Unequivocal proof that DNA is the genetic material was first...

In 1928, Frederick Griffith, a bacteriologist from Britain, in a series of experiments with Streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed miraculous transformation in the bacteria. During the course of his experiment, a living organism (bacteria) had changed in it's physical form.

The unequivocal proof of DNA being the genetic material came from the experiments performed by Alfred Hershey and Martha Chase in the year 1952. They worked with bacteriophages, i.e, the viruses that infect bacteria by using radiolabeled T2 bacteriophages and their and their specific host E.coli cells, they concluded that DNA is the genetic material and not the proteins.