Lesson 4 of 13 · 7 min
The search for genetic material
NCERT § "The Search for Genetic Material"
Long before anyone could sequence a gene, three experiments — on pneumonia bacteria, then on viruses — settled a fight over whether heredity was written in protein or in DNA. The proof that applies to Griffith's bacteria is the same proof that says your β-globin gene, too, is written in DNA.
The lesson in notes
In short
Griffith (1928) used Streptococcus pneumoniae. The S strain has a smooth, shiny mucous (polysaccharide) coat and is virulent; the R strain has rough colonies, lacks the coat and is harmless to mice.
Mice died from live S but not from live R or heat-killed S. Yet a mixture of live R with heat-killed S killed the mice, and live S bacteria were recovered from them.
Griffith concluded that R bacteria had been transformed by some 'transforming principle' from the dead S cells, which let them make the coat and become virulent. He did not identify the chemical.
Between 1933 and 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty purified the biochemicals from heat-killed S cells. Transformation continued in the presence of proteases and RNases but was stopped by DNase, showing DNA to be the transforming substance. Not every biologist accepted this.
Alfred Hershey and Martha Chase (1952) grew bacteriophages either with radioactive phosphorus, which labels DNA (proteins lack P), or with radioactive sulphur, which labels protein (DNA lacks S).
The phages infected E. coli; blending then removed the viral coats and centrifugation separated the bacteria from the free virus particles.
Bacteria infected by ³²P phages were radioactive, while those infected by ³⁵S phages were not, the ³⁵S staying in the supernatant. So DNA, not protein, enters the bacterium and is the genetic material.
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Hershey and Chase step by step
Biology at Ease · English · Lecture · Open on YouTube