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Chromosomes are 'nibbled' before they fuse, researchers report

reakage-fusion-bridge" pathway, a cell interprets chromosomes with short telomeres as being broken, and sets in motion machinery to "fix" the break by fusing it to another exposed end. The unintended consequence of this fix is the connection of two chromosomes. If the fused chromosomes are pulled to opposite sides of a dividing cell, they form a bridge that breaks randomly as the cell divides, and the process begins again.

To test whether this was the correct or only scenario, Hackett inserted genetic markers into a yeast chromosome to reveal where genetic damage most often occurs when telomeres got too short. Instead of random damage, she discovered that the marker at the very end of the chromosome was most likely to be lost, and the marker closest to the chromosome's center the least likely.

"If fusion and breakage was the primary mechanism of gene loss, the pattern of loss would have been random -- each marker would have been just as likely as the others to be lost," explains Greider. "The marker loss we saw was not at all random, so we knew some other mechanism was at work."

Then, Hackett studied the engineered chromosomes in yeast missing an enzyme called exonuclease that normally recognizes and chews up broken chromosomes one strand of DNA at a time. Without the enzyme there were fewer chromosome rearrangements, offering strong evidence that this enzyme is doing the damage.

"Fusion happens, but it's not the primary mechanism that triggers gene loss after telomeres get too short," says Greider. "Instead, exonuclease activity causes the bulk of immediate gene loss."

To prove that fusion does indeed result in a random pattern of marker loss, Hackett made an artificial fused, or di-centric, chromosome, complete with genetic markers to identify which segments were destroyed. Since Hackett engineered it, this fused chromosome could not already have been "nibbled" by an exonuclease.

"We demonstrated th
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Contact: Joanna Downer
jdowner1@jhmi.edu
410-614-5105
Johns Hopkins Medical Institutions
2-Dec-2003


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