course of events that led to the discovery of DNA as the hereditary material . One of these individuals was Oswald Avery . Avery joined the Rockefeller Institute for Medical Research, now the Rockefeller University, in 1913 as part of a team seeking to develop a therapeutic serum for treating lobular pneumonia. Avery believed that knowledge of the chemical composition of the pneumococcus bacterium was essential for understanding and treating the disease . He perfected his biochemical technique by focusing on the chemical composition of the capsule that surrounded virulent S strains of pneumococci. In his early work, Avery helped establish that polysaccharides were a major component of the pneumococcal capsule and that capsules from different serotypes of pneumococci had distinctive polysaccharide compositions. Avery also concerned himself with the role of capsules in pathogenicity, as capsules were notably absent from the surface of nonvirulent R forms of streptococci. Defying the conventional wisdom of the time, Avery hypothesized that the polysaccharides in the capsules were the actual antigens stimulating the production of antibodies in infected patients (Avery & Goebel, 1933).

Purification of the Active Transforming Principle

When Avery first became aware of Griffith's results, he treated them with skepticism. Other researchers and laboratories, however, were quick to reproduce and build upon Griffith's data. Within a few years, Sia and Dawson (1931) showed that transformation could be carried out in liquid cultures of pneumococci as well as in mice, allowing more precise control of environmental variables in transformation experiments. In 1932, Alloway further demonstrated that the active transforming principle was present in sterile, cell-free extracts prepared from heat-treated pneumococci by filtration. These additional findings convinced Avery that the transforming principle could be identified, and he applied his considerable biochemical expertise to its purification from pneumococcal extracts (Avery et al. , 1944).

A critical aspect of any biochemical purification is the development of an assay, or a way to measure the activity of interest. For their experiments, Avery and his colleagues developed conditions under which R cells could be reliably transformed into S cells using extracts of heat-killed Type III S cells. These same conditions could then be used to measure transforming activity in fractions obtained at different steps in the purification process. To quantify the actual amount of transforming principle in a fraction, each sample was tested at a series of increasing dilutions. These data represent four identical experiments in which Avery and his colleagues tested the ability of the purified factor, designated preparation 44, to transform Type II R cells into Type III S cells. The transforming activity was very concentrated in the extract, since it could be diluted ten-thousand-fold without losing its transforming ability. Fractions that maintained transforming activity at the highest dilutions were deemed to possess the highest concentration of transforming activity. Specifically, when at least 0.01μg of the extract was added to cells, transformation was observed. When any less than 0.01μg was added, the transformation was inconsistent (comparing samples 1 and 3 with 2 and 4) (Figure 2).

Physical Characterization of the Transforming Principle

Avery and his colleagues submitted the purified transforming principle to rigorous physical characterization in order to demonstrate that it possessed the properties expected of DNA (Avery et al. , 1944). The elemental composition of the purified transforming compound was close to the theoretical values for DNA (last row, sodium desoxyribonucleate) (Figure 3). Significantly, the purified principle had a high phosphorous content, which is characteristic of DNA, but not of proteins.

Consistent with these results, the factor gave positive reactions in chemical tests for DNA, but negative or weakly positive reactions in tests for proteins and RNA . Other tests indicated that the transforming principle was a very large molecule that absorbed the same spectrum of ultraviolet light as DNA. However, the most definitive proof that the transforming principle was DNA was its sensitivity to specific enzymes, called DNAses, that specifically degrade different kinds of DNA. Avery and his colleagues were able to show that transforming activity was not destroyed by enzymes that degrade proteins or RNA. At the time, Avery could not obtain samples of pure DNAse. Instead, Avery and his colleagues used crude preparations from animal tissues that were known to contain DNAse activity. They then measured the ability of these various crude preparations to destroy the transforming principle in parallel with measurements of phosphatase, esterase, and DNAse activities in the same extracts. In all cases, the ability of the crude extracts to destroy the transforming principle was proportional to their DNAse activity, measured with pure calf thymus DNA as substrate (Figure 4).

DNA Has the Properties Expected of Genes

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Griffith’s Experiment

Table of Contents

Introduction:

• Experiment conducted by Frederick Griffith in 1928
• First experiment to demonstrate genetic material transfer between bacteria
• Laid foundation for discovery of DNA as genetic material and paved way for molecular biology

The Experiment:

• Involved two strains of Streptococcus pneumoniae bacteria: virulent (S) and non-virulent (R)
• Virulent strain caused pneumonia and had a smooth appearance due to polysaccharide capsule
• Non-virulent strain had a rough appearance

Steps of Experiment:

• Mice were injected with virulent strain and they died
• Mice were injected with a mixture of virulent and non-virulent strains and they did not die
• The non-virulent strain had transformed into the virulent strain
• Autopsy showed that the non-virulent bacteria had taken up the virulence factor (polysaccharide capsule) from the dead virulent bacteria
• Further experiments showed that a substance from the dead virulent bacteria (now called the transforming principle) was responsible for the transformation
• The transforming principle was later identified as DNA.

Conclusion:

• The experiment demonstrated that genetic material (DNA) could be transferred from one organism to another
• The experiment was a key step in the discovery of DNA as the genetic material
• The experiment paved the way for the development of molecular biology and genetic engineering.

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Frederick Griffith’s Experiment and the Concept of Transformation

Transformation is a molecular biology mechanism via which foreign and exogenous genetic material is taken up by a cell and incorporated into its own genome. This phenomenon was first described and discovered by British bacteriologist, Frederick Griffith. The concept of transformation and the experiment that led to its discovery are described here.

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Did You Know?

Other processes by which exogenous genetic material is taken up by a cell include conjugation (transfer of DNA between two bacterial cells that are in direct contact) and transduction (injection of viral DNA by a bacteriophage into the host bacterial cell).

