Not really an invention, as such. The DNA molecular chain was figured out in the sixties. I recommend the book " Double Helix ", which documents the procedures. There could be two answers to this question depending on your school of thought, either the correct conditions created simple proteins that eventually became DNA or God. If you are asking who discovered DNA, many people over a number of years have added to our knowledge of it, including...
Friedrich Miescher
Phoebus Levene
William Astbury
Frederick Griffith
Oswald Avery
Colin MacLeod
Maclyn McCarty
Alfred Hershey
Martha Chase ...more