Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Page 3 of 13
DNA in Viruses

Hershey and Chase further proved that DNA is the carrier of genetic material by radioactively labeling the DNA contained in viruses.

In 1952—seven years after Avery's demonstration that genes were DNA—two geneticists, A. D. Hershey and Martha Chase, provided further proof of Avery's discovery. They worked with a DNA virus called T2, which infects the bacteriaEscherichia coli (E. coli). T2 is a bacteriophage, a virus that only infects bacterial cells. A virus is a small infectious agent that contains little more than DNA or RNA, surrounded by a protective protein coat. A virus can replicate only inside the living cells of organisms, and thus is not considered a true cell on its own. Hershey and Chase found that when T2 bacteriophages containing radioactive phosphorus (meaning radioactive DNA) were allowed to infect nonradioactive E. coli bacteria, all the infected cells became radioactive (see above figure) and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages. However, when the bacteria were infected with bacteriophages labeled with radioactive sulfur and incorporated into the phage's proteins, practically no radioactivity could be detected in the infected E. coli cells. From these experiments, it was clear that the DNA component of the bacteriophages is injected into the bacterial cell, while the protein component remains outside. Furthermore, it is the injected component, DNA, that is able to direct the formation of new virus particles complete with protein coats. This experiment further proved that genes (heritable factors) are DNA.

Although Hershey and Chase were cautious in their interpretation, previous, contemporaneous, and subsequent discoveries all served to prove that DNA is the hereditary material. Knowledge of DNA gained from these discoveries led to applications in forensics, crime investigation, and genealogy.