The post-World War I Spanish influenza pandemic influenced Frederick Griffith to study the epidemiology and pathology of bacterial pneumonia in order to attempt creating a successful vaccine. Hence, he carried out experiments, where he injected mice with strains of virulent and avirulent Streptococcus pneumoniae. The experiment he reported in 1928, gave the first description of the phenomenon of transformation, where one bacterial strain could change into the other strain, and this activity was linked to an unidentified element called the transforming factor or transforming principle.

Oswald T. Avery, an American pneumococcal researcher, speculated that Griffith’s experiment lacked appropriate control. However, subsequent, similar experiments carried out in Avery’s laboratory confirmed Griffith’s discovery. The experiments conducted later by Avery, MacLeod, and McCarty, and by Hershey and Chase proved that the transforming factor was DNA and elucidated its exact nature. Thereby, establishing the central role of DNA in inheritance.

Frederick Griffith’s Experiment

For his experiments, Griffith used two strains of Streptococcus pneumoniae that affected mice – type III S (smooth) and type II R (rough). The type III S form has a smooth appearance due to the presence of a polysaccharide layering over the peptidoglycan cell wall of the bacterial cell. This extra coating helps the cell in evading the phagocytosis carried out by the immune cells of the host; hence, allowing the strain to proliferate and become virulent.

In contrast, the type II R form lacks this coating, and hence, has a rough appearance. The absence of the polysaccharide layer leads to its efficient elimination by the host’s immune cells rendering the strain avirulent. While injecting the mice with these bacteria, Griffith devised four sets of inoculation that are as follows:

Type III S bacteria When the mice were inoculated, the bacterial virulence was exhibited, causing pneumonia, and this eventually led to the death of the mice. On examining the blood of the deceased mice, progeny of the inoculated cells were obtained.

Type II R bacteria When injected into the mice, the bacterial cells were successfully eliminated by the immune system, and hence, the mice lived. The blood showed no presence of the inoculated cells.

Type III S heat-killed bacteria When the virulent strain was rendered avirulent by heating and killing it (heat-killed), and then injected into the mice, the strain did not show virulence, and was eliminated by the host’s immune system; hence, the mice survived. Their blood showed no presence of the inoculated cells.

Type II R bacteria + Type III S heat-killed bacteria Injecting the mice with a combination of equal number of cells of type II R strain and heat-killed type III S strain, caused pneumonia which progressed till the mice died. The bacterial cells isolated from the blood of these mice showed the presence of live type III S bacterial cells.

This indicated that the live R strain had assimilated and incorporated the virulent element from the heat-killed S strain in order to transform itself into the virulent S strain. Based on this observation, Griffith concluded that a transforming element from the heat-killed strain was accountable for the transformation of the avirulent strain into the virulent strain. Successive experiments carried out in 1944 by Oswald Avery, Colin MacLeod, and Maclyn McCarty, proved that the element taken up by the harmless strain was genetic in nature.

Concept of Transformation

Transformation is a stable genetic change brought about by the uptake of naked DNA, and the state of being able to take up exogenous DNA is called competence. They occur in two forms―natural and artificial.

Natural Transformation

Only 1% of the bacterial species is capable of taking up DNA. Bacteria also exchange genetic material through a process called horizontal gene transfer, where bacterial cells conjugate and form a bridge via which the genetic material is transferred from one cell to another. A few bacterial species also release their DNA via exocytosis on their death, and this DNA is later taken up by the bacterial cells present in the vicinity. While transformation can occur between various bacterial species, it is most efficient when occurring between closely related species. These cells possess specific genes that code for natural competence allowing them to transport the DNA across the cellular membrane and into the cell. This transport involves the proteins associated with the type IV pili and type II secretion system as well as the DNA translocase complex at the cytoplasmic membrane.

This mechanism differs slightly due to the difference in the structure of the cell membranes of the bacteria. Bacteria are broadly classified into two types based on this difference – Gram-negative and Gram-positive. The general outline is more or less similar. The presence of exogenous DNA is detected by the cell and natural competence is induced, then the foreign DNA binds to a DNA receptor on the surface of these competent cells. This receptor binding allows the activation of the DNA translocase system that allows the passing of DNA into the cell via the cell membrane. During this process, one strand of the DNA is degraded by the action of nucleases. The translocated single strand is then incorporated into the bacterial genome via the help of a RecA-dependent process.

Gram-negative bacteria show the presence of an extra membrane, hence, for DNA to be taken up, a channel is formed on the outer membrane by secretins. The uptake of a DNA fragment is generally not specific to its sequence; however, in some bacterial species, it has been seen that the presence of certain DNA sequences facilitate and enhance efficient uptake of the genetic material.

Artificial Transformation

It is carried out in laboratories in order to carry out gene expression studies. To impart competence, the cells are incubated in a solution containing divalent cations (calcium chloride) under cold conditions, and then, exposed to intermittent pulses of heat. The concentration of the solution depends on the protein and liposaccharide content of the membrane, and the intensity of the heat pulses varies according to the time duration of the pulses, i.e., high intensity pulses should be for very short periods; whereas, low intensity pulses can be for longer durations.

The divalent cations function to weaken the molecular structure of the cell membrane, hence, making it more permeable. The subsequent heat pulses cause the creation of a thermal imbalance, and in the process of regaining balance, the DNA molecules gain entry via the weakened membrane and into the cell.

Artificial competence can be alternatively induced and promoted via the use of a technique called electroporation. It involves applying an electric current to the cell suspension. This causes the formation of pores in the cell membrane. The exogenous DNA is taken up via these holes, which are resealed via the cell membrane repair machinery.

Saccharomyces cerevisiae can be transformed by exogenous DNA using various methods. Yeast cells are treated with certain digesting enzymes that degrade the cell walls. This yields naked cells (devoid of cell wall) called spheroplasts. They are extremely fragile but have a high frequency of foreign DNA uptake.

