Question

(a) Describe the series of experiments of F. Griffith. Comment on the significance of the results obtained.

(b) State the contribution of Macleod, McCarty and Avery. [5]

Answer 1

(a) In 1928 Frederick Griffith, worked on Streptococcus pneumoniae (bacterium responsible for pneumonia).

When Streptococcus pneumoniae bacteria are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R). This is because the S strain bacteria have a mucous (polysaccharide) coat, while R strain does not.

Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia.

$\begin{array}{ccccc}\text{S strain}& ⟶& \text{Inject into mice}& ⟶& \text{Mice die}\\ \text{R strain}& ⟶& \text{Inject into mice}& ⟶& \text{Mice live}\end{array}$

Griffith was able to kill bacteria by heating them. observed that heat-killed S strain bacteria injected into: mice did not kill them.

$\begin{array}{ccccc}\text{S strain}& ⟶& \text{Inject into mice}& ⟶& \text{Mice die}\\ \text{(heat - killed)}& & & & \\ \text{S strain}& & & & \\ \text{(heat-killed)}& & & & \\ +& ⟶& \text{Inject into mice}& ⟶& \text{Mice die}\\ \text{R strain}& & & & \\ \text{(live)}& & & & \end{array}$

When he injected a mixture of heat-killed S and live R bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice. [2]

He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria and had become virulent. [1]

(b) Contribution of Macleod, Mc Carty and Avery : They purified biochemicals (proteins, DNA, RNA) from the heat killed S cells to see which ones could transform live R. cells into S cells. They discovered that DNA alone from S bacteria caused the bacteria to become transformed. They also discovered protein and RNA digesting enzymes did not affect transformation. Hence, they concluded that DNA is the hereditary material. [2]