DNA is the genetic material

Miescher first isolated DNA, which he called "nuclein", from the nuclei of pus cells. For a long time scientists were unsure whether DNA or protein carried the hereditary instructions. Three experiments decided it.

Griffith (1928) worked with the bacterium Streptococcus pneumoniae, which has two strains. The smooth (S) strain has a capsule and kills mice; the rough (R) strain has no capsule and does not.

• Live S → mouse dies. Live R → mouse lives. Heat-killed S → mouse lives.
• But heat-killed S mixed with live R → mouse dies, and live S bacteria were found in it.

Something from the dead S bacteria had changed the live R bacteria into S. Griffith called it the transforming principle.

Avery, MacLeod and McCarty (1944) found out what it was. They treated extract from S bacteria with enzymes that destroy RNA (RNase), protein (protease) or DNA (DNase). Transformation still happened after RNase and protease, but stopped after DNase. So the transforming principle is DNA.

Hershey and Chase (1952) used a virus that infects bacteria (the T2 bacteriophage). They labelled the virus's DNA with radioactive phosphorus (³²P) and its protein with radioactive sulphur (³⁵S), let it infect E. coli, then shook (blended) and spun (centrifuged) the mixture. The ³²P went inside the bacteria (in the pellet); the ³⁵S stayed outside (in the liquid). Only the DNA entered the cells, so DNA carries the instructions to make new viruses.

Later work by Chargaff, Wilkins, Franklin, Watson and Crick revealed DNA's structure and how it stores information.