To track these movements, Susan Gasser of the University of Geneva (Geneva, Switzerland) and colleagues used GFP to tag four positions on two yeast chromosomes. Although the centromeres and telomeres are tethered to the edge of the nucleus and move little, the chromosomal arms are mobile and may traverse one third the diameter of the nucleus in as little as 10 s. Previous workers may not have noticed the movements because they are less obvious in the much larger mammalian nucleus, says Gasser. She showed that adding carbonyl cyanide chlorophenyl hydrazone, which drains the cell's ATP, hampered mobility, thus eliminating the possibility that chromosomes were simply drifting like seaweed in the tide. Movements also became smaller once DNA replication began. “Replication complexes sit like a dead weight on the chromosome and slow it down,” says Gasser.
What does the cell accomplish by expending energy to tug its chromosomes around? According to Gasser, experiments on mutants suggest that movements may be a prelude for transcription, allowing the chromosome to hook up to nucleosome remodeling factors that give transcription factors access to genes. Or they may allow homologous chromosomes to sidle up next to each other and swap snippets of DNA. ▪