Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 86

While bacteria do not contain a special membrane compartment for chromosomal DNA like the nucleus of eukaryotes, even in bacteria the chromosome does not freely diffuse within the cytoplasm. In fact, as we learn more about bacterial chromosomes, we are realizing that they are maintained with an incredible amount of organization. Even with the aid of only a standard laboratory microscope and DNA stain, a mass of chromosomal DNA is easily observed in the center of the cell in a structure called the nucleoid, which is very compact, considering that the DNA is about a thousand times as long as the cell.

Because of the large size of the chromosome, the process of moving the replicating chromosomes to daughter cells, called segregation, is not trivial. Chromosome segregation encounters a number of obstacles. Obvious initial obstacles are viscous forces and torsional stress associated with unwinding the template strands of DNA. Advances in microscopy and techniques that allow the localization of certain regions of the chromosome are revealing the choreography involved in coordinating DNA replication and chromosome segregation. Microscopy experiments using green fluorescent protein (GFP) fused to replication proteins allow the localization of DNA replication forks, and GFP fused to proteins that bind to specific sites on DNA allows localization of the origin and terminus in the cell. These experiments show that soon after the initiation of DNA replication within the nucleoid, the origins start to move to the daughter cells. Once replication is complete, segregation would still be prevented if daughter chromosomes were joined by recombination, interlinked, or otherwise tangled during replication. Even if they were not physically joined, their separation would be very difficult if the two daughter chromosomes were randomly spread out throughout the cell. It is therefore not surprising that bacteria have a number of systems to ensure that their chromosomes segregate properly into the daughter cells during cell division. Molecular systems responsible for chromosome segregation are discussed separately below.