As the chromosome is replicated, the two copies move to opposite ends of the cell by an unknown mechanism.įtsZ proteins assemble into a ring that pinches the cell in two.Ĭhromosomes attach to the nuclear envelope, which remains intact. The single, circular chromosome exists in a region of cytoplasm called the nucleoid. A survey of mitotic assembly components found in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular eukaryotes ( Table 10.3).Ĭell Division Apparatus among Various Organisms While both proteins are found in extant organisms, tubulin function has evolved and diversified tremendously since evolving from its FtsZ prokaryotic origin. In this example, FtsZ is the ancestor protein to tubulin (a modern protein). In addition, both FtsZ and tubulin employ the same energy source, GTP (guanosine triphosphate), to rapidly assemble and disassemble complex structures.įtsZ and tubulin are homologous structures derived from common evolutionary origins. FtsZ proteins can form filaments, rings, and other three-dimensional structures that resemble the way tubulin forms microtubules, centrioles, and various cytoskeletal components. However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules that make up the mitotic spindle fibers that are necessary for eukaryotes. Prokaryotic cells, on the other hand, do not undergo karyokinesis and therefore have no need for a mitotic spindle. Mitotic Spindle ApparatusThe precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. When the new cell walls are in place, the daughter cells separate. A septum is formed between the nucleoids, extending gradually from the periphery toward the center of the cell. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. The formation of a ring composed of repeating units of a protein called FtsZ directs the partition between the nucleoids. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. As the cell elongates, the growing membrane aids in the transport of the chromosomes. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell.

Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously.

The starting point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane ( Figure 10.15). The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The packing proteins of bacteria are, however, related to the cohesin and condensin proteins involved in the chromosome compaction of eukaryotes. Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus no nucleosomes in prokaryotes. The single, circular DNA chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell ( Figure 10.2). Binary Fissionĭue to the relative simplicity of the prokaryotes, the cell division process, called binary fission, is a less complicated and much more rapid process than cell division in eukaryotes. This type of cell division is called binary (prokaryotic) fission. Karyokinesis is unnecessary because there is no nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter cell. In bacterial cells, the genome consists of a single, circular DNA chromosome therefore, the process of cell division is simplified. The genomic DNA must be replicated and then allocated into the daughter cells the cytoplasmic contents must also be divided to give both new cells the machinery to sustain life. To achieve the outcome of cloned offspring, certain steps are essential. In unicellular organisms, daughter cells are individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. For unicellular organisms, cell division is the only method to produce new individuals. Prokaryotes, such as bacteria, propagate by binary fission.

Explain how FtsZ and tubulin proteins are examples of homology.

Describe the process of binary fission in prokaryotes.

Learning Objectives By the end of this section, you will be able to:

Binary fission example organism