The following animation's correspond to figures
2 and 8 in a paper entitled "Peering into Darwin's Black Box: The cell
division processes required for bacterial life" published in the journal
and Design Vol 18:18-32, Spring 2000.
|Figure 2. Hypothetical model of protocell
According to evolutionary theory, it can be postulated that the protocell, the forerunner of contemporary cells, would not be as complex as contemporary cells. Therefore because the coordination of DNA replication with cytokinesis is complex in contemporary cells, these two complex cell events would probably would be uncoordinated in the protocell. In fact, we could predict that in some cells, DNA replication could occur at a faster rate than cytokinesis or alternatively cytokinesis could occur at a faster rate than DNA replication. The animation portrays what cell division might look like if cytokinesis occurred at a faster rate than DNA replication. The red circle corresponds to a single DNA chromosome just before it begins to replicate. Note that the resulting cell populations display "heterploidy" as a result of this kind of "sloppy" division. To replay the animation click on the replay button and then the play button.
|Figure 8. Proposed model for active partitioning
of the bacterial chromosome by a protein mediated process in contemporary
In contrast to the hypothetical "sloppy" division of the protocell, contemporary bacteria utilize several proteins to promote the equal partitioning of DNA chromosomes to each daughter cell. First the protein FtsZ forms a division ring at midcell. FtsZ is postulated to constrict the membrane by a yet undetermined mechanism. As the DNA chromosome is replicated, proteins are postulated to actively "polarize" and separate DNA replication origins (represented as dashes) ensuring that each daughter cell receives one (not both) chromosomes. The polarization of the replication origins appears to occur before the completion of cytokinesis. Therefore DNA partitioning appears to be coordinated with DNA replication and is now postulated to involve additional proteins. It is interesting to note that scientists who describe this process refer to the cytokinesis apparatus and the associated chromosome partitioning process as a "machine."