Chromosome centers defined and sequenced for model plant The serendipitous discovery of a rare mutationand five years of concentrated effort have allowed a team of American andBritish researchers to define the centromeres of the five chromosomes ofArabidopsisthaliana, a flowering plant that has become the primary model for plantgenetics. The centromere is the part of the chromosome that ensures eachdividing cell inherits the correct DNA. The findings, published in the December 24th issue of Science,represent the first time that scientists working with a multi-cellularorganism have been able to identify the genetic boundaries of the centromeres-- which are resistant to standard gene mapping techniques -- and to unraveltheir DNA sequences.
"Precise mapping of these crucial segments of DNA should shed lighton the nature and behavior of chromosomes," said Preuss. "It could helpdefine the centromeres in more complex organisms, boost plant genomics,and speed the development of artificial chromosomes for use in plant engineering,"she added. "This now makes Arabidopsis the leading candidate for thoroughsequencing of the entire genome in a higher organism." The Arabidopsis Genome Project hopes to complete sequencing of the first full plant genome by the endof 2000. The sequences of chromosomes 2 and 4 were published in Nature on December 16. Centromeres are pivotal structures near the center of each chromosome.They are essential for cell division because they form the point of attachmentfor spindles, the fibers that separate DNA strands into equal piles beforecell division. The DNA near the centromeres is highly condensed, packed and coiledso tightly that it can thwart many types of anaysis. The techniques thatallowed genetic and physical mapping of the centromeres in yeast required researchers to analyze the genetic contents of "tetrads" -- clusters offour reproductive cells that are formed from a single parent cell. Unfortunately,tetrads are not stable in most multi-cellular organisms. This has preventedthe precise mapping of centromere boundaries. Other attemps to sequence centromeres have been thwarted by the highlyrepetitive properties of these regions. Consequently, the Human GenomeProject has postponed mapping most of the centromeres until better methodsbecome available.
The researchers studied more than 1,000 meioses to determine the preciseareas at the center of each chromosome in which there was absolutely noexchange of DNA during cell division. They found, as expected, that the actual centromeres were composed oflong arrays of extremely repetitive DNA -- 180 base-pair repeats whichhave a structural function but contain no genes. This central core, however,was flanked by surprisingly complex DNA composed of diverse sequences. The researchers had expected to find more of the repetitious, structuralDNA bordering the centromere, but they found instead that the surroundingregions contained moderately repetitive DNA, plus multiple "mobile elements"-- DNA that appears to have originate elsewhere and been inserted intothe plant's genome -- and several functioning genes.
In fact, the researchers found an entire mitochondrial genome embeddednear the centromere of chromosome 2. This surprising discovery may shedlight on the evolutionary processes that allow DNA from organelles to beexchanged with that from the nucleus. Understanding the centromeres may prove to be a crucial developmentfor advanced genetic engineering of plants, said Preuss. Previously, researcherscould insert only one gene at a time, but by connecting several genes onone long strand to a centromere, it may be possible to move whole setsof genes into an organism. These altered plants, even a weed like Arabidopsis,could then provide an inexpensive source of complex biomolecules for biochemicalor pharmaceutical use. The research was funded by the National Science Foundation, theUnited States Department of Agriculture, the Consortium for Plant BiotechnologyResearch, and the David and Lucille Packard Foundation.
The large-scale DNA sequencing of the centromeres was accomplished bythe
efforts of two teams: one, led by Samir Kaul and Xiaoying Lin at TIGR(The
Institute for Genomic Research) was responsible for characterizingthe
centromere of chromosome 2, and the second, jointly led by Rob Martienssenand
Richard McCombie at Cold Spring Harbor and Mike Bevan at the John
InnesInstitute, Norwich, England, was responsible for sequencing the
centromereof chromosome 4. These teams combined their data with the mapping
workfrom Copenhaver and Preuss, defining the boundaries of the centromeresalong
with their contents.
Copyright (c)1999 The University of Chicago Hospitals& Health System. All rights reserved.
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"The centromere is perhaps the most importantbut also the most inaccessible and
the least understood part of a chromosome,"said Daphne 
The crucial breakthrough for studyingplant centromeres came in 1994 when Preuss
discovered a mutant form ofArabidopsis. The mutation, known as quartet, causes
the four products ofmale meiosis to stick together, forming a tetrad of four
pollen grains.This allows scientists, using genetic markers, to monitor how the
originalfive pairs of chromosomes shuffle the deck of genetic information
throughone replication and two divisions to form the four cells of a
tetrad. 
Being a gene in the centromereregion is "like being a satellite in the asteroid
belt," suggested GregCopenhaver, Ph.D., research associate in Preuss's
laboratory and firstauthor of the paper. The 