The most fundamental property of the genetic material is that it must be able to exist in an almost infinite variety of forms: each cell contains a large number of different genes, each controlling a different heritable trait and each presumably having a structure slightly different from any of the other genes in the cell. Apart from infinite varieties, genetic material must be capable of precisely directing its own replication so that every daughter cell receives an exact copy. Some minor changes should also occur at the rate of one error in a billion.

Although these speculations are quite sound, unfortunately most biologists concluded that protein is the genetic material. Proteins have necessary complexity and structure to form a long polymer. The twenty naturally occurring amino acids can be combined in almost unlimited variety, creating thousands and thousands of different types of proteins virtually. On the other hand, DNA was believed to be relatively small, invariant molecules with identical molecular weights. Each molecule of DNA was thought to be exactly the same as every other molecule of DNA, thus it did not have the variability required of the genetic material. The presence of DNA in chromosomes was attributed to structural functions.