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Griffith Experiment and Search of Genetic Material

The search for Genetic material started during the mid-nineteenth century. The principle of inheritance was discovered by Mendel. Based on his investigation, Mendel concluded that some ‘factors’ are transferred from one generation to another. Mendel’s Law of Inheritance was the basis for the researchers on genetic material. Keeping his conclusions in mind, scientists who came after him, focused on chromosomes in search of genetic material. Even though the chromosomal components were identified, the material which is responsible for inheritance remained unanswered. It took a long time for the acceptance of DNA as the genetic transformation. Let’s go through a brief account of the discovery of genetic material and Griffith experiment.

Griffith Experiment & Transforming Principle

Griffith experiment was a stepping stone for the discovery of genetic material. Frederick Griffith experiments were conducted with Streptococcus pneumoniae.

During the experiment, Griffith cultured Streptococcus pneumoniae bacteria which showed two patterns of growth. One culture plate consisted of smooth shiny colonies (S) while other consisted of rough colonies (R). The difference was due to the presence of mucous coat in S strain bacteria, whereas the R strain bacteria lacked them.

Experiment: Griffith injected both S and R strains to mice. The one which was infected with the S strain developed pneumonia and died while that infected with the R strain stayed alive.

In the second stage, Griffith heat-killed the S strain bacteria and injected into mice, but the mice stayed alive. Then, he mixed the heat-killed S and live R strains. This mixture was injected into mice and they died. In addition, he found living S strain bacteria in dead mice.

Conclusion: Based on the observation, Griffith concluded that R strain bacteria had been transformed by S strain bacteria. The R strain inherited some ‘transforming principle’ from the heat-killed S strain bacteria which made them virulent. And he assumed this transforming principle as genetic material.

DNA as Genetic Material

Griffith experiment was a turning point towards the discovery of hereditary material. However, it failed to explain the biochemistry of genetic material. Hence, a group of scientists, Oswald Avery, Colin MacLeod and Maclyn McCarty continued the Griffith experiment in search of biochemical nature of the hereditary material. Their discovery revised the concept of protein as genetic material to DNA as genetic material .

Avery and his team extracted and purified proteins, DNA, RNA and other biomolecules from the heat-killed S strain bacteria. They discovered that DNA is the genetic material and it is alone responsible for the transformation of the R strain bacteria. They 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.

Frequently Asked Questions on Griffith Experiment

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

Griffith’s experiment was the first experiment which suggested that bacteria can transfer genetic information through a process called transformation.

What is the conclusion of Griffith experiment?

The experiment concluded that bacteria are capable of transfering genetic information through transformation.

What was the most significant conclusion of Griffith’s experiments with pneumonia in mice?

The experiment conducted by Griffith found that bacteria are capable of transfering genetic information through transformation.

What did Frederick Griffith want to learn about bacteria?

Frederick Griffith wanted to learn if bacterial transformation was possible.

How did the two types of bacteria used by Griffith differ?

Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteria: a type III-S and a type II-R.

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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 .

Microbe Notes

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.
• https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/04%3A_Molecular_Biology/4.02%3A_DNA_the_Genetic_Material
• https://byjus.com/biology/dna-genetic-material/
• https://bio.libretexts.org/Bookshelves/Genetics/Book%3A_Online_Open_Genetics_(Nickle_and_Barrette-Ng)/01%3A_Overview_DNA_and_Genes/1.02%3A_DNA_is_the_Genetic_Material
• https://www.toppr.com/guides/biology/the-molecular-basis-of-inheritance/the-genetic-material/
• https://www.nature.com/scitable/topicpage/discovery-of-dna-as-the-hereditary-material-340/
• https://www.biologydiscussion.com/genetics/dna-as-a-genetic-material-biology/56216
• https://www.nature.com/scitable/topicpage/discovery-of-the-function-of-dna-resulted-6494318/
• https://www.ndsu.edu/pubweb/~mcclean/plsc411/DNA%20replication%20sequencing%20revision%202017.pdf
• https://www.britannica.com/biography/Frederick-Griffith
• https://ib.bioninja.com.au/higher-level/topic-7-nucleic-acids/71-dna-structure-and-replic/dna-experiments.html
• https://biolearnspot.blogspot.com/2017/11/experiments-of-avery-macleod-and.html
• 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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Biology Wise

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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Isolating Hereditary Material: Frederick Griffith, Oswald Avery, Alfred Hershey, and Martha Chase

Frederick Griffith Discovers Bacterial Transformation

In the aftermath of the deadly 1918 flu epidemic, governments across the globe rushed to develop vaccines that could stop the spread of infectious diseases. In England, microbiologist Frederick Griffith was studying two strains of Streptococcus pneumoniae that varied dramatically in both their appearance and their virulence , or their ability to cause disease . Specifically, the highly virulent S strain had a smooth capsule, or outer coat composed of polysaccharides, while the nonvirulent R strain had a rough appearance and lacked a capsule (Figure 1). Mice injected with the S strain died within a few days after injection, while mice injected with the R strain did not die.

