Mapping the Universe - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 78

the covalent structure of DNA did not become known with certainty until the late
1940s. Miescher suspected that nuclein or nucleic acid might play a key role in cell
inheritance, but others ruled out such a possibility. It was not until 1943 that the first
direct evidence emerged for DNA as the bearer of genetic information. In that year,
Oswald Avery, Colin MacLeod, and Maclyn McCarty, working at the Rockefeller
Institute provided the early evidence that DNA is the carrier of genetic information
in all living cells. In the 1950s biologists did not know what the DNA molecule
looked like or how the parts of it were arranged.
At King's College in London an English physicist Maurice Wilkins together
with another English scientist Rosalind Franklin spent most of 1951 using an
x-ray method of photography to work out the structural shape and nitrogenous
base arrangements of DNA. Rosalind Franklin was an expert in using X-ray
crystallography to study imperfectly crystalline matter, such as coal. She discovered
the two forms of DNA. The easily photographed A form was dried, while the B
form was wet. While much harder to photograph, her pictures of the B form showed
a helix. Since the water would be attracted to the phosphates in the backbone, and
the DNA was easily hydrated and dehydrated, she guessed that the backbone of the
DNA was on the outside and the bases were therefore on the inside. This was a
major step forward in the search for the structure of DNA.
In May 1952, Franklin got her first good photograph of the B form of DNA,
showing a double helix. This was another major breakthrough, but Franklin missed
it and continued working on the A form. James Watson and Francis Crick started
their work together on DNA in 1951 at Cambridge University. By the end of
1952, Watson approached Maurice Wilkins who gave him one of Franklin's x-ray
photographs. Watson started to build a new model of DNA revealing DNA's
structure as a double helix or spiral. In 1962, together with Maurice Wilkins,
Watson & Crick were awarded a Nobel Prize for their discovery of DNA structure.
Figure 2.32 shows the original structure of DNA's 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 (Watson and Crick
1953 ). This was a key discovery 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. Watson gave a personal account of the discovery in
(Watson 1991 ).