DNA Experiments


Griffith’s Experiment

In 1928, Frederick Griffith conducted one of the first experiments to show that cells possessed genetic material

Griffith's experiment involved the use of two strains of pneumococcus – a deadly virulent strain (S) or a non-virulent strain (R)

  • When Griffith infected mice with the non-virulent bacteria (strain R), the mice survived
  • When Griffith infected mice with the virulent bacteria (strain S), the mice died
  • When Griffith infected mice with heat-killed virulent bacteria (strain R), the mice survived as the bacteria had been killed
  • When Griffith infected mice with a mix of heat-killed strain S and living strain R, the mice were found to have died


From this Griffith’s concluded that the living R cells had somehow been transformed into virulent S cells

  • This indicated that there was some form of transferrable genetic material present within the cells (i.e. DNA)


Griffith’s Experiment – Demonstrating Bacterial Transformation

Griffiths experiment



Avery-MacLeod-McCarty Experiment

Oswald Avery and colleagues expanded upon the findings of Frederick Griffith to demonstrate that DNA is the genetic material

  • They prepared cultures containing the heat-killed S strain and then removed lipids and carbohydrates from the solution
  • Next they treated the solutions with different digestive enzymes (DNase, RNase or protease) to destroy the targeted compound
  • Finally, they introduced living R strain cells to the culture to see which cultures would develop transformed S strain bacteria


Only in the culture treated with DNase did the S strain bacteria fail to grow (i.e. no DNA = no transformation)

  • This indicated that DNA was the genetic component that was being transferred between cells


Despite this finding, the scientific community was reluctant to accept the role of DNA as a genetic material

  • It was only 8 years later, when Hershey and Chase conducted their experiment, that the concept gained traction


Avery-MacLeod-McCarty Experiment – DNA as the Genetic Material of the Cell

Avery experiment


DNA Structure Elucidation

Four scientists are generally recognised as having contributed to the elucidation of DNA structure

  • Rosalind Franklin and Maurice Wilkins used X-ray diffraction techniques to identify key properties of the DNA molecule
  • Wilkins shared this data (without Franklin’s permission) with two other scientists, James Watson and Francis Crick
  • Watson and Crick used this data to help construct an accurate model of DNA structure (as a double helix)
  • In 1962, Watson, Crick and Wilkins were awarded the Nobel prize (Franklin’s critical efforts were not recognised)


Pioneers of DNA Structure

DNA pioneers