Another method that can be used is, exposing the cells to alkaline cations such as lithium (from lithium acetate) and PEG. The PEG helps in pore formation, and the cations in the transport of the DNA fragment inside the cell.

The process of electroporation can also be used for transformation purposes, and efficiency can be enhanced using enzymatic digestion or agitation using glass beads.

The most common method of transforming plant cells is the Agrobacterium mediated transfer. In this method, the tissue of cells to be transformed is cut up into small uniform pieces, and then, treated with a suspension containing Agrobacterium. The foreign DNA gains entry via the cuts on the tissue, and the wound healing compounds secreted from the cuts, activate the virulence operon of the Agrobacterium. his causes the Agrobacterium. to infect the tissue and carry out its normal action of tumor induction. This function allows the transformed plant cells to proliferate. The cells are grown on a selective media till the transformed cells grow into plantlets with shoots and roots. They are then planted in soil and allowed to grow naturally.

Plant cells can also be transformed using viral particles (transduction). Here, the genetic material to be inserted is packaged into a suitable plant virus. This modified virus is then allowed to infect the plant cells. The transfer occurs according to the viral machinery and transformation is achieved. Electroporation can also be used for plant cells.

Introduction of foreign DNA into animal cells is conducted using viral or chemical agents like the ones used in case of plant and bacterial cells. However, since the term transformation is also used to refer to the progression of cancerous growth in animals, here, the term transfection is used.

This concept and technique has seen varied applications in the field of molecular biology with respect to expression studies, gene knockout studies, and cloning experiments. It is also used in the production of genetically modified organisms.

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History of DNA Discovery (6): Frederick Griffith's Experiment, Transforming Principle

Anec  >  Biology  >  Genetic material

Frederick Griffith's transformation experiment about bacteria is a turning point in the exploration path of DNA and genetic material. The transforming principle gradually redirected scientists from the erroneous and confused direction (Tetranucleotide hypothesis) back onto the right path.

Griffith’s Experiment

During the Spanish flu pandemic, pneumonia was one of the leading reasons of death. There are two strains of bacteria responsible for pneumonia: the rough (R) strain, which lacks a polysaccharide capsule on its cell wall, and the smooth (S) strain, which has smooth surface due to the polysaccharide capsule. The polysaccharide capsule itself is non-toxic and non-pathogenic, but it enhances its resistance to white cells. The virulent S bacteria bypass the immune system and proliferate massively within the body . R bacteria, on the other hand, are easily defeated by immune cells and are not lethal.

At this time, the British public health officer Frederick Griffith was engaged in research on infectious diseases and the classification of bacteria. He isolated S and R bacteria from the sputum of pneumonia patients. He found that injecting mice with heat-killed S bacteria or R bacteria alone did not lead to death. To his surprise, when he injected mice with R bacteria and heat-killed S bacteria at the same time, the mice not only became infected with pneumonia and died, but also had a large amount of S bacteria isolated from their blood. Before this, biologists believed that bacterial inherited traits (genes) could only be vertically inherited from parents to offspring, and it was not believed that inherited traits (genes) could be transferred horizontally between bacteria.

Frederick Griffith believed that the R bacteria must have obtained some substances from the dead S bacteria, known as the "transforming principle," making them to produce a polysaccharide capsule. Although he discovered the bacteria transformation, he did not delve into what the "transforming principle" was, thus missing the opportunity to discover genetic material and pioneer a brand new field.

The Significance of the Frederick Griffith transformation Experiment

He is remembered not for his contributions to pathogens and infectious diseases but for the Griffith transformation experiment. It points the way for the exploration of genetic material, that is, to identify what the "transforming principle" is. The Griffith experiment directly led to Avery and his team's exploration of the "transforming principle" which was actually nucleic acid.

Frequently Asked Questions

Why does hot water kill bacteria but not denature their DNA?

The three-dimensional structure of bacterial proteins collapses, and proteins lose activity above 60°C. The denature of proteins is usually irreversible. However, DNA is more stable than proteins. Even in boiling water, broken hydrogen bonds only disintegrate the double helix into single strands. When the solution cools, the single strands recombine into a double helix.

Why didn't Frederick Griffith discover the chemical nature of the "transforming principle"?

In Griffith's era, the nucleic acid structure was still dominated by Phoebus Levene's tetranucleotide hypothesis. Most people, including Griffith, scoffed at the idea that nucleic acid was genetic material. They preferred to believe that information about hereditary traits was stored in proteins. Griffith, a public health officer responsible for infectious diseases, naturally used the transforming principle to explain the outbreak of pneumonia in communities.

How do you explain bacteria transformation in Frederick Griffith experiment, from a modern biological viewpoint?

Heat-killed S bacteria left behind various DNA fragments, including genes controlling polysaccharide capsule. These DNA fragments were released from the S bacteria and were taken up by some R bacteria. Then these DNA fragments were integrated into R bacterial genome. Now, R bacteria acquire the ability to produce a polysaccharide protective coating.

Prev:   History of DNA Discovery (3): Albrecht Kossel, Nucleic Acid, Purine, Pyrimidine

Next:   History of DNA Discovery (7): Oswald Avery proved DNA was Genetic Material

Lessons and Courses on Microbiology

FREDERICK GRIFFITH (1879-1941)

Frederick Griffith was a British bacteriologist who performed transformation experiments that suggested that DNA was the hereditary material. His focus was on the epidemiology and pathology of bacterial pneumonia – which led him to develop the principle of bacterial transformation – which allow scientist to transform a bacterium through the introduction of exogenous DNA carrying the gene of interest.

