Cell Division of Prokaryotes

The events of mitosis and meiosis occur only in eukaryotes. In prokaryotes—bacteria, cyanobacteria, and archaebacteria—mitosis and meiosis do not occur and cytokinesis is much simpler. All the genetic material occurs as one ring of DNA, which does not have any protective histones. The loop of DNA is attached to the cell's plasma membrane, and when the DNA is replicated, the two daughter loops are both attached to the membrane (Fig. 4.21). Because no centromere holds them together, the circles of DNA are pulled apart as the cell and its membranes grow. A new round of replication may begin even before cell division occurs, so each cell may have as many as 20 identical circles.

Cytokinesis occurs by a process of infurrowing: The plasma membrane pulls inward and finally pinches in two, although the contractile mechanism that pulls it inward has not been found yet. As the plasma membrane furrows inward, a new cross wall grows inward, starting from the existing wall. Once complete, the cross wall splits, becoming two walls and releasing the two daughter cells; multicellular aggregates are rarely formed in bacteria. In cyanobacteria the cross walls do not break apart, and multicellular bodies are formed (see Chapter 19). The cell cycle of many bacteria can be short—only 20 minutes under ideal conditions. For many species, however, the cycle may last several days or weeks even under optimal conditions.

FIGURE1:  In prokaryotes, the circles of DNA are attached directly to the plasma membrane. As the cell and its membrane grow, the attached circles of DNA are separated. Cell division then occurs between the circles. 

FIGURE 2 : Micrograph of an algal cell; algae are especially convenient for studying chloroplast division. Because most have only one chloroplast per cell, chloroplast division must be coordinated with cell division. As the chloroplast divides, its central region is pinched inward by a contracting ring of proteins, which constricts the chloroplast into a dumbbell shape.

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