The Gregorian monk Mendel noted that the inheritance of characters by pea plants, such as the character of tallness or of dwarfness, or the character of having purple flowers or white flowers, could be understood on the basis of hereditary units transmitted from the parent to the offspring. Thomas Hunt Morgan and his collaborators identified these units with genes arranged in a linear array in the chromosomes.

Then, toward the end of the lecture, he added the following comment:

The detailed mechanism by means of which a gene or a virus molecule produces replicas of itself is not yet known. In general the use of a gene or virus as a template would lead to the formation of a molecule not with identical structure but with complementary structure. It might happen, of course, that a molecule could be at the same time identical with and complementary to the template on which it is moulded. However, this case seems to me to be too unlikely to be valid in general, except in the following way. If the structure that serves as a template (the gene or virus molecule) consists of, say, two parts, which are themselves complementary in structure, then each of these parts can serve as the mould for the production of a replica of the other part, and the complex of two complementary parts thus can serve as the mould for the production of duplicates of itself [emphasis added].

As we shall soon see, had Pauling remembered his own pronouncement four years later when he was trying to determine the structure of DNA, he might have avoided making a terrible blunder.

Pauling started to turn his attention to DNA only in the summer of 1951. Until the early 1950s, most life scientists subscribed to the protein paradigm: the view that proteins, rather than nucleic acids, formed the foundation for life and were the crucial players in reproduction, growth, and regulation. The roots of this view could be traced to biologist Thomas H. Huxley (“Darwin’s Bulldog”), who believed that the protoplasm—the living part of the cell—was the source of all of life’s attributes. Proteins, which are built up of amino acids in a long chain, make up a large fraction of all living cells, while nucleic acids, as their name implies, were found first in the nuclei of cells.