Scanning the winding coils of naked DNA.Scanning the winding coils of naked DNA When James D. Watson James Dewey Watson (born April 6, 1928) is an American molecular biologist, best known as one of the co-discoverers of the structure of DNA. Watson, Francis Crick, and Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine "for their discoveries concerning the and Francis H.C. Crick Crick , Francis Henry Compton 1916-2004. British biologist who with James D. Watson proposed a spiral model, the double helix, for the molecular structure of DNA. He shared a 1962 Nobel Prize for advances in the study of genetics. first worked out the double-helical structure of DNA DNA: see nucleic acid. DNA or deoxyribonucleic acid One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. , they relied on data in the form of patterns of spots created by X-rays diffracted from crystallized DNA and captured on film. Now, 36 years later, a team of researchers using a custom-built scanning tunneling microscope scanning tunneling microscope, device for studying and imaging individual atoms on the surfaces of materials. The instrument was invented in the early 1980s by Gerd Binnig and Heinrich Rohrer, who were awarded the 1986 Nobel prize in physics for their work. has produced the first direct images of chemically unaltered, uncoated, pure DNA. Magnified 1 million times by the microscope, a typical double-stranded DNA molecule clearly shows its helical structure, and researchers can directly measure the spacing between coils. "It was not obvious that we would succeed," says Miquel B. Salmeron of the Lawrence Berkeley (Calif.) Laboratory (LBL LBL - Lawrence Berkeley Laboratory, Berkeley, CA, USA. ). "The previous experience of other laboratories was a little bit discouraging, but we tried it anyway." Salmeron and his collaborators at LBL and the Lawrence Livermore (Calif.) National Laboratory describe their results in the Jan. 20 SCIENCE. The researchers looked at calf thymus thymus Pyramid-shaped lymphoid organ (see lymphoid tissue) between the breastbone and the heart. Starting at puberty, it shrinks slowly. It has no lymphatic vessels draining into it and does not filter lymph; instead, stem cells in its outer cortex develop into DNA, deposited from a potassium chloride potassium chloride, chemical compound, KCl, a colorless or white, cubic, crystalline compound that closely resembles common salt (sodium chloride). It is soluble in water, alcohol, and alkalies. solution onto a graphite surface. They used their scanning tunneling microscope to trace out a kind of topographic map of the deposited molecules. In the image shown on the left, a looped DNA strand, stretching across an area 400 angstroms wide, rises about 20 angstroms above the surface. A circular feature seen below the strand probably represents an unresolved DNA fragment. The diagram on the right illustrates how the bumps can be interpreted as coils. Measurements show that the distance between adjacent coils varies from 27 to 63 angstroms. "We have not yet pushed the instrument to its maximum potential," Salmeron says. The team plans to investigate whether its microscope can resolve differences among the four nucleotides that serve as the building blocks for a single DNA strand. "DNA was the first molecule that we tried," Salmeron says. "Now we can imagine trying other biological molecules for which there is no other technique that we can utilize to study their structures." |
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