frederick griffith experiment animation

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Animation 17: A gene is made of DNA.

• Description

Oswald Avery explains Fred Griffith's and his own work with Pneumococcus bacteria.

How do you do? I'm Oswald Avery. My colleagues and I did a series of experiments using strains of Pneumococcus bacteria, which cause pneumonia. Pneumococcus grows in the body of the host, but, like other types of bacteria, also can be grown on solid or liquid cultures. In 1928, Fred Griffith published a study on the different strains of Pneumococcus. Two in particular, S and R, look different. The S colonies have a smooth surface, and the R colonies look rough. The S colonies look smooth because each bacterium has a capsule-like coat made of sugars. This coat protects the S bacteria from the host's immune system, and so the S strain is infectious. The coat-less R strain is not. Griffith found that mice injected with the S strain develop pneumonia and die within days. Mice injected with the R strain do not get pneumonia. Griffith noticed that different strains of Pneumococcus could be cultured from one patient. He began to wonder if one strain could change into another. To test this idea, he did a series of experiments using the R and S strains. First, Griffith heated the S strain culture to kill the bacteria. As predicted, when injected into mice, the heat-killed bacteria did not produce an infection. Griffith co-injected the heat-killed S with live R into mice, and, much to his surprise, the mice developed pneumonia and died. Even more astonishing, Griffith was able to isolate live S strain from the blood of infected mice. These cultures could infect other mice. S strain cultured from infected mice remained active — showing that the change was stable and inherited. Griffith concluded that some "principle" was transferred from the heat-killed S to the R strain. The principle transformed the R into the infective S strain with a smooth coat. When I read about Griffith's results, I became very interested in the identity of this transforming "principle." Colin MacLeod, Maclyn McCarty, and I began experimenting using a test tube assay instead of mice. We used detergent to lyse the heat-killed S cells. Then we used this lysate for transformation assays. The test tube assays worked well, and showed us that the heat-killed S lysate could change R to S. The transforming principle was something in the lysate. We tested each of the lysate components for the transforming activity. First, we incubated the heat-killed S lysate with an enzyme, SIII, that completely chewed up the sugar coat. We tested the transforming ability of the sugar coat-less S lysates. The sugar coat-less S lysate was still able to transform. This told us that the R strain was not just assembling a new S sugar coat from the pieces. Next, we incubated the coat-less S extract with protein digesting enzymes — trypsin and chymotrypsin. Next, we incubated the coat-less S extract with protein digesting enzymes — trypsin and chymotrypsin. We tested this lysate's ability to transform. This protein-less lysate was still able to transform. So, the transforming principle is not protein. While we were testing and purifying the lysate, we precipitated nucleic acids — DNA and RNA — with alcohol. We were the first to isolate nucleic acids from Pneumococcus. While we were testing and purifying the lysate, we precipitated nucleic acids — DNA and RNA — with alcohol. We were the first to isolate nucleic acids from Pneumococcus. Since the transforming principle was not the sugar coat, and not protein, we suspected that it may be one of the nucleic acids. We dissolved the precipitate in water, and tested the transforming ability of the solution. Since the transforming prinicple was not the sugar coat, and not protein, we suspected that it may be one of the nucleic acids. We dissolved the precipitate in water, and tested the transforming ability of the solution. First, we destroyed the RNA using the RNase enzyme. We tested this solution for its ability to transform. The solution still had the ability to transform. Therefore, RNA could not be the transforming principle. What we had left was virtually pure DNA. As a final test, we incubated the solution with the DNA-digesting enzyme, DNase. We used this solution to test for transforming ability. This solution was unable to transform. My colleagues and I concluded that DNA is the transforming principle, and we published these results in 1944.

oswald avery, pneumococcus bacteria, liquid cultures, fred griffith, smooth coat, transforming principle, rough coat

• Source: DNALC.DNAFTB

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Oswald Avery, circa 1930.

• Source: DNAi

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In 1944, Oswald Avery and his colleagues, Colin MacLeod and Maclyn McCarty published their landmark paper on the transforming ability of DNA.

16381. Gallery 17: Oswald Avery's letter to his brother, 1943

A page from the May 15, 1943 letter from Oswald Avery to his brother Roy. In the letter Avery speculated on how transformation could happen. Avery never publicly connected genes with DNA and his transformation experiments.

16705. Animation 34: Genes can be moved between species.

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In 1944, Maclyn McCarty and his colleagues, Colin MacLeod and Oswald Avery published their landmark paper on the transforming ability of DNA.

16378. Gallery 17: Oswald Avery, around 1930

Oswald Avery at work in the laboratory, around 1930.

16390. Video 17: Maclyn McCarty, clip 6

How the bacterial transformation experiments provided the first real opportunity to study the chemical nature of the gene.

