As early as 1848, Wilhelm Hofmeister, a German botanist, has observed that cell nuclei resolve themselves into small, rodlike bodies during mitosis. Later these structures were found to absorb certain dyes and so came to be called chromosomes (colored bodies).
In 1869, Friedrich Miescher, a Swiss physician, isolated a substance from cell nuclei, that he called nuclein - now known as DNA.
During the early years of the 20th century, cellular studies using ordinary light microscopes clarified the behavior of chromosomes during mitosis and meiosis, which led to the conclusion that chromosomes are the carriers of genes – the basic units of heredity.
The chromosomes of eukaryotes contain a variety of proteins in addition to DNA. Soon the question arose whether the nucleic acids (DNA) or proteins, or both together, are the carriers of the genetic information. Until the early 1950s most biologists were inclined to believe that the proteins were the chief carriers if heredity. Nucleic acids contain only four different unitary building blocks, but proteins are made up of 20 different amino acids. Proteins therefore appeared to have a greater diversity structure, and the diversity of the genes seemed first likely to rest on the diversity of the proteins.
In 1928, Frederick Griffith, an English army doctor, wanted to make a vaccine against a bacteria named Streptococcus pneumoniae, which caused a type of pneumonia. Since the time of Pasteur, about 50 years before, vaccines had been made using killed microorganisms which could be injected into patients to elicit the immune response of live cells without risk of disease. Though he failed in making the vaccine he stumbled on a demonstration of the transmission of genetic instructions by a process we now call the "transformation principle".
He found that the bacterium had two forms when grown on agar plates, a smooth (S) and a rough (R) form. The R bacteria were harmless, but the S bacteria were lethal when injected into mice. Heat-killed S cells were also harmless - the same effect seen by Pasteur. However, surprisingly when live R cells were mixed with killed S cells and injected into mice the mice died, and the bacteria rescued from the mice had been "transformed" into the S type.
This experiment strongly implied that genetic material had been transferred from the dead to the live cell. It was hard to be certain of this, or to know what exactly genetic material was transferred and was responsible for the transformation process.
Sixteen years later, in 1944, the team of Avery, MacLeod and McCarty revisited this experiment and attempted a more definitive experiment. They extracted from S bacteria nucleoid purified DNA, proteins and other materials and mixed R bacteria with these different materials, and only those mixed with DNA were transformed into S bacteria.
This Experiment strongly implied that DNA is the “transforming factor” and not proteins or other materials and by this was demonstrated what is known to us as the transforming principle - that genes are made of DNA.
While Avery and his team were nominated several times for the Nobel Prize, they never received it. It is a universally recognized omission of the Nobel committee since their work was a blazing discovery that changed everything. For more about this topic see the link section - "The Nobel Prize Omission"
Those experiments are intended for high motivated high school students since both experiments, and especially Avery's need some extra biochemistry knowledge.
Before you start, please pay notice that these experiments can be dangerous since you will have to experiment with bacteria that can be infectious and mouse, when not given appropriate care, can transmit disease.
As a rule: this experiment should be performed under the supervision of teachers or adults familiar with biochemistry/biomedical safety procedures. Consult your teacher or other knowledgeable adults and experts about how to obtain the mentioned materials and supplies and how to use them properly and safely in these experiments. Do not do this experiment alone! Notify your parents and school authorities before you begin - wait for their approval.
If you have decided to embark on this challenge, before you begin, consult the link section of this page and further resources provided. Ensure that you understand the basic principals. Surf the web further and consult your local library, your teachers and other knowledgeable adults and experts.
Good Luck
Griffith's and Avery, MacLeod and McCarty's Experiments
Genetics and Genomics Timeline:1944 - Genome News Network
The Oswald T. Avery Collection - U.S. National Library of Medicine
Avery, Macleod McCarty's Article from 1944 - MicroBiology
Two Interviews with Avery Oswald - The Lasker Foundation
An Interview with Maclyn McCarty - The Lasker Foundation
Griffith's and Avery, MacLeod and McCarty's Experiments
Linus Pauling and the Race for DNA - Oregon State University
The Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types
The Transforming Principal - Maclyn McCarty
Structure and Replication of DNA - The Chinese University of Hong Kong
The Experiment That Transformed Biology - The Journal of Experimental Medicine
Structures of DNA and RNA - Molecular Biology and Biochemistry
How Can DNA be the Genetic Material? - Jeff Elhai
The Nobel Prize Omission
Maclyn McCarty Dies - The New York Times
How Rhetorical Choices Can Influence the Impact of a Scientific Paper
Prematurity in Scientific Discovery - Ernest B. Hook, Editor
Science Fair Projects & Experiments
DNA / RNA Science Fair Projects
DNA Experiments
DNA Identification
 Oswald Avery and the Story of DNA (Unlocking the Secrets of Science) |
 Francis Crick and James Watson: And the Building Blocks of Life |
 Francis Crick and James Watson: Pioneers in DNA Research (Unlocking the Secrets of Science) |
