Although the prokaryotic cells of bacteria have no nuclear envelopes or organelles, folds of the plasma and other membranes apparently perform some of the functions of the organelles of eukaryotic cells. Ribosomes that are about half the size of those in the cytoplasm of eukaryotic cells are also present. Bacterial cells have a nucleoid, which is a single, long, very condensed DNA molecule in the form of a ring. The nucleoid is usually attached at one point to the plasma membrane. In addition, up to 30 or 40 small, circular DNA molecules called plasmids may be present. The plasmids replicate (produce duplicate copies of themselves) independently of the large DNA molecule or chromosome, and the entire complement of plasmids often consists of copies of one or, at most, very few different plasmids. The chromosome and sometimes all or part of the plasmids replicate before the cell divides (Fig. 17.1).
The mitosis of eukaryotic cells does not occur in bacteria. Instead, there is an internal reorganization of material during which the two DNA molecules migrate to opposite ends of the cell. Then, at approximately the middle of the cell, the plasma membrane and cell wall, as a result of forming a transverse wall, divide the cell in two. Finally, the two new cells separate and enlarge to their original size, or in some bacteria, the cells
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