Frederick showed that streptococcus pneumonia could transform from one strain into a different strain, an effect he attributed to an unidentified transforming principle or transforming factor . In 1928, Frederick reported what is now known as Griffith’s Experiment , which was the first widely accepted demonstration of bacterial transformation.

Frederick’s student, Avery Oswald , later demonstrated this principle of bacterial transformation conclusively. Frederick Griffith’s work established the foundation of molecular genetics, a field which is thriving today in the biomedical, biological and medical sciences.

His work showed that DNA is the genetic material.

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Nester E.W, Anderson D.G, Roberts C.E and Nester M.T (2009). Microbiology: A Human Perspective. Sixth edition. McGraw-Hill Companies, Inc, New York, USA.

Salyers A.A and Whitt D.D (2001). Microbiology: diversity, disease, and the environment. Fitzgerald Science Press Inc. Maryland, USA.

Slonczewski J.L, Foster J.W and Gillen K.M (2011). Microbiology: An Evolving Science. Second edition. W.W. Norton and Company, Inc, New York, USA.

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Griffith Experiment

Griffith experiment: an introduction.

It may come as a surprise that less than a century ago, even the most educated members of the scientific community were unaware that DNA was a hereditary material. Frederick Griffith conducted a series of experiments with Streptococcus pneumonia bacteria and mice in 1928 and concluded that the R-strain bacteria must have picked up a "transforming principle" from the heat-killed S bacteria, allowing them to "transform" into smooth-coated bacteria and become virulent.

In this article, we'll look at one of the classic experiments that led to the discovery of DNA as a genetic information carrier.

Who was Frederick Griffith?

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

Griffith combined living non-virulent bacteria with a heat-inactivated virulent form in this experiment.

He was the first to discover the "transforming principle," which led to the discovery of DNA as a carrier of genetic information.

He suggested that bacteria can transfer genetic information via a process known as transformation.

Griffith's goal was not to identify the genetic material but to create a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria known as R and S.

Griffith's work was expanded by Avery, MacLeod, and McCarty.

R Strain And S Strain Bacteria

Streptococcus pneumonia comes in several types or strains. Griffith chose two different strains for his experiment.

One strain of bacteria has smooth surfaces and is known as the smooth strain (S strain), while the other has rough surfaces and is known as the rough strain (R strain).

Bacteria of the S strain have smooth surfaces because they produce a polysaccharide protective coating that forms the outermost layer.

Apart from the morphological differences, Griffith discovered another significant difference between the S and R strains of bacteria, i.e., the S strain is the "virulent" strain capable of causing death in mice, whereas the R strain is the "nonvirulent" strain that will not cause death in mice.

Griffith observed that when he injected these bacteria into mice, the mice infected with the virulent S strain died from pneumonia, whereas the mice infected with the nonvirulent R strain survived.

R Strain and S Strain of Streptococcus Pneumonia

Griffith’s Transformation Experiment

Griffith was researching the possibility of developing a pneumonia vaccine.

He used two strains of pneumococcus (Streptococcus pneumonia) bacteria that infect mice – a virulent (causing disease) S (smooth) strain and a non-virulent type R (rough) strain.

The S strain produced a polysaccharide capsule that protected itself from the host's immune system, resulting in the host's death, whereas the R strain lacked that protective capsule and was defeated by the host's immune system.

Griffith attempted to inject mice with heat-killed S bacteria as a part of his research (i.e., S bacteria that had been heated to high temperatures, causing the cells to die). The heat-killed S bacteria, but unsurprisingly, did not cause disease in the mouse.

When harmless R bacteria were combined with harmless heat-killed S bacteria and injected into a mouse, the experiments took an unexpected turn.

Not only did the mouse develop pneumonia and die, but Griffith discovered living S bacteria in a blood sample taken from the dead mouse.

He concluded that some factor or biomolecule from the heat-killed S bacteria had entered the living R bacteria, allowing them to synthesise a polysaccharide coating and become virulent. As a result, this factor "transformed" the R bacteria into S bacteria.

Griffith called this factor the "transforming principle," concluding that it carried some genetic material from the S bacteria to the R bacteria.

This process is now known as bacterial transformation and is used in a variety of significant genetic engineering applications.

Griffith Experiment Diagram

Impact of The Griffith Experiment

One of the characteristics of hereditary material is a changing phenotype . Griffith referred to the phenotypic-changing factor as the transforming principle.

His work on the transforming principle received the most attention, but only after a group of Canadian and American scientists set out to investigate the chemical nature of the transforming principle in Oswald Avery's laboratory.

Avery's group concluded in their studies that deoxyribonucleic acid was the molecule identified by Griffith as the transforming principle after conducting numerous experiments.

The implications of this discovery are farfetched because it was made at a time when scientists considered protein molecules to be genetic material.

DNA, or deoxyribonucleic acid, is now recognised as the molecule that encodes all cell functions and transmits genetic information from parent to offspring in almost every living species .

In the 1940s, however, DNA was thought to be a less qualified candidate for genetic material. Avery and colleagues' research on Griffith's experiment provided the first solid evidence that DNA could be the genetic material.

Griffith's ultimate goal was to find a way to cure pneumonia. Griffith inoculated mice with various strains of pneumococci to see if they would infect and eventually kill the mice. Griffith concluded that heat-killed virulent bacteria transformed living, non-virulent bacteria into virulent bacteria. He performed his experiment on the two strains of Streptococcus pneumonia, which differ from each other due to the presence of a polysaccharide coat.

Griffith's findings were published in the Journal of Hygiene. In 1928, his experiments with mice led to his major discovery of bacterial transformation. Griffith's experiment discovered that bacteria can transfer genetic information through transformation.

