Chromosome Structure
Chromosome Structure
Chromosome Structure
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<strong>Chromosome</strong> <strong>Structure</strong>separate, cell wall synthesis can form a permanent barrierbetween daughter cells.Bacteria can constrict the cell cycle to allow doublingtimes that are shorter than the 40-min period necessary tomake one complete chromosomal copy. Under conditionsofrapid growth, initiation at oriC speeds up so thatdaughter cells inherit one fully replicated chromosome plustwo or more partially replicated chromosomes at the timeofcell division (dichotomous replication).Eukaryotic SegregationFollowing DNA replication in eukaryotes, which occurs ina part ofthe growth cycle called the S phase, cells movethrough a mechanical cycle called mitosis to distribute thereplicated chromosomes to each daughter cell (Figure 5).Mitosis proceeds through four stages. The first is theprophase in which, after replication, each chromosomebecomes condensed. Stage 2 is metaphase, where twochanges occur: each pair ofreplicated chromosomes movesto the cell centre and then the nuclear membrane begins todissolve. In stage 3, anaphase, one centromere ofeach pairofchromosomes is attached to a set offibres called thespindle, and molecular motors pull one ofeach chromosomepair to opposite cell poles. In stage 4, telophase, thenuclear membrane is reformed and daughter cells areseparated by the synthesis ofa new septum.TranscriptionA fundamental problem in biology is understanding howorganisms respond to the wide variety ofenvironmentalconditions that a cell or a population ofcells encounters.As noted above, different bacterial species grow over anamazing range oftemperatures and environmental niches,but even a single organism can adapt to widely varyinggrowth states, from aerobic growth on rich nutrients toanaerobic growth in minimal salts and a single carbon andProphaseS phaseMetaphaseTelophaseAnaphaseFigure 5 <strong>Chromosome</strong> segregation in eukaryotes is completed in the mitotic cycle. Mitosis proceeds through four stages, prophase, metaphase,anaphase and telophase, as described in the text.ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net7