Question

explain how scientists were able to suggest a molecule associated with heredity using strains of pneumonia-causing bacteria. In your explanation include, one control experiment and at least one experiment as proof of such a hereditary molecule.

Answer 1

The experiments conducted to show that DNA was the molecule that passed on hereditary information were performed using Streptococcus pneumoniae. S. pneumoniae had been known to exist as different types or strains.

Two such strains were the Rough and Smooth type strains, so named because of the smooth colonies of the strain. This strain was also Virluent - meaning, it was able to infect Mice. The rough strain did not have smooth colonies and was unable to infect mice.

In these experiments, the Smooth strains were killed by heat and its constituents (Proteins, Polysaccharides, DNA) were separated by biochemical methods. The scientists showed that Mice injected with the Rough strain of S. pneumoniae, survived and thus the Rough strain was unable to cause death of injected mice.

In later experiments, scientists injected mice with either the heat-killed smooth strain cells, or a mixture of Rough strain cells + Smooth strain protein, or a mixture of Rough strain cells + Smooth strain DNA, or a mixture of Rough Strain cells + Smooth strain polysaccharide.

Of all these mixtures, neither the Rough strain alone or the heat killed Smooth strain cells (these were the controls) were able to kill mice. The mixture of Rough strain cells with either Protein or Polysaccharide was also unable to kill the injected mice.

However, when a mixture of Rough strain S. pneumoniae cells with DNA from heat killed Smooth strain cells was injected into Mice, the mice were injected and perished. When cells were isolated from these dead mice, the strain observed was the smooth strain. This showed that DNA was able to 'transform' Rough strain cells into Smooth strain cells and alter it's morphology, since the information pertaining to the synthesis of the Smooth colony phenoype was inherited by the Rough strain cells when Smooth strain DNA was present.