11. Cell division and chromosome segregation

12.11 Cell division and chromosome segregation

Key terms defined in this section
Septum constitutes the material that forms in the center of a bacterium to divide it into two daughter cells at the end of a division cycle.

Chromosome segregation in bacteria is especially interesting because the DNA itself is involved in the mechanism for partition. (This contrasts with eukaryotic cells, in which segregation is achieved by the complex apparatus of mitosis.) The bacterial apparatus is quite accurate, however; anucleate cells form <0.03% of a bacterial population.


The division of a bacterium into two daughter cells is accomplished by the formation of a septum, a structure that forms in the center of the cell as an invagination from the surrounding envelope. The septum forms an impenetrable barrier between the two parts of the cell and provides the site at which the two daughter cells eventually separate entirely. Two related questions address the role of the septum in division: what determines the location at which it forms; and what ensures that the daughter chromosomes lie on opposite sides of it?




Figure 12.25 Duplication and displacement of the periseptal annulus give rise to the formation of a septum that divides the cell.
Animated figure

The formation of the septum is preceded by the organization of the periseptal annulus. This is observed as a zone in E. coli or S. typhimurium in which the structure of the envelope is altered so that the inner membrane is connected more closely to the cell wall and outer membrane layer. As its name suggests, the annulus extends around the cell. Figure 12.25 summarizes its development (for review see de Boer et al., 1990).


The annulus starts at a central position in a new cell. As the cell grows, two events occur. A septum forms at the midcell position defined by the annulus. And new annuli form on either side of the initial annulus. These new annuli are displaced from the center and move along the cell to positions at 1/4 and 3/4 of the cell length. These will become the midcell positions after the next division. The displacement of the periseptal annulus to the correct position may be the crucial event that ensures the division of the cell into daughters of equal size. (The mechanism of movement is unknown.) Septation begins when the cell reaches a fixed length (2L), and the distance between the new annuli is always L. We do not know how the cell measures length, but the relevant parameter appears to be linear distance as such (not area or volume).


The septum consists of the same components as the cell envelope: the periplasmic layer contains a rigid layer of peptidoglycan, made by polymerization of tri- or pentapeptide Vdisaccharide units in a reaction involving connections between both types of subunit (transpeptidation and transglycosylation). The rod-like shape of the bacterium is maintained by a pair of activities, PBP2 and RodA, which are responsible for extending the envelope; mutations in the gene for either protein cause the bacterium to lose its extended shape, becoming round. This demonstrates the important principle that shape and rigidity can be determined by the simple extension of a polymeric structure. The enzyme responsible for generating the peptidoglycan in the septum is PBP3 (penicillin-binding protein 3), a membrane-bound protein that has its catalytic site in the periplasm. The septum initially forms as a double layer of peptidoglycan, and the protein EnvA is required to split the covalent links between the layers, so that the daughter cells may separate.




Figure 12.26 Attachment of bacterial DNA to the membrane could provide a mechanism for segregation.

The behavior of the periseptal annulus suggests that the mechanism for measuring position is associated with the cell envelope. It is plausible to suppose that the envelope could also be used to ensure segregation of the chromosomes. A direct link between DNA and the membrane could account for segregation. If daughter chromosomes are attached to the membrane, they could be physically separated when the septum forms. Figure 12.26 shows that the formation of a septum could segregate the chromosomes into the different daughter cells if the origins are connected to sites that lie on either side of the periseptal annulus.



Reviews
de Boer, P. A. J., Cook, W. R., and Rothfield, L. I. (1990). Bacterial cell division. Ann. Rev. Genet. 24, 249-274.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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