Unraveling the DNA Double Helix

Despite proof that DNA carries genetic information from one generation to the next, the structure of DNA and the mechanism by which genetic information is passed on to the next generation remained the single greatest unanswered question in biology until 1953. It was in that year that James Watson, an American geneticist, and Francis Crick, an English physicist, working at the University of Cambridge in England proposed a double helical structure for DNA. This was the culmination of a brilliant piece of detective work - and a discovery that has proven to be the key to molecular biology and modern biotechnology. Using information derived from a number of other scientists working on various aspects of the chemistry and structure of DNA, Watson and Crick were able to assemble the information like pieces of a jigsaw puzzle to produce their model of the structure of DNA.
It had already been established by chemical studies that DNA was a polymer of nucleotide subunits, each nucleotide comprising a sugar (deoxyribose), phosphate and one of four different bases - the purines, adenine (A) and guanine (G) together with the pyrimidines, thymine (T) and cytosine (C). A most important clue was the discovery in the late 1940s by Erwin Chargaff and his colleagues at Columbia University that the four bases may occur in varying proportions in the DNAs of different organisms, but the number of A residues is always equal to the number of T residues; similarly equal numbers of G and C residues are present. These quantitative relationships are important, not only in establishing the three-dimensional structure of DNA, but also in providing clues on how genetic information is encoded in DNA and passed on from one generation to the next.