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Griffith's experiment, [1] performed by Frederick Griffith and reported in 1928, [2] was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation. [3] [4] Griffith's findings were followed by research in the late 1930s and early 40s that isolated DNA as the material ...
- Overview
- Introduction
- Protein vs. DNA
- Frederick Griffith: Bacterial transformation
- Avery, McCarty, and MacLeod: Identifying the transforming principle
- The Hershey-Chase experiments
- Remaining questions
Experiments by Frederick Griffith, Oswald Avery and his colleagues, and Alfred Hershey and Martha Chase.
Introduction
Our modern understanding of DNA's role in heredity has led to a variety of practical applications, including forensic analysis, paternity testing, and genetic screening. Thanks to these wide-ranging uses, today many people have at least a basic awareness of DNA.
It may be surprising, then, to realize that less than a century ago, even the best-educated members of the scientific community did not know that DNA was the hereditary material!
Our modern understanding of DNA's role in heredity has led to a variety of practical applications, including forensic analysis, paternity testing, and genetic screening. Thanks to these wide-ranging uses, today many people have at least a basic awareness of DNA.
It may be surprising, then, to realize that less than a century ago, even the best-educated members of the scientific community did not know that DNA was the hereditary material!
The work of Gregor Mendel showed that traits (such as flower colors in pea plants) were not inherited directly, but rather, were specified by genes passed on from parents to offspring. The work of additional scientists around the turn of the 20th century, including Theodor Boveri, Walter Sutton, and Thomas Hunt Morgan, established that Mendel's heritable factors were most likely carried on chromosomes.
Scientists first thought that proteins, which are found in chromosomes along with DNA, would turn out to be the sought-after genetic material. Proteins were known to have diverse amino acid sequences, while DNA was thought to be a boring, repetitive polymer, due in part to an incorrect (but popular) model of its structure and composition1 .
In 1928, British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus pneumoniae bacteria and mice. Griffith wasn't trying to identify the genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria, known as R and S.
•R strain. When grown in a petri dish, the R bacteria formed colonies, or clumps of related bacteria, that had well-defined edges and a rough appearance (hence the abbreviation "R"). The R bacteria were nonvirulent, meaning that they did not cause sickness when injected into a mouse.
•S strain. S bacteria formed colonies that were rounded and smooth (hence the abbreviation "S"). The smooth appearance was due to a polysaccharide, or sugar-based, coat produced by the bacteria. This coat protected the S bacteria from the mouse immune system, making them virulent (capable of causing disease). Mice injected with live S bacteria developed pneumonia and died.
As part of his experiments, Griffith tried injecting mice with heat-killed S bacteria (that is, S bacteria that had been heated to high temperatures, causing the cells to die). Unsurprisingly, the heat-killed S bacteria did not cause disease in mice.
The experiments took an unexpected turn, however, when harmless R bacteria were combined with harmless heat-killed S bacteria and injected into a mouse. Not only did the mouse develop pnenumonia and die, but when Griffith took a blood sample from the dead mouse, he found that it contained living S bacteria!
Diagram illustrating Frederick Griffith's experiment with S and R bacteria.
In 1944, three Canadian and American researchers, Oswald Avery, Maclyn McCarty, and Colin MacLeod, set out to identify Griffith's "transforming principle."
To do so, they began with large cultures of heat-killed S cells and, through a long series of biochemical steps (determined by careful experimentation), progressively purified the transforming principle by washing away, separating out, or enzymatically destroying the other cellular components. By this method, they were able to obtain small amounts of highly purified transforming principle, which they could then analyze through other tests to determine its identity2 .
Several lines of evidence suggested to Avery and his colleagues that the transforming principle might be DNA2 :
•The purified substance gave a negative result in chemical tests known to detect proteins, but a strongly positive result in a chemical test known to detect DNA.
•The elemental composition of the purified transforming principle closely resembled DNA in its ratio of nitrogen and phosphorous.
•Protein- and RNA-degrading enzymes had little effect on the transforming principle, but enzymes able to degrade DNA eliminated the transforming activity.
In their now-legendary experiments, Hershey and Chase studied bacteriophage, or viruses that attack bacteria. The phages they used were simple particles composed of protein and DNA, with the outer structures made of protein and the inner core consisting of DNA.
Hershey and Chase knew that the phages attached to the surface of a host bacterial cell and injected some substance (either DNA or protein) into the host. This substance gave "instructions" that caused the host bacterium to start making lots and lots of phages—in other words, it was the phage's genetic material. Before the experiment, Hershey thought that the genetic material would prove to be protein4 .
To establish whether the phage injected DNA or protein into host bacteria, Hershey and Chase prepared two different batches of phage. In each batch, the phage were produced in the presence of a specific radioactive element, which was incorporated into the macromolecules (DNA and protein) that made up the phage.
•One sample was produced in the presence of 35S , a radioactive isotope of sulfur. Sulfur is found in many proteins and is absent from DNA, so only phage proteins were radioactively labeled by this treatment.
•The other sample was produced in the presence of 32P , a radioactive isotope of phosphorous. Phosphorous is found in DNA and not in proteins, so only phage DNA (and not phage proteins) was radioactively labeled by this treatment.
Each batch of phage was used to infect a different culture of bacteria. After infection had taken place, each culture was whirled in a blender, removing any remaining phage and phage parts from the outside of the bacterial cells. Finally, the cultures were centrifuged, or spun at high speeds, to separate the bacteria from the phage debris.
The work of the researchers above provided strong evidence for DNA as the genetic material. However, it still wasn't clear how such a seemingly simple molecule could encode the genetic information needed to build a complex organism. Additional research by many scientists, including Erwin Chargaff, James Watson, Francis Crick, and Rosalind Franklin, led to the discovery of DNA structure, clarifying how DNA can encode large amounts of information.
[Attribution and references]
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.
- 20 min
- Griffith’s experiment was the first experiment which suggested that bacteria can transfer genetic information through a process called transformation.
- The experiment concluded that bacteria are capable of transfering genetic information through transformation.
- The experiment conducted by Griffith found that bacteria are capable of transfering genetic information through transformation.
- Frederick Griffith wanted to learn if bacterial transformation was possible.
- Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteria: a type III-S and a type II-R.
The “Griffith's Experiment,” conducted in 1928 by English bacteriologist Frederick Griffith described the conversion of a non-pathogenic pneumococcal bacteria to a virulent strain.17 In this experiment, Griffith mixed the living non-virulent bacteria with a heat inactivated virulent form.
This video explains Griffith's experiment to prove the existence of a "transformation principle" via experimentation with mice and two kinds of pneumonia bac...
- 4 min
- 117.8K
- BOGObiology
Sep 30, 2008 · The discovery of DNA as the hereditary material was built upon decades of clinical research with the pneumococcus bacterium. Frederick Griffith's 1928 discovery of transformation galvanized ...
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Griffith's Experiment was an experiment done in 1928 by Frederick Griffith. It was one of the first experiments showing that bacteria can get DNA through a process. Griffith used two strains of Streptococcus pneumoniae. He then uses the bacteria to infect the mice, which have many different characteristics to humans.
