That genes, located on chromosomes, are the hereditary material was known to scientists in the early twentieth century. That genes are segments of DNA became evident from the work of Griffith on bacterial transformation.
The bacterium Streptococcus pneumoniae when grown in the lab forms smooth colonies and when injected into mice kill them. A mutant of this bacterium forms rough colonies and is harmless to mice. In 1928, Frederick Griffith found that if the smooth virulent form of Streptococcus is killed and mixed with the harmless rough form of Streptococcus the latter becomes virulent (killer). This change (or transformation) of the bacteria from harmless to virulent is termed bacterial transformation.
In 1944, Avery, Mcleod and McCarty extracted DNA from the virulent smooth Streptococcus and mixed it with the non-virulent rough variety. The non-rough variety became virulent and had a smooth coat. This did not happen when DNA of the virulent form was digested with the enzyme DNase and then mixed. Thus it became clear that DNA was the transforming principle.
Later Hershey and Chase in 1952 used T2 bacteriophage, a virus which infects bacteria for their experiments. They labelled the protein coat of the virus with radioactive isotope of sulphur 35S. When the virus was introduced into the bacteria, no radioactivity was found inside the bacteria as the viral coat was left outside. When they labelled viral DNA with 52P32 or radioactive phosphorus, radioactivity was found inside the bacteria. It bacame clear that new generations of the virus were reproduced inside bacteria because of viral DNA.
These experiments confirmed that DNA is the genetic material and genes are made of Deoxyribonucleic Acid or DNA.