Biology 12

222 MHR • Unit 3 Molecular Genetics

then uses a tail-like projection to inject its genetic

material into the bacterial cell. The genetic material

reprograms the cell, causing it to produce new

viruses. These new viruses accumulate within the

bacterial cell until the cell ruptures and releases

the viruses to infect nearby cells.

Figure 7.8 illustrates the Hershey-Chase

experiment. The scientists knew that virtually all

of the phosphorus present in the T2 virus was in

its DNA, while sulfur was found only in its protein

coat. Consequently, they prepared two different

samples of the T2 virus, one tagged with radioactive

phosphorus and the other tagged with radioactive

sulfur. Each sample was then added to a separate

suspension of non-radioactive E. coli. After a

period of growth, the two resulting mixtures were

individually agitated in a blender to shake off the

part of the virus that remained attached to the

exterior cell wall of the bacterium after the cell was

infected. Finally, the infected bacteria were separated

from each mixture, leaving the viral protein coats

in suspension. The results were then analyzed.

In both cases, the bacteria became infected by

the phage. In the first sample, in which the viral

DNA was radioactive, the infected bacteria were

http://www.mcgrawhill.ca/links/biology12

Griffith’s discovery of the transforming principle led some

researchers to propose that transformation could occur in

eukaryotes as well. What effect would this viewpoint have on

theories about inheritance and development? Use the Internet

to research the history of scientific thought from 1900 to 1950.

What other world events might have influenced how scientific

theories were applied during these years? Go to the web site

above, and click on Web Links. Write down some ideas to

discuss with your class.

WEB LINK

radioactive

sulphur in

protein coat

radioactive

phosphorus in

DNA

non-radioactive

DNA

non-radioactive

protein coat

“ghosts” sheared

off by blender

“ghosts” and bacteria

separated by centrifuge

radioactive non-radioactive non-radioactive radioactive

Sample 1 Sample 2

Figure 7.8The experiments conducted by Hershey and Chase demonstrated that

when a virus infects a bacterium, only the DNA of the virus enters the host cell.

Two batches of phages are cultured. One has

radioactively tagged DNA, while the other has a

radioactively tagged protein coat. A sample of

each type of virus is added to a separate

suspension of non-radioactive E. coli.

A

B The viruses inject their DNA into the bacteria.

Each suspension is shaken in a blender to

separate the virus heads or “ghosts” from the

outside of the cell walls of the infected bacteria.

C

The bacterial cells infected by the virus with

radioactive DNA are found to be radioactive,

indicating that the viral DNA entered the host

cell. In contrast, the bacterial cells infected by

the virus with radioactive protein are found to be

non-radioactive, indicating that no viral protein

entered the host cell.

D