Question

In the early 20th century, there was significant debate about what molecules were the hereditary material in living cells. Choose Frederick Griffith’s experiments with lethal S and nonlethal R strains of bacteria or Avery, McCarty, and MacLeod’s continued experiments with the bacteria and answer the following:

a. Summarize the experimental procedures and findings.
b. Explain how the experimental results supported the idea that DNA is the hereditary molecule in cells.

Answer 1

1. Griffith in his experiment used two related strains of bacteria (Streptococcus pneumonia), known as R and S and mice, trying to develop a vaccine against pneumonia. R strain-formed nonvirulent, rough-edged colonies

S strain- rounded and smooth colonies, with sugar protection coat, virulent

Mice that were injected with S strain developed pneumonia and died.

But, when mice were injected with heat-killed S strain it did not cause disease in mice.

The next part of experiment is the injection of combined harmless R bacteria with harmless heat-killed S bacteria. The result was that the mouse developed pnenumonia and in blood sample from the dead mouse, living S bacteria were found.

2. From his experiment, Griffith concluded that injected together, R strain and S strain bacteria most likely “communicate”. The R-strain bacteria took "transforming principle" (we know today that this is genetic material DNA) from the heat-killed S bacteria which allowed them to "transform" into virulent bacteria.