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more related articles at http://www.stealthskater.com/Science.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.

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."

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 "left-handed" 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.

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 URL ]

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