The document summarizes key evidence and experiments that helped establish the nature of genes as made of DNA: 1. In the 1940s-50s, experiments showed that genes were made of DNA, not proteins, including Avery's finding that a bacterial agent causing genetic change was inactivated by DNAase. 2. Analysis of DNA content found it was constant in all body cells of an organism and diploid in somatic cells versus haploid in germ cells. 3. Hershey and Chase's experiment in 1952 demonstrated that the genetic material injected by bacteriophages into bacteria was DNA, not proteins. 4. All these findings helped Watson and Crick propose their 1953 double helix