Biology 12

220 MHR • Unit 3 Molecular Genetics

Several decades passed before Levene’s conclusion

was finally corrected.

Mounting Evidence for the

Role of DNA in Heredity

One important piece of evidence that DNA was, in

fact, the material of heredity came in 1944, when

the team of Oswald Avery, Colin MacLeod, and

Maclyn McCarty published the results of their

experiments with bacteria. These experiments,

which built on the 1928 work of British researcher

Fred Griffith, were conducted over a period of nearly

15 years. As illustrated in Figure 7.5, Griffith showed

that when a heat-killed, pathogenic (disease-

causing) strain of the bacterium Streptococcus

pneumoniaewas added to a suspension containing

a non-pathogenic strain, the non-pathogenic strain

was somehow transformed to become pathogenic.

Avery and his colleagues undertook several

important steps to isolate the agent behind this

transformation, which Griffith had called the

transforming principle. When they treated a

suspension of the pathogenic bacteria with a

protein-destroying enzyme, they noticed that the

transformation of non-pathogenic bacteria into a

pathogenic strain still took place. When the

pathogenic bacteria were treated with a DNA-

destroying enzyme, however, the transformation

did not take place. Finally, when the bacteria were

treated with an enzyme that destroyed RNA but

not DNA, the transformation occurred again. This

demonstrated that the substance responsible for

the transformation of the non-pathogenic bacteria

into a pathogenic strain was DNA.

Although the work of Avery, MacLeod, and

McCarty provided strong evidence for the role of

DNA in determining cell function, the results were

Figure 7.5Griffith’s discovery of the “transforming

principle” in 1928 was accidental. He employed heat-killed,

pathogenic bacteria as a control in an experiment on

infection, but did not treat the cells at a high enough

temperature to denature their DNA. In so doing, he

discovered that the dead cells’ pathogenic properties

could be passed on to living bacterial cells. Griffith died of

injuries suffered in an air raid during World War II before he

could discover what caused this transformation. In 1944,

Avery and his team were the first to demonstrate that the

transforming principle was DNA.

mice live

heat-killed

pathogenic strain

of S. pneumoniae

mixture of heat-killed

pathogenic and live

live pathogenic

strain of

S. pneumoniae

live nonpathogenic

strain of

S. pneumoniae

nonpathogenic strain

of S. pneumoniae

mice die

mice die

mice live

+

When a heat-killed, pathogenic strain of

Streptococcus pneumoniaeis injected

into mice, the mice live.

A

When live, pathogenic S. pneumoniae

bacteria are injected into mice, the

mice die.

B

When a live, non-pathogenic mutant

strain of the same S. pneumoniae

bacteria is injected into mice, the

mice live.

C

When heat-killed, pathogenic bacteria are

added to a suspension containing the live,

non-pathogenic strain of bacteria,

transformation occurs and the colony

of non-pathogenic bacteria become

pathogenic. When these bacteria are

injected into mice, the mice die.

D