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Science Frontiers ONLINE No. 111: May-Jun 1997 |
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Chromosome Choreography
Every biology student has seen sketches of the "dance of the chromosomes" that is performed when eukaryote (nucleuscontaining) cells divide. Because chromosomes are composed of genes and their DNA -- the information carriers of inheritance -- it reasonable to suppose that they are the "dance-masters." This expectation is enhanced if one holds that the genes are "selfish;" that is, they have their own evolutionary agendas, and all life forms exist only to execute their "will."
But cell division would not occur at all without the action of the cell's bipolar spindle. This spindle is composed of microtubules -- rods of the protein "tubulin." Somehow, when cells are about to divide, they synthesize these microtubules, which then seem to organize themselves into orderly arrays (the bipolar spindles). Then, the microtubules sort out and separate the two sets of chromosomes required for the two new cells. So, far, our description conforms to what biologists have known and accepted for decades; but there is something more mysterious going on.
In 1996, researchers discovered that they can actually substitute DNAcovered beads for the chromosomes, and the microtubules will still go through the motions of sorting and separating the chromosome-less strands. Actually, the microtubules will perform their act even without the DNA-covered beads. In a sense, the bipolar spindle is a puppetmaster, and the microtubules are the strings. The puppet show can go on without the puppets (chromosomes).
(Hyams, Jeremy; "Look Ma, No Chromosomes," Nature, 382:397, 1996. Also: Heald, Rebecca, et al; "Self-Organization of Microtubules into Bipolar Spindles around Artificial Chromosomes...," Nature, 382:420, 1996. Travis, John; "Mitotic Mischief," Science News, 150:14, 1996.)
Comments. Obviously, neither the chromosomes, the genes, nor the DNA are the master of the show. They are merely passive players in cell division. It is fair, therefore, to ask how these microtubules originate and operate. Whence the information and instructions ("program") required to put on the show? "Self-organization" is the current answer of mainstream biologists, but this term has no explanatory value.
 | Microtubules separating chromsomes prior to cell division. Left: early anaphase. Right: late anaphase. |