Scientists have used a variety of evidence from research to build our understanding of the chemical and structural nature of DNA.
One of the first pieces of evidence came from studies conducted by Frederick Griffith in the 1920s and 1930s. Griffith's experiments involved infecting mice with different strains of Streptococcus pneumoniae bacteria. He discovered that when he injected mice with a lethal strain of the bacteria that had been killed, along with a non-lethal strain of live bacteria, the mice still died. Upon examining the bacteria found in the dead mice, Griffith found that they had been transformed into the lethal strain. This transformation suggested that genetic material was being transferred between the different strains of bacteria.