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Documents/DNA_08.doc [pdf]
more related articles at
http://www.stealthskater.com/Sc
ience.htm#Franklin
note: because important websites are frequently "here today but gone tomorrow", the following was
archived from http://www.pbs.org/wgbh/nova/photo51/picturing.html on November 15, 2006 .
This is NOT an attempt to divert readers from the aforementioned website. Indeed, the reader
should only read this back-up copy if it cannot be found at the original author's site.
Picturing the Molecules of Life
by Rebecca Deusser
PBS/NOVA
In science as in art, a picture is worth a thousand words. Scientists who study molecular structures
have known this ever since James Watson saw "Photo 51" and went on to deduce DNA's structure with
Francis Crick. Researchers today continue to rely on images of important molecules to confirm their
educated guesses. Like Rosalind Franklin, these scientists secure such images using X-ray diffraction
techniques, which enable them to study the arrangement of atoms in crystals or crystal-like structures by
spraying molecules with X-rays to produce patterns of diffraction.
Today, X-ray diffraction specialists feed such patterns into computer programs that generate colorful
3-D images. As the technology has improved, scientists have also tackled ever more complex
biochemical structures from RNA to ribosomes. The resulting images -- like "Photo 51" -- reveal clues
about the structures, their interactions with other molecular structures, and their role in human genetics.
Franklin's "Photo 51" shows the
mysterious "X" shape that inspired Watson
and Crick to visualize the double helix
structure of DNA.
She was able to get this remarkable
image -- the clearest image of DNA ever
created up until that time -- with her
advanced techniques of X-ray diffraction.
Using Franklin's image as physical
evidence, Watson and Crick then went on
to publish their Nobel Prize-winning
theoretical structure of DNA in Nature in
1953.
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In 1973, Alexander Rich of the
Massachusetts Institute of Technology
obtained the first atomic-level image of
transfer RNA. (This form of ribonucleic
acid transfers a particular amino acid to a
cell's ribosome during protein synthesis.)
At the atomic level of detail, scientists
can see all the molecules of a large nucleic
acid.
Rich's image was a breakthrough,
because it showed that it might be possible
to get a similar picture of DNA, which
could confirm or deny the structure
proposed by Watson and Crick 20 years
earlier.
This image also provided
important clues about the role of transfer
RNA in protein synthesis.
Rich used the same diffraction technique
to get an image of a small piece of DNA
(6 base pairs) in 1979.
The image
confirmed Watson and Crick's proposed
structure -- except for one major
difference.
The photo showed that the helical
structure bore a left-handed turn while
Watson and Crick's structure called for a
right-handed turn.
Rich's image thus threatened the
validity of the 20th Century's most
celebrated theoretical structure.
Rich
remembers a telephone conversation he
had with Crick after looking over his
results. "I told him that the structure was
left-handed. And he was silent," he said.
"Francis Crick is rarely silent."
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Less than a year later in 1980, Richard
Dickerson obtained another atomic level
image at UCLA.
Dickerson used a larger piece of DNA
with 10 base pairs, or a full helical unit. This
image revealed a right-handed helical
structure.
Other scientists, including Alexander
Rich, were able to duplicate the result and
confirm that Watson and Crick's proposed
structure was correct after all. (Rich's "lefthanded" DNA turned out to be a special form
of DNA now known as Z-DNA.)
In 1984, John Rosenberg of the University
of Pittsburgh completed the first visualization of
a protein/DNA complex -- a molecule of DNA
and a molecule of protein bound together.
This complex involved the EcoR1 protein -an important enzyme that is known to bind to
DNA.
Protein/DNA complexes play an important
part in many biological processes.
The structure imaged here has helped
scientists understand how protein and DNA
molecules recognize each other.
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In 2001, several research groups were able to get structures of a ribosome -- a very complex nucleic
acid structure and an enormous protein-RNA complex that is responsible for synthesizing proteins.
Many scientists believed that getting an atomic-level image of a ribosome would be impossible
because its structure is so complicated. (Ribosomes contain more than 50 proteins and thousands of
RNA nucleotides.)
Some scientists think these images -- which were produced by Harry Noller at the University of
California Santa Cruz; Venki Ramakrishnan at the University of Cambridge, England; and Thomas
Steitz at Yale University -- may be worthy of a Nobel Prize.
[StealthSkater note: more on Rosalind Franklin from PBS/NOVA is archived at doc pdf
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