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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 Transformation Experiment

Pneumococcus bacteria include two strains, a virulent S strain with a S mooth glycoprotein coat that kills mice (left), and a non-virulent R   R ough strain that does not (middle). Heating destroys the virulence of S (right).

In the critical experiment, Frederick Griffith ( 1928 ) mixed heat-killed S with live R and injected the combination into mice: the mouse died.The dead mouse's tissues were found to contain liv e bacteria with smooth coats like S . These bacteria were subsequently able to kill other mice, and continued to do so after several generations in culture.

Griffith concluded that something in the heat-killed S bacteria ' transformed'   the hereditary properties of the R bacteria. The nature of this ' transforming principle ' was unknown.

HOMEWORK . What do each of the "Control" experiments control for? Suppose the combination of heat-killed S and live R killed the first generation of mice, but not the second or subsequent generations. What would you conclude about transformation?

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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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The Griffith Experiment: How Frederick Griffith Discovered the Transfer of Genes Between Two Strains of Bacteria

• Sonal Panse
• Categories : Genetics , Science
• Tags : Science genetics topics genomic research

Bacteria Microbiology

Frederick Griffith’s experiment showed that bacteria were capable of transferring their genetic information by a process that he called transformation. Frederick Griffith was an English army doctor and his experiment, which consisted of testing the effects of killed bacteria on live cells, was actually intended to help develop a vaccine against a future outbreak of “Spanish flu,” the pandemic that killed millions of people after world War I.

For his experiment, which was conducted in 1928, Griffith used two of the three strains of Pneumococcus bacteria that had been discovered by the German bacteriologist Fred Neufeld. These bacteria infect and cause pneumonia in mice -

• The virulent Type III-S strain - Type III-S has a smooth polysaccharide capsule covering that protects it from attacks from the host’s immune system.
• The non-virulent Type II-R strain - Type II-R does not have a polysaccharide capsule covering, has a rough appearance and it can be destroyed by the host’s immune system.

Bacterial Transformation

Here’s what Griffith did and what he observed -

• He injected mice with the Type II-R strain and the mice survived.
• He injected mice with the Type III-S strain and the mice died.
• He heat killed the Type III-S strain and then injected the mice with the dead bacteria and the mice lived.
• He injected dead Type III-S strain and live Type II-R strain into the mice and the mice died. He then detected the presence of live Type III-S strain bacteria with live Type II-R strain bacteria in the blood of the dead mice.

This experiment led Griffith to conclude that the dead the Type II-R bacteria had been transformed by the Type III-S bacteria enabling it to develop a polysaccharide cover and take on its virulent properties.

This meant that the bacterial strains did not have fixed and non-interchangeable properties, which was the thinking at the time. They were capable of transformation and this clearly indicated gene transfer. Frederick Griffith was not, however, able to discover how this transformation took place; he knew about chromosomes and about nuclein (DNA and RNA) that Frederick Miescher had detected in 1869, but no one knew for sure if the genetic information was contained in nucleic acids or proteins.

Bacterial Transformation - What Really Happened?

What actually happened in Griffith’s experiment was that, after the Type III-S bacteria was heat killed, its DNA survived and was taken up by the II-R bacteria strain. The Type III-S DNA enabled the II-R bacteria to grow a protective capsule, gain virulent properties and defeat the host’s immune system.

However it was much later, in 1944, that three researchers from the Rockefeller Institute - Maclyn McCarty, Oswald Avery and Colin MacLeod - followed up on Griffith’s research and discovered that gene transfer from the Type III-S bacteria to the Type II-R bacteria was responsible for the transformation process.

Resources -

https://education.llnl.gov/bep/science/10/tLect.html

https://www.newton.dep.anl.gov/askasci/bio99/bio99074.htm

https://io.uwinnipeg.ca/~simmons/DNA/sld003.htm

https://juliantrubin.com/bigten/dnaexperiments.html

https://jem.rupress.org/cgi/reprint/179/2/379.pdf

https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=7968924

https://profiles.nlm.nih.gov/CC/A/A/O/F/ _/ccaaof.pdf

https://profiles.nlm.nih.gov/CC/A/A/A/M/ _/ccaaam.pdf

https://profiles.nlm.nih.gov/CC/

https://202.114.65.51/fzjx/wsw/newindex/wswfzjs/pdf/24avery.pdf

https://www.cuhk.edu.hk/bio/course _homepage/BIO2310/lecture/gen03DNARep%5B1%5D.pdf

https://www.people.vcu.edu/~elhaij/bio105/geneticmaterial.html

https://www.rci.rutgers.edu/~molbio/Courses/MBB _408_512/Lec_16_17.doc

• Biology Article
• Griffith Experiment Genetic Material

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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