The Avery and Hershey-Chase Experiments: The Active Principle is DNA! LeAnne, Carly, and Drabe.

The Avery Experiments Avery provided conclusive evidence that DNA is the heredity material for the bacterial specimens under investigation. Oswald Avery (with his co-workers MacLeod and McCarty) characterized what they called the “transforming principle” from Griffith’s experiment. They prepared a mixture of dead S Streptococcus and live R Streptococcus. (That Griffith had used). Avery and his colleagues achieved 99.98% purity by removing as they could form their mixtures. The transforming activity was NOT reduced.

Avery Experiments (cont.) The properties of the transforming principle resembled those of DNA in several ways: When the purified principle was analyzed chemically, the array of elements agreed closely with DNA When spun at high speeds, the transforming principle migrated at the same level (density) as DNA Extracting the lipid and protein from the purified transforming principle did not reduce its activity Protein-digesting enzymes did not affect the principle’s activity, nor did RNA-digesting enzymes The DNA-digesting enzyme DNase destroyed all transforming activity

Avery Experiment: Conclusion The researchers concluded that “a nucleic acid of the deoxyribose type is the fundamental unit of the transforming principle of Pneumococcus Type III” – basically: DNA is the hereditary material for this bacterial species.

The Hershey-Chase Experiment Hershey and Chase provided further evidence that heredity material in bacteriophages was found in DNA, not in proteins. Many did not accept Avery’s conclusions until 1952 when Alfred Hershey and Martha Chase conducted this experiment with bacteriophages (viruses that attack bacteria)

The Hershey Chase Experiment In the Hershey-Chase experiment, bacterial viruses called phage, were used to demonstrate that DNA is the genetic material. The phage used in this experiment consisted of a DNA molecule surrounded by a protein coat. When phage infect bacteria, they attach to the surface of the bacterium and inject the DNA into the cell. The protein coat remains on the outside of the cell.

Hershey-Chase (cont.) In the first part of the experiment, phage were produced in a medium containing S-35 radioactively labeled amino acids. This resulted in a phage population with S-35 labeled proteins but no radioactive label in the DNA The phage were then allowed to infect the bacteria. They attached to the bacterial cell and injected their DNA, but the radioactively-labeled protein coat remained on the outside of the cell

Hershey-Chase (cont.) The phage produced in these cells contained no radioactivity. Vigorous shaking caused the empty protein coats to be removed, but did not interfere with production of new phage. In the second part, phage were produced in a medium containing P-32 labeled deoxyribonucleotides. This resulted in phage population with P-32 labeled DNA, but no radioactive label in the protein.

Hershey-Chase (cont.) When the phage infected the bacteria, the P-32 labeled DNA entered the cell and could not be found in phage subsequently produced in the infected bacteria. This demonstrated that the DNA, but not the protein, carries the genetic information for a new generation of phage! SWEET!