Avery's results were not widely accepted at first, because many biologists preferred to believe that protiens were the respository of hereditary informaton. Additional evidence supporting Avery's conclusion was provided in 1952 by alfred Hershey and martha chase, who experimented with bacteriophages, viruses that attack bacteria. Consist of either DNA or RNA surrounded by a protien coat. When a lytic(potentially cell-rupturing) bacteriophage infects a bacterial cell, it first binds to the cell's outer surface and then injects its hereditary information into the cells. There, the herditary information directs the production of thousands of new viruses within the baterium. The bacterial cell eventually ruptures, or lyses, releasing the newly made viruses.
To idenify the hereditary material injected into bacterial cells at the start of an infection, Hershey and chase used the bacteriophage T2, which contains DNA rather than RNA. They labeled the two parts of the viruses-the DNA and the protien coat-with different radioactive istopes that would serve as tracers. In some experiments, the viruses were gown on a medium containing an istopes of phosphorus, 32P, and the isotope was incorporated into the phosphate groups of newly synthesized DNA molecules. Because phosphorus is not in protien, this label is specific for DNA. In other experiments, the viruses were grown on a medium containing 35S, an isotope of sulfur, which is incorporated into the amino acids of newly synthesized proein coats. Because sulfur is not found in DNA, this label is specific for protien. The 32p and 35s isotopes are easily distinguished from each other because they emit particles with different energies when they decay.
After the labeled viruses were permitted to infect bacteria, the bacterial cells were agitated violently to remove the protein coats of the infecting viruses from the surface of the bacteria. This procedure removed nearly all the 35s label(and thus nearly all of the viral protein) from the bacteria. However, the 32p label(and thus the DNA) had transferred to the interior of the bacteria and was found in viruses subsequently released from the infected bacteria. Hence, the hereditary information injected into the bacteria that specified the new generation of viruses was DNA, not protein.
Comments
Post a Comment