Re: How do we know that DNA holds the genetic code?

A very good question! The Griffith and Avery experiments are "classic" in showing that DNA is the genetic material. Not long after Avery's work, Alfred Hershey and Martha Chase did their famous blender experiment which also showed that DNA is the genetic material. They used phage, viruses that infect bacteria. Hershey and Chase labelled the inside of the phage (DNA) with radioactive phosophorus and labelled the outside (proteins) with radioactive sulfur. They then infected bacteria with the labelled phage and used a Waring blender to make sure that only the viral DNA made it into the bacteria. Afterwards, they checked to see where the radioactivity went. The infected bacteria always contained radioactive phosphorus and new phage particles also retained some radioactive phosophorus. Thus, DNA and not proteins carried the genetic instructions for infecting bacteria.

You can see an animated version of this explanation in DNA from the Beginning (you will need the Macromedia Flash Player). The first part of the animation in Concept 18 deals with Joshua Lederberg's experiments that showed how one bacterium can pass genes to another -- also indicative of DNA being the genetic material. The second half of the animation is the Hershey-Chase experiment.

Of course, these days scientist can make recombinant DNA and express it in a variety of model organisms. In the 1970s, Stanley Cohen and Herbert Boyer were among the first to make a bacteria that expressed a "new" gene. And we know from mutation studies that changes in the DNA sequence can cause big changes in the protein sequence and thus affect the organism.

I hope this answers your question.