Simulation · Biology · Class 12
Hershey and Chase: is it DNA or protein?
From the lesson The search for genetic material in Molecular Basis of Inheritance. Change the values and watch what happens.
Hershey and Chase: is it DNA or protein?Biology · Class 12
The idea behind it
NCERT § "The Search for Genetic Material"
- 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.