Discovery of and Evidence for DNA as the Genetic Material


DNA was first discovered in 1869 by a Swiss biochemist, Johann Friedrich Miescher. He extracted a gelatinous material that contained organic phosphorus from cells in human pus that was obtained from the bandages of wounded soldiers. He named this material nuclein. Ten years later Albrecht Kossel explored the chemistry of nuclein (for which he received the Nobel Prize) and discovered that it contained the organic bases adenine, thymine, guanine, and cytosine. In 1889 Richard Altman removed the proteins from the nuclein in yeast cells and named the deproteinized material nucleic acid. It was not until about 1910 that it was realized that there were two types of nucleic acid, DNA and RNA. A great deal of chemistry during the early part of the twentieth century focused on characterizing the composition of and the linkages in both DNA and RNA. A chemical test for deoxyribose, developed by Robert Feulgen during the 1920s, was the first test capable of distinguishing DNA from RNA. Because of the simplicity of the composition of DNA, which has only four bases (and early reports indicated erroneously that there were equimolar quantities of each), it was originally thought that DNA molecules functioned in chromosomal stability and maintenance. It was only after Erwin Chargaff, in 1950, showed that the molar amounts of the bases varied widely in different organisms that the notion that DNA might be the genetic material became an attractive idea.

The general consensus prior to the mid-1940s was that proteins (which contain twenty different amino acids) were the most logical candidate for the genetic material. Three later, however, findings pointed toward the conclusion that DNA was the genetic material. During the 1920s Frederick Griffith examined the activity of cell extracts in an attempt to identify a "transforming principle" (and a specific molecule related to this principle) in experiments with the bacterium Streptococcus pneumoniae. Unfortunately, his experiments failed to identify a specific molecule. In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty partially purified cell extracts and presented evidence that the genetic component of these cells was DNA. In 1952 Alfred Hershey and Martha Chase investigated the infection of Escherichia coli cells with phage T2 (a virus) and their results were further corroboration that DNA was the genetic material. Since that time, a large body of evidence has confirmed the (nearly) universal truth that DNA is the genetic, heritable material in organisms. (The only exception to this is the case of RNA viruses, such as the AIDS virus, in which RNA is the only nucleic acid present in the virus and the genetic material.)

Modern research took a giant step forward when James Watson and Francis Crick, analyzing the collected findings of a number of laboratories, proposed the double helix structure of DNA in 1953. Their announcements motivated scientists to find corroboration for this proposal. During the 1980s detailed x-ray crystallographic analyses of DNA became acknowledged as proof of the structural arrangement that had been described by Watson and Crick, including the Watson–Crick complementary base-pairing arrangements. The elucidation of the structure of DNA led to an enormous and rapid expansion of our understanding of DNA's function in the living cell.