Part 1: Crick and Watson
About the Program
Although deoxyribonucleic acid, or DNA, was discovered in the late 1860s,
the substance was largely ignored for nearly a century because it seemed
too simple to serve any significant purpose. This view changed dramatically
in the 1940s. At this time scientists discovered that chromosomes, which
were known to carry hereditary information, consisted of DNA and proteins.
Experiments conducted throughout the 1940s showed that, contrary to the
prevailing opinion that proteins carry the genetic information, DNA actually
seemed to be the genetic material. However, it was still not known what
the structure of DNA was, and how such a simple molecule could contain all
the information needed to produce a human being or other living organisms.
This program examines the celebrated partnership between James
Watson and Francis Crick, a postdoctoral
fellow and a graduate student, who together managed to solve the mystery
of DNA structure. The partnership began when Watson, an American, took a
research position at Cambridge University in England in 1951. Crick was
also at Cambridge, studying protein structure with a technique called Xray
crystallography. By their own admission, both were more interested in the
prevailing scientific problems of that day than in their own work, and the
structure of DNA was definitely an interesting problem. Over the next few
years, Watson and Crick would collect, by coincidence, hard work, and a
little luck, key pieces of information that they would use to solve the
DNA puzzle.
Crick and Watson already knew the main components of DNA, phosphates,
sugars, and four nitrogenous bases: adenine, thymine, cytosine, and guanine.
But how did the elements fit together? One clue came from Xray photographs
of DNA taken by Rosalind Franklin. These photographs suggested that the
structure of DNA was a helix. Another impetus came from Linus Pauling, who
had already built a model of a helical protein. Watson and Crick were inspired
by the fact that Pauling had used his imagination along with molecular models
to deduce the structure of this protein. And they thought that if an eminent
scientist such as Pauling could model a structure with little experimentation,
then they might be able to do the same with DNA.
Using wire and pieces of metal, Crick and Watson began building scale
models of DNA. After several embarrassing failures, the two men recognized
another clue to the puzzle from biochemist Erwin Chargaff. Chargaff had
found that the amounts of adenine and thymine were approximately equal and
the amounts of guanine and cytosine were also approximately equal. This
information helped solidify the idea that the bases might be paired in a
specific way. What if DNA consisted of two strands of phosphates on the
outside, and paired bases on the inside? At first, Watson imagined that
the bases paired like with like, for example adenine with adenine, and cytosine
with cytosine. But the resulting model did not fit the Xray data. Then Watson
and Crick discovered that thymine and guanine could adopt a slightly different
chemical shape, and that they had been trying to make the models from one
version of the bases. Using the new forms, Watson discovered that he could
make two base pairs, one consisting of adenine and thymine, and the other
consisting of guanine and cytosine, that had exactly the same size. This
discovery was the final key to the DNA structure.
Viewing Objectives:
- To learn that collaboration as well as competition are important factors
in a scientific endeavor.
- To understand what compelled Watson and Crick to study DNA structure.
- To recognize that Watson and Crick relied on information from several
different sources to determine the structure of DNA.
- To reinforce an understanding of the general structure of a DNA molecule.
Discussion Questions
- In the 1940s and early 1950s, identifying the chemical basis of heredity
was a major problem in biology. Many scientists believed that proteins,
not DNA, were the carriers of genetic information. What led them to this
conclusion?
- When Watson and Crick first began to look for the structure of DNA,
many other, more established scientists were trying to solve the same mystery.
Why did Watson and Crick succeed in finding the structure ahead of these
other scientists?
- How does the Watson and Crick model of DNA provide an explanation for
genetic variability, Chargaff's rules, and DNA replication?
- What role did Rosalind Franklin play in the discovery of the double
helix? Why was her role important? Why did she not receive a Nobel Prize
with Watson, Crick, and Wilkins?
Related Activities
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