FAQs on Griffith Experiment

1. Explain the Oswald Avery Experiment.

Avery and his colleagues conducted additional research on the virulent S strain of Streptococcus pneumonia. They were aware that the potential carriers of genetic material were proteins, RNA, or DNA. When the mixtures were treated with protein-digesting or RNA-digesting enzymes, the DNA remained intact and was capable of transforming R bacteria into S bacteria. However, when the DNA in these mixtures was broken down with DNase, the genetic material could not be passed from the heat-killed S bacteria to the live R bacteria, preventing transformation. As a result, Avery and his colleagues concluded that the transforming principle described by Griffith had to be DNA.

2. What are the characteristics of genetic material?

Any substance that forms the genetic material must fulfil some essential requirements:

It must be stable.

It should be able to carry and transcribe information which is required to control the processes.

It should be able to replicate itself and remain unchanged while passing down from one generation to another.

It must be able to mutate itself to provide variations.

A genetic material must be able to store the information, transmit it, replicate it and provide variation.

DNA fulfils all the above-mentioned requirements and hence acts as genetic material.

3. Define Horizontal Gene Transfer.

Horizontal gene transfer (HGT) is the exchange of genetic information between organisms, which includes the spread of antibiotic resistance genes among bacteria (except those passed down from parent to offspring), thereby, fueling pathogen evolution.

Bacterial horizontal gene transfer occurs via three mechanisms: transformation, transduction, and conjugation. Conjugation is the primary mechanism for the spread of antibiotic resistance in bacteria, and it is critical in the evolution of the bacteria that degrade novel compounds such as pesticides created by humans, as well as in the evolution, maintenance, and transmission of virulence.

How Did Scientists Prove That DNA Is Our Genetic Material?

Griffith experiment, avery, macleod and mccarty experiment, hershey and chase experiment.

Three seminal experiments proved, without doubt, that DNA was the genetic material, and not proteins. These experiments were the Griffith experiment, Avery, MacLeod, and McCarthy Experiment, and finally the Hershey-Chase Experiment.

DNA is the fundamental component of our being. The human body is merely the carrier for this genetic material, passing it down from generation to generation. Our purpose is to ensure the survival of the species. Humans are to DNA like a fruit is to a seed. We are just an outer covering to ensure the safe passage and protection of the source code of our existence through time. Makes you feel pretty useless, doesn’t it?

However, that’s not what I want you to focus on. The main focus is, how did we discover that DNA is the carrier of information? How did we determine that it wasn’t something else, like proteins? After all, proteins are also present in every cell.

For a long time this debate had been going on. Even after Gregor Mendel formed the 3 laws of inheritance , it wasn’t accepted by the scientific community for 45 years. The reason? There was no concept of DNA or genes being the information carriers! The whole debate was finally put to rest by 3 main experiments carried out by independent researchers, which formed the basis of all our evolutionary and molecular biology studies.

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The first step was taken by Frederick Griffith in the year 1928. He was a bacteriologist who focused on epidemiology.  Griffith was studying how Streptococcus pneumoniae caused an infection. He was working with 2 strains of the bacteria called the S and R strains. S strain organisms, when cultured in the lab, gave rise to bacterial colonies with a smooth appearance. This was due to a shiny, polysaccharide coat, which is supposed to be their virulence factor. A virulence factor is any quality or factor of a pathogen that helps it in achieving its goal – causing a disease! The other strain was the R strain. This strain gave rise to colonies that didn’t possess the polysaccharide coat, and therefore had a ‘rough’ appearance. Therefore, the S strain was virulent and the R strain was avirulent.

Griffith took 4 mice and injected them with different solutions. The first one was injected with the S strain organisms; the second one was injected with the R strain organisms; the third mouse was injected with heat-killed S strain organisms; and the last one was injected with a mixture of heat-killed S strain and live R strain organisms. The result? The first and fourth mice died due to the infection, while the second and third mice survived. When he extracted the infectious agent from the dead mice, in both cases, he found S strain organisms.

Let’s break it down. The first 2 mice showed that S strain is the virulent strain, while the R strain is avirulent. The third mouse proved that heat-killed S strain organisms cannot cause an infection. Now here is where it gets interesting. The death of the 4 th mouse, and the retrieval of live S strain organisms showed that, somehow, the heat-killed S strain organisms had caused the transformation of live R strain organisms to live S strain organisms.

This was called the transformation experiment… not particularly creative in the naming department.

Also Read: Does Human DNA Change With Time?

While Griffith’s experiment had provided a surprising result, it wasn’t clear as to what component of the dead S strain bacteria were responsible for the transformation. 16 years later, in 1944, Oswald Avery, Colin Macleod and MacLynn McCarty solved this puzzle.

They worked with a batch of heat-killed S strain bacteria. They divided it into 5 batches. In the first batch, they destroyed the polysaccharide coat of the bacteria; in the second batch they destroyed its lipid content; they destroyed the RNA of the bacteria in the third batch; with the fourth batch, they destroyed the proteins; and in the last batch, they destroyed the DNA. Each of these batches was individually mixed with live R strain bacteria and injected into individual mice.

From all 5 mice, all of them died except the last mouse. From all the dead mice, live S strain bacteria was retrieved. This experiment clearly proved that when the DNA of the S strain bacteria were destroyed, they lost the ability to transform the R strain bacteria into live S strain ones. When other components, such as the polysaccharide coat, lipid, RNA or protein were destroyed, transformation still took place. Although the polysaccharide coat was a virulent factor, it wasn’t responsible for the transfer of the genetic matter.

Even after the compelling evidence provided by the Avery, Macleod and McCarty experiment, there were still a few skeptics out there who weren’t convinced. The debate still raged between proteins and DNA. However, the Hershey – Chase experiment permanently put an end to this long-standing debate.

