(Follow along on page 226 of your textbook.)

The year was 1928… Frederick Griffith, an army medical officer, was attempting to develop a vaccine against Streptococcus pneumoniae. Griffith never did develop a vaccine. But his work unexpectedly opened a door to the molecular world of heredity.

Griffith’s Bacteria Griffith isolated and cultured two different strains of the bacterium. He noticed that colonies of one strain had a rough surface appearance, but those of the other strain appeared smooth. He designated the strains “R” and “S” and used them in a series of four experiments.

Laboratory mice were injected with living “R” cells. The mice did not develop pneumonia. Experiment #1

Laboratory mice were injected with living “R” cells. The mice did not develop pneumonia. Experiment #1 (The “R” cells were harmless.)

Laboratory mice were injected with living “S” cells. Experiment #2

The mice died. Experiment #2 Blood samples taken from them teemed with live “S” cells. (The “S” strain was pathogenic.)

“S” cells were killed by exposure to high temperatures. Mice injected with these cells did not die. Experiment #3

“S” cells were killed by exposure to high temperatures. Mice injected with these cells did not die. Experiment #3 (The heat-killed “S” cells were harmless.)

Live “R” cells were mixed with heat-killed “S” cells and injected into mice. Experiment #4

The mice died. Experiment #4 Blood samples taken from them teemed with live “S” cells.

What Happened? Maybe the heat-killed “S” cells in the mixture weren’t really killed. But what if that were true? What should have happened to the mice in Experiment #3?

What Else Might Have Happened? Maybe the harmless “R” cells in the mixture mutated into a killer form. But what if that were true? What should have happened to the mice in Experiment #1?

The Simplest Explanation: Heat did kill the “S” cells but did not destroy their hereditary material, including the part that specified “how to cause infection.” Somehow, that material had been transferred from dead “S” cells to living “R” cells where it was put to use. “R”“S”

The Importance of Griffith’s Work Essentially, what Griffith had done was similar to putting a harmless live kitten and a stuffed toy tiger cub in a box and suddenly finding yourself with a live roaring tiger!!!!

The Importance of Griffith’s Work Griffith called the process he had observed “transformation.” He had shown that something had been transferred from the heat-killed disease- causing bacteria to the live harmless bacteria. Scientists soon began searching for this unknown material.

Today, we know this material is:

Oswald Avery, 1944

Hershey and Chase (see Online Activity 11.1)