Prokaryotic(Bacterial) Cell Division

Bacterial Cell Division 

Bacteria divide as a way of reproducing themselves. Although bacteria exchange DNA, they do not have a sexual cycle like eukaryotes. Thus, all growth in a bacterial population is due to division to produce new cells. The reproduction of bacteria is clonal—that is, each cell produced by cell division is an identical copy of the original cell.

Bacterial Cell Division By Binary Fission

In all organisms, cell division produces two new cells, each with the same genetic information as the parent cell. Despite the host of differences between prokaryotes and eukaryotes, the essentials of the cell division process are the same: duplication and segregation of genetic information into daughter cells, and division of cellular contents. We begin by looking at the prokaryotic process, binary fission, as it occurs in bacteria.

Binary Fission 

The cell division in wich the parent cell divides into two daughter individuals is known as binary fission.In bacterial binary fission, replication produces two exact copies of the bacterial circular DNA (genome) and septation divides the cytoplasm (whole cell)  into two daughters that would be the exact copies of the original bacterial cell.

Bacterial Genome

Most bacteria have a genome made up of a single, circular DNA molecule. In spite of its apparent simplicity, the DNA molecule of the bacterium Escherichia coli is actually on the order of 500 times longer than the cell itself! Packaged very tightly to fit into the cell, the DNA is located in a region called the nucleoid. Although distinct from the cytoplasm around it, the nucleoid is not surrounded by a membrane.

The compaction and organization of the nucleoid are achieved by a class of proteins called structural maintenance of chromosome, or SMC, proteins. These are ancient proteins that have diversified over evolutionary time to fulfill a variety of roles related to DNA organization in different lineages. The eukaryotic cohesin and condensin proteins  are also SMC proteins.

Replication of Bacterial Circular  DNA

One key feature of bacterial cell division is that replication and partitioning of the chromosome occur as a concerted process.

For many years it was thought that newly replicated E. coli DNA molecules were passively segregated by attachment to and growth of the membrane as the cell elongated. To test this hypothesis, experiments followed the movement of the origin of replication from its position at midcell prior to replication, tracking it as the newly replicated origins moved toward opposite ends of the cell. Surprisingly, this movement is faster than the rate of cell elongation, showing that membrane growth alone is not enough. The replication origins are captured at the one-quarter and three-quarter positions relative to the length of the cell, which will be midcell of the resulting daughter cells.

During replication, first the origin and then the rest of the newly replicated chromosomes are moved to opposite ends of the cell as two new nucleoids are assembled. The final event of replication is decatenation (untangling) of the final replication products. The force behind chromosome segregation has been attributed to DNA replication itself, to transcription, and to the polymerization of actin-like molecules. Currently, no single model appears to explain the process, and it may involve more than one of these.

Septation of Bacterial Cell

The cell’s other components are physically partitioned between the daughter cells by the production of a septum. This process, termed septation, usually occurs at the midpoint of the cell. It begins with the formation of a ring composed of many copies of the protein FtsZ. A number of other proteins then accumulate, including ones embedded in the membrane. Thisstructure contracts inward radially until the cell divides into two new cells.

The FtsZ protein is found in most prokaryotes, including archaea. It can form filaments and rings, and recently solved three-dimensional models show a high degree of similarity to eukaryotic tubulin. However, its role in bacterial division is quite different from the role of tubulin in mitosis.
The evolution of eukaryotic cells included much more complex genomes. These complex genomes may be due to the evolution of mechanisms that delay chromosome separation after replication. Although it is unclear how this ability to keep  chromosomes together evolved, it does seem more closely  related to binary fission than we once thought (see figure).

Figure:A comparison of protein assemblies during cell division among different organisms. The prokaryotic protein FtsZ has a structure that is similar to that of the eukaryotic protein tubulin, the protein component of microtubules, which are the fibers eukaryotic cells use to construct the spindle apparatus used to separate chromosomes.

During binary fission, the bacterial chromosome is replicated and the two products partitioned to each end of the cell, prior to the physical division of the cell. Binary fission begins with the replication of the bacterial DNA at a specific site—the origin of replication—and proceeds in both directions around the circular DNA to a specific site of termination (figure ). As this occurs, the cell body proceeds to divide in two (“binary fission”), division occurring roughly at midcell.

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