Alfred Hershey and Martha Chase in 1952, performed an experiment that proved, without a doubt, that DNA was the carrier of information. For their experiment, they employed the use of the bacteriophage T2. A bacteriophage is a virus that only infects bacteria. This particular virus infects Escherichia coli . T2 had a simple structure that consisted of just 2 components – an outer protein casing and the inner DNA. Hershey and Chase took 2 different samples of T2. They grew one sample with 32 P, which is the radioactive isotope of phosphorus, and the other sample was grown with 35 S, the radioactive isotope of sulphur!

The protein coat has sulphur and no phosphorus, while the DNA material has phosphorus but no sulphur. Thus, the 2 samples were labelled with 2 different radioactive isotopes.

The viruses were then allowed to infect the E. coli . Once the infection was done, the experimental solution was subjected to blending and centrifugation. The former removed the ghost shells, or empty shells of the virus from the body of the bacteria. The latter separated the bacteria from everything else. The bacterial solution and the supernatant were then checked for their radioactivity .

In the first sample, where 32 P was used, the bacterial solution showed radioactivity, whereas the supernatant barely had any radioactivity. In the sample where 35 S was used, the bacterial solution didn’t show any radioactivity, but the supernatant did.

This experiment clearly showed that DNA was transferred from the phage to the bacteria, thus establishing its place as the fundamental carrier of genetic information.

Until the final experiment performed by Hershey and Chase, DNA was thought to be a rather simple and boring molecule. It wasn’t considered structured enough to perform such a complicated and extremely important function. However, after this experiment, scientists started paying much more attention to DNA, leading us to where we are in research today!

Also Read: A History Of DNA: Who Discovered DNA?

• How was DNA shown to be the genetic material?. The University of Texas at Austin
• The Genetic Material - DNA - CSUN. California State University, Northridge
• Home - Books - NCBI. National Center for Biotechnology Information

Mahak Jalan has a BSc degree in Zoology from Mumbai University in India. She loves animals, books and biology. She has a general assumption that everyone shares her enthusiasm about the human body! An introvert by nature, she finds solace in music and writing.

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Frederick Griffith

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Frederick Griffith (born October 3, 1877, Eccleston, Lancashire, England—died 1941, London) was a British bacteriologist whose 1928 experiment with bacterium was the first to reveal the “transforming principle,” which led to the discovery that DNA acts as the carrier of genetic information.

Griffith studied medicine at the University of Liverpool and later worked at the Pathological Laboratory of the Ministry of Health. He developed a reputation for his thorough and methodical research. In 1928 he conducted an experiment involving two strains of the bacterium Streptococcus pneumoniae ; one strain was lethal to mice (virulent) and the other was harmless (avirulent). Griffith found that mice inoculated with either the heat-killed virulent bacteria or the living avirulent bacteria remained free of infection, but mice inoculated with a mixture of both became infected and died. It seemed as if some chemical “transforming principle” had transferred from the dead virulent cells into the avirulent cells and changed them. Furthermore, the transformation was heritable—i.e., able to be passed on to succeeding generations of bacteria. In 1944 American bacteriologist Oswald Avery and his coworkers found that the transforming substance—the genetic material of the cell—was DNA.

In 1941 Griffith died during a German bombing raid on London .

Frederick Griffith Experiment – Bacterial transformation

Griffith experiment & transforming principle.

Frederick Griffith, a British bacteriologist, conducted a series of studies with Streptococcus pneumoniae bacteria and mice in 1928. Griffith was not attempting to detect genetic material; rather, he was attempting to produce a vaccine for pneumonia. Griffith utilised two strains of bacteria identified as R and S in his tests.

Griffith, as part of his trials, attempted to inject mice with heat-killed S bacterium (that is, S bacteria that had been heated to high temperatures, causing the cells to die). Unsurprisingly, mice were not infected by heat-killed S bacterium.

When innocuous R bacteria were paired with harmless heat-killed S bacteria and injected into a mouse, the research took an unexpected turn. The mouse not only developed pnenumonia and died, but when Griffith took a blood sample from the deceased mouse, he discovered living S bacteria!

Avery, McCarty, and MacLeod: Identifying the transforming principle

In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod, three Canadian and American scholars, set out to find Griffith’s “transforming principle.”

Several lines of evidence suggested to Avery and his colleagues that DNA may be the transformative factor.

All of these results pointed to DNA as the most plausible transformative agent. However, Avery interpreted his data with caution. He concluded that it was still plausible that a small bit of a contaminated chemical, and not DNA, was the actual transforming agent.

DNA as Genetic Material

The Griffith experiment was a pivotal moment in the discovery of genetic material. However, it failed to explain genetic material’s biology. Oswald Avery, Colin MacLeod, and Maclyn McCarty continued the Griffith experiment in quest of the biochemical nature of the genetic material. Their finding replaced the notion of protein as genetic material with that of DNA as genetic material.

Avery and his group isolated and purified proteins, DNA, RNA, and other macromolecules from bacteria of the S strain that had been destroyed by heat. They determined that DNA is the sole genetic material responsible for the transformation of R strain bacteria. They discovered that protein- and RNA-digesting enzymes did not inhibit transformation, but DNase did. Although it was not acknowledged by others, they concluded DNA as genetic material.

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Griffith Experiment – Transformation in Bacteria, DNA as Genetic Material

Griffith’s Experiment in 1928 demonstrated bacterial transformation, where non-virulent bacteria turned virulent upon exposure to heat-killed virulent strains. Avery, MacLeod, and McCarty experiment later confirmed in 1944 that DNA, not proteins, was the genetic material responsible for this transformation. Griffith Experiment in conclusion recognized DNA’s significant role in heredity. In this article, we will study the Frederick Griffith Experiment – steps, strain of bacteria, and Griffith Experiment summary.

Table of Content

Griffith Experiment & Transforming Principle

Griffith experiment diagram, r strain and s strain bacteria.

• Griffith’s Experiment – Transformation in Bacteria

Impact of the Griffith Experiment

Dna as genetic material.

