Name: _____________________________________________Date: _____________ Period: ________
Determining the structure for DNA was one of the outstanding science achievements of the 20th
century. The story told here begins in the 1860s and ends with the 1962 Nobel Prize.
Pre Discovery
In the 1860s chemist Johann Miescher (1844-1895) studied human pus and isolated a substance
he called " nuclein." Later he would isolate a similar substance from fish sperm. This substance
became known as nucleic acid or DNA. At about the same time as Miescher, Gregor Mendel
studied the breeding pattern of peas and proposed that observed traits are determined by discrete
factors (now called genes). In 1902 the principles formulated by Mendel were rediscovered and
the branch of biology known as genetics was launched; however, the nature of the genetic
material was unknown. In the 1940s Erwin Chargaff from Columbia University studied samples
of DNA taken from different sources and found the amount of adenine was almost equal to the
amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine.
The discovery of A=T and G=C became known as Chargaff�s Rule. So by 1950 the following
was known about DNA:
 Probably the genetic material and very large molecule
 Composed of nucleotides (units containing phosphate, 5 carbon sugar, and nitrogen base)
 Nitrogen bases consist of adenine(A), cytosine (C), guanine (G), and thymine (T)
 Pauling suggested DNA was helical
 Chargaff's data indicated A = T & G = C
Name: _____________________________________________Date: _____________ Period: ________
Discovery
Watson & Crick Examine Model of DNA
At Cambridge in the early 1950s, James Watson (biochemist post doctorate) and Francis Crick
(physics doctoral candidate) formed one of the most famous partnerships in the history of
science. The objective was to unravel the structure of DNA and ultimately the secret of life.
Their plan was to emulate Linus Pauling's method of model building to determine how atoms
joined together to form molecules.
At the same time that Watson & Crick formed their partnership, another group was doing
research at Kings College in England. Maurice Wilkins (biophysicist) had isolated extremely
pure samples of DNA. Rosalind Franklin was "brought" in to obtain x-rays of this DNA;
Franklin had an uncanny ability to obtain excellent x-rays from poorly crystallized materials.
But this partnership presented a problem: Wilkins assumed Franklin was working for him while
Franklin believed she was doing her own research (the problem was never resolved). Franklin
did obtain excellent x-rays of the DNA. While visiting with Wilkins, Watson was shown these xrays. From the previous information available and Franklin's x-rays, Watson & Crick pieced
together the structure of DNA, built their famous model, and published results in Nature
magazine. They proposed:
 DNA was not a single stranded molecule but rather a duplex
 Molecule was helical in nature
 The two strands of DNA were anti parallel
 The two strands were held together by hydrogen bonds
 Nitrogen bases paired in complementary fashion (A&T and C&G to account for Chargaff's
data)
Name: _____________________________________________Date: _____________ Period: ________
Three papers on DNA appeared in the famous April 1953 issue of Nature: the first by Watson &
Crick, the second by Wilkins, and the third by Franklin. The structure proposed by Watson &
Crick suggested not only how DNA might replicate itself, but also how it might store genetic
information as well as mutate. But keep in mind, the key data to support the helical structure
came from Franklin's x-rays--without Franklin's data, Watson & Crick merely had a theory
without evidence.
1962 Nobel Prize
Based on the will of Alfred Nobel, the Nobel Prize may be split by as many as three individuals
and cannot to be awarded posthumously. When the Nobel Prize for Medicine or Physiology was
announced in 1962, Franklin had already died (1958) and the award was given to Crick &
Watson for piecing together the puzzle of DNA and to Wilkins as a direct result of his work
Name: _____________________________________________Date: _____________ Period: ________
with Franklin. Only Wilkins mentioned Franklin in the Nobel speeches saying, "Rosalind
Franklin made some very valuable contributions to the x-ray analysis."