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646 Chapter 19 NEL

Confirming the Chemical of Heredity

Frederick Griffith’s work in the 1920s began because he was trying to develop a vaccine

against pneumonia caused by Streptococcus pneumoniae. However, his unexpected exper-

imental observations, followed by the work of Avery, McCarty, and MacLeod, led scientists

to begin questioning the initial assumption within the scientific community that the

material of heredity was protein. What was now needed was experimental evidence that

would clearly and conclusively indicate that DNA was indeed the material of heredity.

This evidence was to come some six years after the work of Avery’s team as the result of

an innovative experiment.

Alfred D. Hershey and Martha Chase

It was not until 1952 that DNA was accepted as the hereditary material. That year,

American scientists Alfred Hershey and Martha Chase conducted experiments using a

virus (bacteriophageT2) that infects a bacterial host (Figure 7). Bacteriophages (com-

monly called phages) consist of two components: DNA and a protein coat. A bacterio-

phage infects a bacterial cell by attaching to the outer surface of the cell and injecting its

hereditary information into it. This leads to the production of thousands of new viruses,

which then burst out of the cell, resulting in its death. The results of Hershey and Chase’s

experiments showed that only the DNA from the bacteriophage, and not the protein

coat, enters the bacteria to direct the synthesis of new viral DNA and new viral protein coats.

bacteriophagea virus that infects
bacteria

Figure 7
Micrograph of a bacteriophage
injecting its DNA into a bacterial
cell

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Canadian Achievers—Avery and MacLeod

Canadian-born scientists Dr. Oswald Avery and Dr. Colin MacLeod spent their early years as

scientists in Nova Scotia, where they were born. They met in New York, where, together with

American scientist Maclyn McCarty, they painstakingly isolated components of pneumococci

(Streptococcus pneumoniae) for over a decade before identifying DNA as the transforming

principle. You can find more information on this classic experiment in an animation by

accessing the Nelson science Web site.

Figure 5
Dr. Oswald Avery

Figure 6
Dr. Colin MacLeod