Through a series of experiments, Griffith established that the virulence of the S strain was destroyed by heating the bacteria. Thus, he was surprised to find that mice died when they were injected with a mixture of heat-killed S bacteria and living R bacteria (Figure 2), neither of which caused mice to die when they were injected alone. Griffith was able to isolate live bacteria from the hearts of the dead animals that had been injected with the mixed strains, and he observed that these bacteria had the smooth capsules characteristic of the S strain. Based on these observations, Griffith hypothesized that a chemical component from the virulent S cells had somehow transformed the R cells into the more virulent S form (Griffith, 1928). Unfortunately, Griffith was not able to identify the chemical nature of this " transforming principle " beyond the fact that it was able to survive heat treatment.

DNA Is Identified as the “Transforming Principle”

The actual identification of DNA as the "transforming principle" was an unexpected outcome of a series of clinical investigations of pneumococcal infections performed over many years (Steinman & Moberg, 1994). At the same time that Griffith was conducting his experiments, researcher Oswald Avery and his colleagues at the Rockefeller University in New York were performing detailed analyses of the pneumococcal cell capsule and the role of this capsule in infections. Modern antibiotics had not yet been discovered, and Avery was convinced that a detailed understanding of the pneumococcal cell was essential to the effective treatment of bacterial pneumonia. Over the years, Avery's group had accumulated considerable biochemical expertise as they established that strains of pneumococci could be distinguished by the polysaccharides in their capsules and that the integrity of the capsule was essential for virulence. Thus, when Griffith's results were published, Avery and his colleagues recognized the importance of these findings, and they decided to use their expertise to identify the specific molecules that could transform a nonencapsulated bacterium into an encapsulated form. In a significant departure from Griffith's procedure, however, Avery's team employed a method for transforming bacteria in cultures rather than in living mice, which gave them better control of their experiments.

Avery and his colleagues, including researchers Colin MacLeod and Maclyn McCarty, used a process of elimination to identify the transforming principle (Avery et al. , 1944). In their experiments (Figure 3), identical extracts from heat-treated S cells were first treated with hydrolytic enzymes that specifically destroyed protein , RNA , or DNA. After the enzyme treatments, the treated extracts were then mixed with live R cells. Encapsulated S cells appeared in all of the cultures, except those in which the S strain extract had been treated with DNAse, an enzyme that destroys DNA. These results suggested that DNA was the molecule responsible for transformation.

Avery and his colleagues provided further confirmation for this hypothesis by chemically isolating DNA from the cell extract and showing that it possessed the same transforming ability as the heat-treated extract. We now consider these experiments, which were published in 1944, as providing definitive proof that DNA is the hereditary material. However, the team's results were not well received at the time, most likely because popular opinion still favored protein as the hereditary material.

Hershey and Chase Prove Protein Is Not the Hereditary Material

From these experiments, Hershey and Chase determined that protein formed a protective coat around the bacteriophage that functioned in both phage attachment to the bacterium and in the injection of phage DNA into the cell. Interestingly, they did not conclude that DNA was the hereditary material, pointing out that further experiments were required to establish the role that DNA played in phage replication . In fact, Hershey and Chase circumspectly ended their paper with the following statement: "This protein probably has no function in the growth of intracellular phage. The DNA has some function. Further chemical inferences should not be drawn from the experiments presented" (Hershey & Chase, 1952). However, a mere one year later, the structure of DNA was determined , and this allowed investigators to put together the pieces in the question of DNA structure and function.

