For the first development of a biochemical answer, credit goes to Oswald T. Avery and his colleagues at the Rockefeller Institute for Medical Research. Avery had devoted much of his career to understanding the most frequent causative agent of pneumonia, a bacterium then called the pneumococcus. Pneumococci come in many types, each one of which breeds true in laboratory cultures. However, if one mixes an extract of dead bacteria of one type with a living culture of another type, some of the living bacteria are "transformed" to the type of the donor of the extract and then breed true. Avery (who was in his sixties and already on emeritus status) and two young colleagues (Maclyn McCarty and Colin MacLeod) set out to uncover the chemical nature of the transforming factor. They found that it was an already known substance called deoxyribonucleic acid, found mainly in the nucleus of cells. This jawbreaker name is commonly shortened to DNA. These findings were published in 1944 and Avery correctly proposed that DNA was the "stuff" of inheritance.

figure 5
Short Length of a DNA molecule
(Riboses in red, nucleic acids in blue)

When it became apparent that DNA carried the messages necessary for inherited characters, genetic research began to center on DNA. Only understanding the structure of this oversized and apparently complicated molecule would allow scientists to work out how Mendel's "discrete packets" were passed from one generation to another. In a physics laboratory in Cambridge, England, James D. Watson and Francis Crick won the race in 1953 by deducing from X-Ray crystallography and molecular modeling that the structure was a long repeating double helix chain of simple sugar (deoxyribose) molecules and only four similar nucleic acids. Others soon found that the genetic code for proteins was contained in the order of the nucleic acids. Each triplet of nucleic acids encodes for one amino acid in a protein. Four nucleic acids in triplets make 64 possible combinations; more than enough to provide a distinct code for each of the 20 amino acids found in nature. So DNA encodes for the proteins, both short and long, that are essential building blocks for the functions of life.

Mutations are the product of changes, which may be in a single nucleic acid, that in turn change a single amino acid in a protein, so that the protein's character is changed. Random changes in these nucleic acids are a major source of the random variations of Darwin's theory. Thus we find ourselves with an elegant chemical explanation of how evolutionary change comes about. Mutations that have survival value are passed on to descendants; those that are not helpful fail to be perpetuated. In nature mutations occur at random. Animal experiments going back to Morgan's fruit flies show that the rate of mutation can be increased by exposure to X-irradiation.

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