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DNA doubles in a four-stranded huddle.

DNA's elegant, double helical structure provides the basis for its information storage and copying functions. Using X-ray crystallography, a team of researchers has observed DNA in a novel shape--a quadruplex. Two closed, twisted loops of single-stranded DNA, synthesized by the researchers, nestle side by side, forming a structure with four adjacent strands.

In this image, one loop is on the left, the other on the right. Each contains eight bases: two adenines (green), four thymines (yellow), and two cytosines (blue), arranged ATTCATTC. Base pairs of adenine and thymine run diagonally between the two loops, with unpaired thymines above and below. The cytosines keep to the fringes. A central sodium atom "helps to moderate the electrostatic repulsion" between the bases in the crystallized material, says Stephen A. Salisbury of the University of Cambridge Crystallographic Data Centre in England.

He and his colleagues at the University of Gottingen in Germany and the University of Barcelona in Spain report their findings in the May 27 Proceedings of The National Academy of Sciences.

The researchers didn't set out to look for the quadruplex, says Cambridge crystallographer Harold R. Powell. "That was a bit of a surprise to us." Originally, they synthesized the loops to examine the structure of hairpin turns in DNA.

After determining that the DNA formed a quadruplex, the researchers learned that another group had previously seen a similar four-stranded structure held together by cytosine-guanine base pairs. "That's when we started looking at it more closely and felt that it might be a more general structure" rather than an anomalous result of the crystallization, Powell says.

Scientists have evidence that when chromosomes exchange genes, four strands from two DNA double helixes come together to form an intermediate structure called a Holliday junction, which looks like the street curbs of a four-way intersection. Salisbury and his colleagues speculate that the quadruplex could be an alternative way for DNA to associate during gene swapping.

"It's an interesting motif that requires experimental challenge" to determine its biological significance, says Nadrian C. Seeman of New York University.
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Title Annotation:researchers find DNA in quadraplex shape
Author:Wu, Corinna
Publication:Science News
Article Type:Brief Article
Date:May 31, 1997
Words:341
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