References and Recommended Reading

Avery, O. T., et al . Studies on the chemical nature of the substance inducing transformation of pneumococcal types. Journal of Experimental Medicine 79 , 137–157 (1944)

Griffith, F. The significance of pneumococcal types . Journal of Hygiene 27 , 113–159 (1928)

Hershey, A. D., & Chase, M. Independent functions of viral protein and nucleic acid in growth of bacteriophage. Journal of General Physiology 36 , 39–56 (1952)

Steinman, R. M., & Moberg, C. L. A triple tribute to the experiment that transformed biology . Journal of Experimental Medicine 179 , 379–384 (1994)

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1944: DNA is \"Transforming Principle\"

1944: dna is "transforming principle".

Avery, MacLeod and McCarty identified DNA as the "transforming principle" while studying Streptococcus pneumoniae , bacteria that can cause pneumonia. The bacteriologists were interested in the difference between two strains of Streptococci that Frederick Griffith had identified in 1923: one, the S (smooth) strain, has a polysaccharide coat and produces smooth, shiny colonies on a lab plate; the other, the R (rough) strain, lacks the coat and produces colonies that look rough and irregular. The relatively harmless R strain lacks an enzyme needed to make the capsule found in the virulent S strain.

Griffith had discovered that he could convert the R strain into the virulent S strain. After he injected mice with R strain cells and, simultaneously, with heat-killed cells of the S strain, the mice developed pneumonia and died. In their blood, Griffith found live bacteria of the deadly S type. The S strain extract somehow had "transformed" the R strain bacteria to S form. Avery and members of his lab studied transformation in fits and starts over the next 15 years. In the early 1940s, they began a concerted effort to purify the "transforming principle" and understand its chemical nature.

Bacteriologists suspected the transforming factor was some kind of protein. The transforming principle could be precipitated with alcohol, which showed that it was not a carbohydrate like the polysaccharide coat itself. But Avery and McCarty observed that proteases - enzymes that degrade proteins - did not destroy the transforming principle. Neither did lipases - enzymes that digest lipids. They found that the transforming substance was rich in nucleic acids, but ribonuclease, which digests RNA, did not inactivate the substance. They also found that the transforming principle had a high molecular weight. They had isolated DNA. This was the agent that could produce an enduring, heritable change in an organism.

Until then, biochemists had assumed that deoxyribonucleic acid was a relatively unimportant, structural chemical in chromosomes and that proteins, with their greater chemical complexity, transmitted genetic traits.

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Last updated: April 23, 2013

Transformation in Bacteria

Transformation in bacteria was first studied by a scientist Frederick Griffith in 1928 . According to Griffith, the DNA or gene transfer can occur either naturally or artificially from one type of bacteria to another. For example, Transformation of non-virulent strain to a virulent cell or vice versa.

To explain the transformation principle, Griffith performed certain experiments on the mice by taking pathogenic bacteria Streptococcus pneumoniae . Transformation results in gene alteration in the recipient cell, due to the incorporation of free DNA from its surrounding through the cell membrane.

The transformation process is widely used in gene cloning, DNA linkage, generation of cDNA libraries and protein expression. Here, we will discuss the definition, stages, competence in the transformation of bacteria. You will also get to know the transformation principle of the bacteria through the Griffith experiment (transformation experiment).

Content: Transformation in Bacteria

Definition of transformation, streptococcus pneumoniae, strains used in griffith experiment, transformation experiment, transforming factor, stages of transformation.

Transformation can define as the process of taking up of an extracellular or free DNA strand of one bacterial cell ( donor’s cell ) by the competent bacterial cell ( recipient’s cell ). The taking up of the DNA strand occurs either by natural or artificial means. The transformation occurs mostly in the closely related species. Therefore, transformation merely refers to the direct insertion , incorporation and expression of the exogenous DNA in the competent bacterial cell (gets transformed by the inclusion of free DNA).

Classification :

• Kingdom : Bacteria
• Phylum : Firmicutes
• Class : Bacilli
• Order : Lactobacillales
• Family : Streptococcaceae
• Genus : Streptococcus
• Species : pneumoniae

To demonstrate the transformation principle, Frederick Griffith had taken the pathogenic bacteria Streptococcus pneumoniae . Further, he observed two different strains of Streptococcus pneumoniae and named it as S-III and R-II strain.

• S-III strain : It is the smooth strain of Streptococcus pneumonia , which is encapsulated with the polysaccharide. S-III strain will act as virulent or wild strain , as the polysaccharide is a virulent factor.
• R-II strain : It is the rough strain of Streptococcus pneumonia , which lacks the polysaccharide covering. The R-II strain will act as mutant or avirulent strain , as the polysaccharide is absent.

