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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.
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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!