Frederick Griffith conducted an experiment that demonstrated the transfer of genetic information between bacteria. The experiment showed that a heat-killed virulent strain could transform a non-lethal strain of bacteria . Griffith called the material that was transferred the “transforming principle”. Griffith’s experiment involved mixing living non-virulent bacteria with a heat-inactivated virulent form. The bacteria used in the experiment were Streptococcus pneumoniae, which showed two growth patterns. One culture plate had s mooth, shiny colonies (S), while the other had rough colonies (R) .

Griffith’s experiment proved that some organisms can acquire new properties from their environment and from one another. However, it took almost 20 years for Avery, McLeod, and McCarty to confirm that nucleic acids, not proteins , are the molecules of heredity

Also Read : Mendel’s Laws of Inheritance

The diagram of griffith experiment is shown below:

The R strain and S strain bacteria are two variants of the bacterium Streptococcus pneumonia, used by Frederick Griffith in his experiment. S strains are pathogenic, meaning they can cause disease. R strains are non-pathogenic, meaning they do not cause disease. Some other differences between R and S strains are:

• Appearance:   S strains have a smooth capsule , or outer coat, made of polysaccharides. R strains lack a capsule and have a rough appearance.
• Colonies:  S strains produce rough colonies, while R strains produce smooth colonies.
• Virulence:  S strains are virulent, while R strains are non-virulent.
• Immune responses:  The capsule of S strains allows the cell to escape the immune responses of the host mouse.
• Mice:  Mice injected with S strains die within a few days, while mice injected with R strains do not die.

In Griffith’s experiment, when he injected mice with the heat-killed S strain and live R strain , the mice unexpectedly died. This revealed a transformation process where the R strain had taken up genetic material from the heat-killed S strain and become virulent. This observation helped in understanding bacterial transformation and the role of DNA as genetic material.

Also Read: Genetic Code – Molecular Basis of Inheritance

Griffith Experiment of Transformation in Bacteria

In 1928, English bacteriologist Frederick Griffith conducted an experiment that demonstrated how bacteria can change their function and form through transformation. The experiment was the first to suggest that bacteria can transfer genetic information through transformation. The experiment involved two strains of the bacterium Streptococcus pneumoniae: a virulent (disease-causing) strain (S) and a non-virulent (non-disease-causing) strain (R).

Transformation is the process of one thing changing into another. In molecular biology and genetics, transformation is the genetic alteration of a cell. It’s one of three processes that lead to horizontal gene transfer , along with conjugation and transduction. The detail description of the Griffith’s Experiment – Transformation in Bacteria is as follows:

Also Read : Bacterial Genetics 

Griffith Experiment Steps

In the experiment, Griffith injected two types of Streptococcus pneumoniae into mice.

• Griffith then subjected the virulent, smooth strain (S) to heat that killed the bacteria. This heat-killed strain (S) was no longer capable of causing disease.
• Griffith injected mice with the heat-killed virulent strain (S). Surprisingly, the mice survived, indicating that the heat-killed bacteria alone were not harmful.
• Griffith mixed the heat-killed virulent strain (S) with the live non-virulent, rough strain (R) and injected this mixture into mice.
• The mice developed pneumonia and died, even though the strain injected was previously non-virulent.

Observations and Conclusion

Griffith concluded that some factor or biomolecule in the heat-killed virulent bacteria (S) had transformed the live non-virulent bacteria (R) into a virulent form. This phenomenon was termed “transformation,” though Griffith could not identify the nature of the transforming substance.

Significance

Griffith’s experiment laid the groundwork for understanding genetic transformation and proved that DNA , rather than proteins, carried genetic information. This discovery was fundamental to the development of molecular genetics and is also used in a variety of genetic engineering applications.

Also Read : Mutation

Impact of The Griffith Experiment are:

• Griffith’s experiment led to the discovery of the “transforming principle”. This discovery led to the discovery of DNA as a carrier of genetic information.
• The experiment introduced the concept of genetic transformation, demonstrating that genetic material could alter an organism’s characteristics.
• The understanding of genetic material transfer contributed to advancements in biotechnology, genetic engineering, and recombinant DNA technology.
• Transformation experiments were the basis for proposing the chromosomal theory of inheritance .
• Griffith’s experiment provided how external factors, such as genetic material transfer, could influence the pathogenicity of the bacteria.
• Griffith’s research led to the study of disease prevention and treatment by vaccines and immune serums.

Also Read: Difference between Vaccination and Immunization

Frederick Griffith experiment suggested that a hereditary material from heat-killed bacteria could transform live bacteria. Griffith did not identify the transforming substance. In the 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith’s experiment to identify the transforming substance.

• They isolated cellular components including proteins, DNA, RNA from the heat-killed virulent bacteria (S strain) and tested each component’s ability to transform the harmless bacteria (R strain).
• They used enzymes to selectively break down different cellular components of the heat-killed virulent bacteria (S) to determine which component was essential for transformation.
• They treated the heat-killed virulent bacteria (S) with enzymes that specifically degrade either proteins, RNA , or DNA.
• The treated bacterial extracts were then mixed with live non-virulent bacteria (R), and the mixtures were injected into mice.
• Enzymatic degradation of proteins and RNA did not prevent the transformation. However, when the DNA-degrading enzyme was used, the transforming ability was lost.
• This led Avery, MacLeod, and McCarty to conclude that the transforming substance responsible for genetic transformation in bacteria was DNA.

The discovery revolutionized the understanding of genetics and molecular biology. It established DNA as the molecule responsible for transmitting hereditary information and laid the foundation for the molecular biology. Their research paved the way for subsequent studies that explained the structure of DNA (Watson and Crick, 1953) and contributed to the development of molecular genetics, genetic engineering, and modern biotechnology.