To explain the theory of transformation principle, Frederick Griffith performed a series of experiments where he injected two different strains of Streptococcus pneumoniae into the mice and reported the particular strain’s effect into the mice.

• In his first experiment, Griffith used a rough strain of Streptococcus pneumonia ( R-II ) and injected it into the mice. After doing this, he observed that the R-II strain of bacteria did not affect the mice and the mice lived . Therefore, Griffith named R-II strain as an “ Avirulent strain ”.
• In his second experiment, Griffith used a smooth strain of Streptococcus pneumonia ( S-III ) and injected it into the mice. After doing this, he observed that the S-III strain of bacteria killed the mice. Therefore, Griffith named S-III strain as a “ Virulent strain ”.
• In the third experiment, Griffith used smooth or virulent S-III and subjected it to the heat to destroy the virulence. Then, he injected the heat-killed S-III strain into the mice. After doing this, he observed that the heat-killed S-III strain did not affect the mice and the mice lived . Therefore, Griffith concluded that the virulence of the S-III strain becomes ineffective the heat exposure.
• In the fourth experiment, Griffith used rough R-II strain plus dead or heat-killed S-III strain and injected into the mice. After that, he observed the death of mice. Then, he concluded that the S-III strain had transferred something which transformed the R-II strain into the virulent strain (R-III) and caused the death of mice.

To explain the transformation factor (whether it was a protein or some other component), Avery , Macleod and McCarty performed a series of experiments. Their experiment to identify the transformation of R-II to virulent type can be summarized into the following sequential steps:

• First, they extracted different components like protein, polysaccharide, lipid, RNA and DNA from the heat-killed S-III strain.
• After that, they added R-II strain individually into each test tubes.
• In the third step, they used specific enzymes for the digestion of specific components.
• Then, they injected it to the mice.

After doing this experiment, they observed the death of four mice except for the last one. They concluded that the DNA is the transformation factor which has transformed the R-II strain to the virulent type. Therefore, the DNA is the heritable material that has transferred the virulence from the dead or heat-killed S-III strain to the R-II strain.

As the DNA of S-III or virulent strain is destroyed by the enzyme DNase , there will not be any transformation between the heat-killed S-III strain and the R-II strain, and thus there will be no effect on the mice. They also concluded that even though the polysaccharide is a virulent factor, but still it is not involved in the transformation as it is not heritable.

There are three stages of transformation which include :

• Competence is the first stage where a cell must be competent to take up the DNA. To develop competence, the cell responds to the environmental signal, allowing the binding and penetration of the free DNA .
• The DNA binding is the second stage of transformation in which the exogenous or free DNA binds to the recipient’s cell wall due to  developed competence. This stage occurs at the time of incubation of bacterial cell culture on ice. The DNA will bind to the recipient cell wall of bacteria by forming calcium chloride plus a DNA complex.
• DNA integration is the incorporation of the exogenous DNA that has entered to the recipient cell cytoplasm. Therefore, the insertion of foreign DNA into the chromosome of the recipient cell will cause transformation.

To carry out the transformation process, the bacteria should be competent to take up the free DNA. Competence can define as the physiological state , where a recipient cell is in a state where it can respond to the environmental conditions such as starvation and cell density. Therefore, when a cell becomes competent, it can take up the exogenous DNA from the donor’s cell.

In the process of transformation, competence can be of two types :

Natural competence

Artificial competence.

It is a type where a transformation occurs naturally in response to environmental signals and extreme conditions. About 1% of bacteria can develop competence naturally. A set of genes are carried by the naturally competent bacteria. The genes (DNA) then migrate across the cell membrane naturally and infuse within the recipient’s cell.

In this type, a transformation is induced artificially by some chemical or physical methods. Thus, the transformation process is forced  or do not occur naturally. Artificial competence can be achieved by both chemical and physical methods. The artificial competence can be achieved by the chemical method through the divalent cation method and physical method through the electroporation.

Divalent cation method : It was first introduced by the two scientists Mandel and Higa in 1970 . In the divalent cation method, the E.coli in the log phase of growth are taken from the culture. Then, E.coli culture is centrifuged. From the E.coli culture, the pellet of bacteria is resuspended in the divalent ion solution like calcium chloride . After that, the culture is kept under cold conditions that result in the weakening of bacteria’s cell surface and allow the binding of free DNA molecule.

