By: Rosalind Franklin

X-ray crystallography: Diffraction Photo of

“B” - DNA

Important Molecular Images

1953

The The clearest imageclearest image of DNA ever created up until that of DNA ever created up until that time, with her advanced techniques of X-ray diffraction. time, with her advanced techniques of X-ray diffraction. Using Franklin's image as physical evidence, Watson Using Franklin's image as physical evidence, Watson and Crick then went on to publish their Nobel Prize-and Crick then went on to publish their Nobel Prize-

winning winning theoretical structure of DNA in theoretical structure of DNA in NatureNature in 1953 in 1953. .

Photo 51

1973 Alexander Rich of the MIT obtained the first atomic-level image of tRNA. 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.

1979 Rich used the same diffraction technique to get an image of a small piece of DNA (six 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.)

1980

1984 John Rosenberg of the University of Pittsburgh completed the first visualization of a protein/DNA complex. 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.

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. Some scientists think these images—which were produced at the University of California Santa Cruz, University of Cambridge, England, and at Yale University—may be worthy of a Nobel Prize.

Web Site Credits

• Picturing the Molecules of Life—Photos: (1952) © Franklin, R. and Gosling, R.G./Nature; (1973, 1984) Courtesy of Shana Kelley, Boston College; (1979, 1980) © Protein Data Bank; (2001) © Center for Molecular Biology of RNA, UC-Santa Cruz

Photo 51

Notes & QuestionsNotes & Questions

• What does the “X” pattern tell us?What does the “X” pattern tell us?

• What does the diamond pattern tell What does the diamond pattern tell us? us?

• What do the smears tell us?What do the smears tell us?

• What do the missing smears tell us?What do the missing smears tell us?

1. Set up Helium/Neon laser as demonstrated by instructor.

2. Using the light bulb filament, focus a diffraction pattern on the white board.

3. Sketch the diffraction pattern.

4. Accurately measure distance from laser to white board.

LAB

Protocol

d

r

Experimental Laser Results 5. Calculate the

diffraction angle, , using the white board to light bulb filament distance and the ring radius. = tan-1(r/d)

Calculator Operation1. r/d=2. 2nd 3. tan-1=

6. Calculate the Diameter (d) of the light bulb filament using Braggs law

Given:

Laser light (HeNe) = 0.6328 m

Special Thanks To: Heather Heide (class of ’05) for helping to put this lab together.

Calculator Operation1. Sin of =2. 1/Sin of = 3. /2=4. (/2) x (1/Sin of ) = d