Dna

Prokaryotic DNA is a single closed loop of double-stranded DNA attached at one point to the cell membrane. Unlike eukaryotic DNA, prokaryotic DNA is not enclosed in a nucleus. Along with this single main chromosome, some prokaryotes have plasmids. Plasmids are small, circular, self-replicating loops of double-stranded DNA. Plasmids are usually not necessary for the cell's growth and reproduction, but some plasmids carry genes that enable the bacterium to cause disease. Other plasmids carry genes that provide the bacterium with resistance to certain antibiotics.

Capsules and Pili

Many bacteria have an outer covering of polysaccharides called a capsule. These sugars bind to the cell wall and protect the cell against drying or harsh chemicals. The capsule also helps protect a pathogenic (disease-causing) bacterium from the host's white blood cells, which could otherwise engulf the bacterium. A capsule made up of a fuzzy coat of sticky sugars is called a glycocalyx (GLiE-koh-KAY-liks). This structure allows bacteria to connect to the surface of host cells and tissues.

Pili (PIL-ee) (singular, pilus) are short, hairlike protein structures on the surface of some bacteria. Pili help bacteria connect to each other and to surfaces, such as those of a host cell. Pili can also serve as a bridge to pass genetic material between bacteria.

Endospores

Some Gram-positive bacteria can form a thick-coated, resistant structure called an endospore when environmental conditions become harsh. The harsh conditions may destroy the original cell, but the endospore containing the cell's DNA can survive. Endospores can resist high temperatures, strong chemicals, radiation, drying, and other environmental extremes. When good conditions return, the endospore gives rise to a normal bacterial cell. Species of the genera Bacillus and Clostridium can form endospores. Figure 23-8 shows a C. botulinum cell with an endospore. The endospores of C. botulinum can germinate in improperly sterilized canned foods. The bacteria make a toxin that if eaten can cause the nerve disease botulism. While rare, botulism is often fatal if it is not treated promptly.

Prokaryotic Movement

Many prokaryotes have long flagella that allow the prokaryotes to move toward food sources or away from danger. Movement toward or away from a stimulus is called taxis (TAKS-is). In chemotaxis, prokaryotes react to chemical stimuli by moving toward food or away from a toxin. Prokaryotic flagella rotate and move the prokaryote in a "run-and-tumble" motion. Prokaryotes can have a single whiplike flagellum or many flagella.

Some bacteria do not have flagella but can move by other means. Species of the genus Myxobacteria, for example, form a layer of slime. Wavelike contractions of the outer membrane move the organisms through the slime. Some spiral-shaped bacteria move by a corkscrew-like rotation. Filaments inside the organism's cell walls contract and cause the bacterium to turn and move ahead.

figure 23-8

Unfavorable conditions can cause some bacteria, such as the Grampositive rod Clostridium botulinum, to form endospores. After forming the endospore, the bacterial cell disintegrates, and the endospore containing the cell's DNA is released. Endospores can survive in harsh environments for a long time.

figure 23-8

Unfavorable conditions can cause some bacteria, such as the Grampositive rod Clostridium botulinum, to form endospores. After forming the endospore, the bacterial cell disintegrates, and the endospore containing the cell's DNA is released. Endospores can survive in harsh environments for a long time.

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