The final proof for DNA as the genetic material came from the experiments of:

While Griffith observed transformation and Avery et al. identified the "transforming principle" as DNA, the unequivocal (final) proof came from Hershey and Chase (1952). They used radioactive isotopes (${}^{35}S$ and ${}^{32}P$) and bacteriophages to show that only DNA, not protein, enters the bacteria to direct reproduction.