In prokaryotes, cell division results in the reproduction of the entire single-celled organism. The cell grows in size, replicates its DNA, and then separates the cytoplasm and DNA into two new cells in a process called binary fission.

REPRODUCTIVE SIGNALS External factors such as environmental conditions and nutrient concentrations are common signals for the initiation of cell division in prokaryotes. For example, the bacterium Bacillus subtillis divides every 120 minutes if abundant sources of carbohydrate and mineral nutrients are available. When nutrient levels are low, it slows down its growth and eventually stops dividing. When conditions improve, growth and division resume. The bacterium Escherichia coli, which is widely used in genetic studies, divides every 20 minutes if abundant sources of carbohydrates and mineral nutrients are available.

REPLICATION OF DNA As you saw in Key Concept 5.3, a chromosome consists of a long, thin DNA molecule with proteins attached to it. When a cell divides, all of its chromosomes, which contain the genetic information for the organism, must be replicated, and one copy of each chromosome must find its way into each of the two new cells.

Most prokaryotes have just one main chromosome. If the E. coli chromosome were spread out into a full circle, the circle would be about 500 micrometers (µm) in diameter, more than 200 times larger than the cell! To fit into the cell, bacterial DNA must be compacted. The DNA folds in on itself, and positively charged (basic) proteins bound to the negatively charged (acidic) DNA contribute to this folding.

Two regions of the prokaryotic chromosome play functional roles in cell reproduction:

Chromosome replication takes place as the DNA is threaded through a replication complex of proteins near the center of the cell. Replication begins at the ori site and moves bidirectionally toward the ter site. While the DNA replicates, anabolic metabolism is active, resulting in cell growth. When replication is complete, the two daughter DNA molecules separate and segregate from one another at opposite ends of the cell. In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell divisions.

SEGREGATION OF DNA MOLECULES Replication begins near the center of the cell, and as it proceeds, the ori regions move toward opposite ends of the cell (Figure 11.2A). DNA sequences adjacent to the ori region bind proteins that are essential for this segregation. This is an energy-requiring process, since the binding proteins hydrolyze ATP.

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CYTOKINESIS Immediately after chromosome replication is finished, cytokinesis begins. At first, the cell membrane pinches in to form a ring of fibers similar to a purse string. The major component of these fibers is a protein called FtsZ that is related to eukaryotic tubulin (which makes up microtubules; see Figure 5.14). As the membrane pinches in, new cell wall materials are deposited, which finally separate the two cells (Figure 11.2B).