Biology 9.1 Identifying Genetic Material Genetic Material: Viral Genes and DNA

Transformation Mendel’s experiments answered many questions about why you resemble your parents. You resemble your parents because you have copies of chromosomes from each. Chromosomes are each made up of many genes. The question that remained was what were these genes made up of?

Griffith’s Experiments In 1928, an experiment completely unrelated to the study of genetics led to an astounding discovery about DNA. Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against pneumonia.

Griffith’s Experiments A vaccine is a substance that is prepared from a killed or weakened disease-causing agent, including certain bacteria.

Griffith’s Experiments The vaccine is introduced into the body to protect the body against future infections by the disease causing agent.

Griffith’s Experiments Griffith worked with two types of strains of S. pneumonia. The first strain was enclosed in a capsule of polysaccharides. The capsule protects the bacterium from the body’s defense systems. This helps make the microorganism virulent or able to cause disease.

Griffith’s Experiments The second strain of the bacterium lacks the capsule and does not cause the disease. Those bacterium grown in the capsule developed smooth edges. Those in the second strain grown freely developed rough-edge colonies.

Griffith’s Experiments Griffith knew that mice infected with the virulent material grew sick and died, while mice infected with the non-lethal material were unharmed. To determine if the capsule on the lethal bacteria were causing the mice to die, Griffith injected the mice with dead S bacteria. The mice remained healthy and did not die.

Griffith’s Experiments Griffith than prepared a vaccine of weakened S bacteria by raising the temperature to a point where the bacteria were “heat-killed”, meaning that they could no longer reproduce. When Griffith injected mice with the heat-killed S bacteria, the mice lived.

Griffith’s Experiments He than mixed the harmless live R bacteria with the harmless heat-killed bacteria. Mice injected with these previously harmless materials died. When Griffith examined the blood of the dead mice, he discovered that the live R bacteria had acquired capsules. S R heat killed S R + Heatkilled S

Griffith’s Experiments Somehow the harmless R bacteria had changed and become virulent S bacteria. Griffith had discovered what is now called transformation. Transformation is a change in a genotype caused when cells take up foreign genetic material.

Avery’s Experiments Avery’s Experiments: The search for the substance responsible for transformation continued until 1944. Than, a series of experiments showed that the activity of the material responsible for transformation is not affected by protein-destroying enzymes.

Avery’s Experiments Avery’s Experiments: The activity is stopped instead by a DNA-destroying enzyme. Oswald Avery and his fellow scientists at Rockefeller Institute in New York City demonstrated that DNA is the material responsible for transformation.

Avery’s Experiments Although Avery’s experiments proved that the genetic material is composed of DNA, many scientists remained skeptical. In 1952, Alfred Hershey and Martha Chase performed an experiment that settled the controversy.

Avery’s Experiments It was known at the time that viruses, which are much simpler than cells, are composed of DNA or RNA surrounded by a protective protein. A bacteriophage (also known as a phage) is a virus that infects bacteria.

Avery’s Experiments It was also known that when these phages infect bacterial cells, the phages are able to produce more viruses, which are released when the bacteria cells rupture. Hershey and Chase concluded that the DNA of viruses is injected into bacterial cells, while most of the viral proteins remain outside. This meant that the DNA, rather than the proteins, is the heredity material, at least in viruses.

Avery’s Experiments These important experiments have shown that DNA is the molecule that stores genetic information in living cells.

Computer Lab: Today’s assignment, after completing your worksheet 9.1, is to use the internet to research and answer the following with a short written report. Answer all of the following in your report. DO NOT COPY, CUT,OR PASTE Who was Frederick Griffith and what did his experiments with viruses discover? Who was Oswald Avery and what did his experiments with viruses discover? Who were Alfred Hershey and Martha Chase and what did their experiments discover?