Then, the bacterial suspension is suddenly subjected to the high temperature (42 Degrees Celsius) for 30 seconds in the boiling water bath, and the process called heat shock . It results in the thermal imbalance within the bacterial cell and forces the binding of free DNA into the cell.

Electroporation : It is an alternative method of chemical transformation. In electroporation, the bacterial cell is subjected to a high voltage of 15 kV/cm for a 5 µ sec under the influence of an electric field, and the process is called electroshock . The electric shock enhances the ability to take up the free DNA strand. In 1982, a technique of introducing free DNA into the mice was carried out by a scientist Neumann where he treated it with the short pulses at high voltage.

Neumann concluded that the electric shock increases the cell’s membrane potential and thereby increases the cell permeability to take up the charged molecule like DNA.

We can conclude that the non-competent cell has to be competent to carry out the transformation. The competence is developed by the environmental signals like temperature, pH, heat etc., making the cell competent by enhancing the ability to take up the free DNA.

Related Topics:

• Major Groups of Microorganisms
• Bacterial Conjugation
• Nuclear Pore Complex
• Nutrition in Bacteria

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Transforming Principle

Frederick Griffith, established that there was a transforming principle in bacterial genetics in a ground-breaking experiment, performed in 1928.

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He postulated that information could somehow be transferred between different strains of bacteria. This was long before the discovery of DNA and was an inspired piece of scientific detective work.

Methodology

There is one major difference between these two types; the III-S strain has a smooth polysaccharide coat which makes it resistant to the immune system of mice, whereas the II-R strain lacks this coat and so will be destroyed by the immune system of the host.

For the first stage of the transforming principle experiment , Griffith showed that mice injected with III-S died but when injected with II-R lived and showed few symptoms.

The next stage showed that if the mice were injected with type III-S that had been killed by heat, the mice all lived, indicating that the bacteria had been rendered ineffective.

The interesting results came with the third part of the experiment, where mice were injected with a mixture of heat killed III-S and live II-R.

Interestingly enough, the mice all died, indicating that some sort on information had been passed from the dead type III-S to the live type II-R. Blood sampling showed that the blood of the dead mice contained both live type III-S and live type II-R bacteria.

Somehow the type III-S had been transformed into the type III-R strain, a process he christened the transforming principle.

Follow up experiments performed by Avery, McLeod and McCarty and by Hershey and Chase established that DNA was the mechanism for this transferal of genetic information between the two bacteria.

In turn, this lead to the discoveries of Crick and Watson, who discovered the exact structure of DNA, and the mechanisms used for storing and transferring information.

Considering that Griffith did not know the chemical and biological processes behind the transforming principle, it was inspirational research which built on the theories of scientists such as Mendel . The study opened up avenues of research into the biochemical principles behind the genetic transference of information.

Genetic engineering, involving the transferring of DNA between organisms, is now more commonplace, but built upon the research performed by Griffith. Most biology students have heard of Mendel, and Crick and Watson, but must not forget the work of the other inspiring scientists in between.

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Martyn Shuttleworth (Sep 3, 2008). Transforming Principle. Retrieved Sep 07, 2024 from Explorable.com: https://explorable.com/transforming-principle

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Home > MARKUS_LIBRARY > Exhibits > DNA

The Transforming Principle: DNA, The Molecule of Heredity

The story of DNA is one of the most fascinating of modern science. Contrary to popular belief, the discovery of the chemical structure and biological function of deoxyribonucleic acid (DNA) did not occur within several years in the twentieth century and was not accomplished by a small, select group of scientists. Solving the problems of DNA was similar to the painstaking work in assembling the many isolated pieces of a large jigsaw puzzle. A great number of scientists working in a variety of fields contributed to the final outcome, but few ever received anything more significant than the personal satisfaction of having been a participant.

In 1928, Frederick Griffith, a British geneticist, discovered what he called a transforming principle in which a nonvirulent bacteria was turned into a virulent one. It was not until sixteen years later that Griffith’s “transforming principle” was identified as DNA by Avery, MacLeod, and McCarty.

The first in a new series “Bridging Science and Medicine”, this exhibit features Oswald Avery’s research that led to the development of the first vaccine for pneumococcal pneumonia, but it also led him and colleagues Colin M. MacLeod and Maclyn McCarty to make an unexpected discovery in 1944: that DNA is the substance that transmits hereditary information, a finding that would set the course for biological research for the rest of the century.

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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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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.

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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.

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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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