What did Rosalind Franklin do?
By 1952, much was known about DNA, including its exclusive role
as genetic material - the sole substance capable of storing
practially all the information needed to create a living being.
What was not yet known was what the elusive DNA molecule looked
like, or how it performed this amazing hereditary function. This
would change in the course of a single year. The now familiar
double helical structure of DNA, a twisted ladder with base-pairs
rungs essential to its hereditary function, was deciphered in 1953.
The individuals most commonly associated with this remarkable
accomplishment are James Watson and Francis Crick. Maurice Wilkins
played a role as well, for which he shared the 1962 Nobel Prize for
Physiology and Medicine with Watson and Crick. Yet there was one
other person whose truly essential contribution to this discovery
could not be recognized by the Nobel Committee in 1962. That person
was Rosalind Franklin. Born in July of 1920, Rosalind Franklin
graduated with a Ph.D. from Cambridge University in 1945. In 1951,
she went to work as a research associate for John Randall at King's
College in London. A chemist by training, Franklin had established
herself as a world expert in the structure of graphite and other
carbon compounds before she moved to London. In James Watson's
account of the discovery of the structure of DNA, entitled The
Double Helix, Rosalind Franklin was depicted inaccurately as an
underling of Maurice Wilkins at King's College. In fact, Maurice
Wilkins and Rosalind Franklin were peers. Franklin had discovered
that DNA could crystallize into two different forms, an A form and
a B form. John Randall gave Franklin the A form and Wilkins the B
form, assigning them each the task of elucidating their molecular
structure. The technique with which Wilkins and Franklin set out to
do this is called X-ray crystallography. With this technique a
crystal is exposed to x-rays in order to produce a diffraction
pattern. If the crystal is pure enough and the diffraction pattern
is acquired very carefully, it is possible to reconstruct the
positions of the atoms in the molecules that comprise the basic
unit of the crystal called the unit cell. By the early 1950s,
scientists were just learning how to do this for biological
molecules as complex as DNA. Progress in discerning the structure
of DNA was blocked because the A and B forms of DNA were mixed
together in preparations, yielding impure crystals and "muddy"
diffraction patterns that were near impossible to interpret. After
discovering the existence of the A and B forms of DNA, Rosalind
Franklin also succeeded in developing an ingenious and laborious
method to separate the two forms, providing the first DNA crystals
pure enough to yield interpretable diffraction patterns. She then
went on to obtain excellent X-ray diffraction patterns of
crystalline B-form DNA and, using a combination of crystallographic
theory and chemical reasoning, discovered important basic facts
about its structure. She discovered that the sugar-phosphate
backbone of DNA lies on the outside of the molecule, not the inside
as was previously thought. She discovered the helical structure of
DNA has two strands, not three as proposed in competing theories.
She gave quantitative details about the shape and size of the
double helix. The all- important missing piece of the puzzle, that
she could not discover from her data, was how the bases paired on
the inside of the helix, and thus the secret of heredity itself.
That discovery remained for Watson and Crick to make. After Randall
presented Franklin's data and unpublished conclusions at a routine
seminar, aspects of her results were informally communicated to
Watson and Crick by Maurice Wilkins and Max Perutz, without her or
John Randall's knowledge. It was Watson and Crick who put all the
pieces of the puzzle together from a variety of sources including
Franklin's results, to build their ultimately correct and complete
description of DNA's structure. Their model for the structure of
DNA appeared in the journal Nature in April, 1953, alongside
Franklin's own report. Rosalind Franklin never knew that Watson and
Crick had gotten access to her results. At the time of the Watson
and Crick publication and afterwards, Franklin appears not to have
been bitter about their accomplishment. In her own publications
about DNA structure, she agreed with their essential conclusions
but remained skeptical about some details of their model. Franklin
moved on to work on an even more challenging problem: the structure
of an entire virus, called the Tobacco Mosaic Virus. Her subsequent
publications on this topic would include four more papers in the
journal Nature. Rosalind Franklin was friendly with both James
Watson and Francis Crick, and communicated regularly with them
until her life and career were cut short by cancer in April of
1958, at the age of 37. She died with a reputation around the world
for her contributions to knowledge about the structure of carbon
compounds and of viruses. After her death, Watson and Crick made
abundantly clear in public lectures that they could not have
discovered the structure of DNA without her work. However, because
the Nobel Prize is not awarded posthumously, Rosalind Franklin
could not be cited for her essential role in the discovery of the
physical basis of genetic heredity
