Tests for the chemical nature of genes
The Feulgen reaction for DNA:
In 1912 Feulgen discovered that if DNA were treated with warm acid(hydrolysis), there would be a reddish-purplish staining reaction after Schiff's reagent is added. This reaction had been found to be specific for DNA, and neither RNA nor any other cell substance will give such response. When the reaction was carried out in cells, only the chromosomes show the Feulgen reaction, and all other areas remain relatively colourless.
The specificity of the Feulgen reaction for chromosomes and DNA has enabled a number of quantitative studies to be made by measuring the amount of light transmitted through Feulgen stained nuclei(cytophotometry). Cytophotometry experiments have shown a constancy of the amount of DNA for all nuclei of an organism.
Transformation
Griffith's experiment:
Evidence that a chemical is the genetic material came in 1928 from the investigation of Frederick Griffith, a British medical officer on the causative bacterium, now renamed Streptococcus pneumonia. In 1923 he discovered that besides capsule forming virulent strains, there were other types of Streptococcus pneumonia, that did not have a capsule and did not cause pneumonia. Pneumococci Type II(R) bacteria are avirulent i, e they do not kill the mice when injected. The Type III(S) is virulent because of presence of a polysaccharide capsule surrounding the cell, which protects the cell from destruction by white blood cells. When grown on blood agar medium in petri plates, pneumococci with capsules form large smooth colonies and are thus designated Type S. He injected(1) live virulent cocci Type III(S) into one group of mice, (2) live avirulent cocci Type II(R) into another group and (3) heat killed(and therefore no longer virulent) cocci type III(S) into mixed with live avirulent cocci Type II(R) into a fourth group. The mice of first group died and second and third groups were not affected by the injections as expected but the mice in the fourth group became ill and died in spite of the fact that the only live bacteria were harmless ones. Griffith carried out biopsies of these dead mice and discovered that their respiratory tracts contained Type III(S) virulent bacteria. The change in the cocci could not have arisen by mutation. So something from the dead virulent cocci Type III(S) was transferred to the live avirulent cocci Type II(R) during their coexistence within the mouse transforming type II(R) into virulent cells- was called transformation. That component of the dead bacteria responsible for transformation was called the transforming principle. It was further shown that the newly formed Type III(S) cells reproduced true to type for many generations, indicating that the transformation had directly affected their genetic material. Griffith's work provided indirect evidence that a chemical was the genetic material.
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| Griffith's experiment |