Conclusion – Griffith Experiment

Frederick Griffith’s 1928 experiment on Streptococcus pneumoniae demonstrated bacterial transformation through a transfer of hereditary traits between strains. In Griffith experiment conclusion, the result showed that the harmless R strain could be transformed into a virulent form when exposed to the heat-killed S strain. Subsequent work by Avery, MacLeod, and McCarty in 1944 identified DNA as the transforming substance, establishing it as the genetic material. The discovery laid the foundation for molecular genetics, confirming the role of DNA in transmitting hereditary information.

Also Read: Inherited Traits Lethal Allele​ – Examples, & its Types Difference Between Phenotype and Genotype Ratio Importance of Variation

FAQs on Frederick Griffith Experiment

What was griffith’s experiment and why was it important.

Frederick Griffith conducted an experiment that suggested bacteria can transfer genetic information through transformation. The experiment was important because it showed that bacteria can change their function and form through transformation.

What is the Griffith Experiment Conclusion?

Frederick Griffith experiment concluded that bacteria can transfer genetic information through a process called transformation.

What was the Most Significant Conclusion of Griffith’s Experiments with Pneumonia in Mice?

Griffith conducted experiments with mice and Streptococcus pneumonia bacteria. He concluded that heat-killed bacteria can convert live avirulent cells to virulent cells. Griffith called this phenomenon transformation.

What did Frederick Griffith Want to Learn about Bacteria?

Frederick Griffith, a British bacteriologist, wanted to learn how bacteria could acquire new traits and how certain types of bacteria produce pneumonia.

How did the Two Types of Bacteria Used by Griffith Differ?

The two types of bacteria used by Griffith were the R strain, lacking a virulent capsule and non-pathogenic, and the S strain, possessing a smooth capsule and causing pneumonia in mice, making it pathogenic.

What was Oswald Avery’s Experiment?

The experiment demonstrated that DNA was the only molecule that transformed from one bacterial strain to another.

What is Griffith’s Transforming Principle?

Griffith performed an experiment with bacteria and mice and discovered that bacteria can incorporate foreign genetic material from their environment, which he called the transforming principle.

Why is Chapter Griffith Experiment Class 12 Important?

The Griffith Experiment in Class 12 biology is important as it describes bacterial transformation, highlighting the role of genetic material in heredity and laying the foundation for modern molecular biology and genetics research.

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Griffith Experiment: Search of Genetic Material

Collegedunia Team

Content Curator

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 .

R Strain And S Strain Bacteria

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

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).

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, di dn'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

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

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

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.

Sample Questions

Ques. What was Griffith’s experiment?  (1 Mark)

Ans.  Griffith's experiment was the first to show that bacteria can transfer genetic information through a process known as transformation.

Ques. Define Horizontal Gene Transfer. (1 Mark)

Ans.  Horizontal gene transfer (HGT) is the interchange of genetic information across organisms, including the spread of antibiotic resistance genes among bacteria (other than those passed down from parent to offspring), which fuels pathogen development.

Ques. Who conducted Griffith’s experiment and in which year?   (1 Mark)

Ans.  Griffith’s experiment was conducted by an English bacteriologist, Frederick Griffith in the year 1928.

Ques. How did Frederick Griffith conduct the Griffith experiment?   (1 Mark)

Ans.  Frederick Griffith conducted the Griffith experiment on two strains of the bacterium.

Ques. Which animal did Frederick Griffith use to perform the Griffith experiment?   (1 Mark)

Ans.  Frederick Griffith used mice as animals to perform the Griffith experiment.

Ques. What are the two types of bacteria used in Griffith’s experiment?   (2 Marks)

Ans.  The two types of bacteria used in Griffith’s experiment are:

• Smooth (S) or Capsulated type: The smooth strain contains a capsule and is said to be virulent and causes pneumonia.
• Rough (R) or Non-Capsulated Type: The rough strain does not contain a capsule and is therefore said to be non-virulent and does not cause pneumonia.

Ques. Name the Bacterium used in Griffith’s experiment.  (1 Mark)

Ans.  The name of the bacterium used in Griffith’s experiment is Diplococcus or Streptococcus pneumonia or Pneumococcus.

Ques. Explain how the Griffith experiment was conducted. Write the steps.  (3 Marks)

Ans.  Griffith’s experiment was conducted by following the steps mentioned below:

• Smooth-type bacteria were injected into mice. As a result, the mice died of pneumonia caused by the bacteria.

Live S strain à injected into mice à Mice died

• Rough-type bacteria were injected into mice. As a result, the mice lived and pneumonia was not caused.

Live R strain à injected into mice à Mice lived

• Smooth-type bacteria that normally cause disease were heat-killed and then injected into mice. As a result, the mice lived and pneumonia was not caused.

S strain (heat-killed) à injected into mice à Mice lived

• Rough-type bacteria (living) and smooth heat-killed bacteria (both were known not to cause disease) were injected together into mice. As a result, the mice died due to pneumonia and even virulent smooth living bacteria could be recovered from their bodies.

S strain (heat-killed) + R strain (living) à injected into mice à Mice died

Ques. What was the conclusion of Griffith’s experiment?  (2 Marks)

Ans.  Griffith’s experiment concluded that bacteria are actually capable of transferring genetic information through transformation. However, in the end, Frederick Griffith was not able to identify the biochemical nature of the genetic material from his experiment.

Ques. Name the scientists who repeated the Griffith experiment. Why did they do so and what did they conclude?  (2 Marks)

Ans.  Oswald Avery, Colin McLeod, and Maclyn McCarty are the groups of scientists who repeated the Griffith experiment in the year 1944.

They did so to identify the biochemical nature of the genetic material and they concluded DNA was the biochemical nature of the genetic material in the Griffith experiment.

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