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Bacterial chromosomes run to the poles.

Over the years, biologists have developed a detailed picture of how dividing eukaryotic cells-cells with a nucleus-copy their chromosomes and parcel identical sets to the resulting pair of cells. Crucial to this dance of chromosomes is the mitotic spindle , an elaborate skeleton of proteins that segregates the threads of DNA appropriately (SN: 8/31/96, p. 140).

An unanswered question is whether bacteria-which, unlike human cells, don't have a nucleus-contain a spindle or some other cellular machinery to segregate chromosomes when they divide. Investigators probing this issue have been stymied because bacteria ar e so small and their chromosomes are more difficult to view than those of their eukaryotic counterparts.

Two research groups, both with reports in the March 7 Cell, now offer some promising insights into how bacteria divvy up their chromosomes. Dane A. Mohl and James W. Gober of the University of California, Los Angeles describe how two recently discovered p roteins, ParA and ParB, may help partition chromosomes in a dividing bacterium.

The researchers show that after a bacterium makes a copy of its single chromosome, the two proteins both gradually shift from the interior of the organism to opposing poles of the cell. Since ParA and ParB bind to specific DNA sequences on a chromosome, t heir movement hints that they may be part of the machinery that guides a pair of bacterial chromosomes to opposite ends of the cell before it splits in two. Indeed, when the scientists added extra ParA and ParB to dividing bacteria, they frequently disrup ted the normal segregation of chromosomes.

In the other report, Chris D. Webb of Harvard University and his colleagues have made use of a method that induces specific bacterial DNA sequences to fluoresce. The scientists used this technique to label an area near the origin of replication, a well-kn own site on bacterial chromosomes. They then took snapshots of bacteria that had just copied their chromosomes. The pictures showed that the two chromosomes' origins of replication were usually at opposite ends of a dividing bacterium, implying that this region is involved when the bacterial chromosomes pull away from one another.

The next step in this research is to make "movies" that show how the origins move during a single cycle of bacterial division, says Webb.
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Title Annotation:Biology; ParA and ParB proteins may aid partition chromosomes in dividing bacterium
Author:Travis, John
Publication:Science News
Article Type:Brief Article
Date:Apr 19, 1997
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