The structure of genetic material and how it encodes for the synthesis of proteins, was explained by several scientists in several steps of discovery. Chromosomes are visible during cell division using a light microscope and scientists recognized as early as 1900 that genes were carried on chromosomes which appeared to consist of polymers of both nucleic acids and proteins. It was first thought that the proteins were the genetic material from which all life got its traits, and that nucleic acids were simply a structural component of the chromosome. However, it was soon demonstrated that this was not the case.
During the early 1900s, the pieces of the gene puzzle were gradually being assembled as several laboratories reported results of investigations into the nature of genetic material. However, historical accounts in biology, biochemistry and microbiology textbooks tend to hold up the following two lines of experimentation as being critical cornerstones for the discovery of DNA as the encoding material of genes:
Experiments by Fred Griffith in 1928, and Oswald Avery over 10 years later, showed that the genetic material encoding virulence of Diplococcus (now Streptococcus) pneumoniae strains in mice, consisted of DNA.
In 1952, A.D. Hershey and Martha Chase used radiolabeled sulfur (35S) and phosphate (32P) to demonstrate that viral DNA, and not the protein-based shell, or “capsid”, was the infectious agent responsible for viral replication in infected bacterial cells where it became part of the progeny phage.