Who discovered that dna was the genetic material or transforming factor that could convert nonvirulent r-type streptococcus pneumoniae bacterium to the virulent s-type?

Discovery
               In 1928 it was discovered by Frederick Griffith in an experiment generally known as transformation.

Experiment
                   In his experiment he considered two strains of streptococus pneumonia, one was R-type which was non-virulent and cause no disease in mice, other was virulent and S-type which cause disease and at last death of mice.
This experiment was comprised of four steps which are as follow:

 Step 1:
           First he injected living strain of S into mice, after sometime mouce died.

Step 2:
          He injected living strain of R into mice, the mice alive as he did not got any disease.

Step 3:
         He injected heat killed strain of S into mice and mice remain alive.

Step 4:
           He mixed living R strain with heat killed S strain and then inject into mice. As a result the mice died.

Conclusion:
                  It was found that genetic material from heat killed S stain were transferred to living R (non-virulent) strain, as a result R become virulent and cause the death of mice.

Covalent bond is the correct answer