18.1 Studying Viruses and Prokaryotes

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How are bacteriahelpful to humans?colored SEM; magnification unknownThese bacteria live in the lining of anesophagus, the tube that leads fromyour mouth to your stomach. Only a fewyears ago, it was thought that nothing couldsurvive in an esophagus, but the entiredigestive tract is home for many types ofbacteria. Being a home to bacteria isn’t bad,though. Some doctors hypothesize thatthese bacteria may protect us from somethroat and stomach cancers.ConnectingCONCEPTSBiotechnology Hundreds ofbacteria can fit on the tip of apin, as shown at left. About thesame number of bacteria willfit on a silicon chip. Scientistshave made a device that cancollect signals from bacteriajoined to silicon chips. Thesebacteria have been speciallyaltered to glow when near pollutants.The chips then measurethe amount of light emitted—themore light, the morepollution present. (colored SEM;magnification 1100)Chapter 18: Viruses and Prokaryotes 543
18.1 Studying Virusesand ProkaryotesKEY CONCEPT Infections can be caused in several ways.MAIN IDEA• Viruses, bacteria, viroids, and prions can allcause infection.VOCABULARYvirus, p. 544pathogen, p. 544viroid, p. 544prion, p. 545Reviewprokaryote, archaeaConnect Bacteria are everywhere, including in and on your own body—such asthe bacteria that live in our digestive tracts. The relationship between you and themicroorganisms in your body is usually mutually beneficial. Under certainconditions, however, normally harmless microorganisms can cause disease, andsome types of microorganisms are particularly nasty—they always make you sick.FIGURE 18.1 Prokaryotes, such asthis Escherichia coli bacterium,are single cells that have all of thecharacteristics of living things.(colored TEM; magnification 6000)MAIN IDEAViruses, bacteria, viroids, and prions can all causeinfection.You are probably familiar with the terms virus and bacteria, but you may notknow exactly what they are. A virus is an infectious particle made only of astrand of DNA or RNA surrounded by a protein coat. Bacteria, on the otherhand, are one-celled microorganisms that can also cause infection. Any livingorganism or particle that can cause an infectious disease is called an infectiousagent, or pathogen.In Chapter 1, you learned that all living things share certain key characteristics:the abilities to reproduce, to use nutrients and energy, to grow anddevelop, and to respond to their environments. They also contain geneticmaterial that carries the code of life. Prokaryotes—such as the bacteriumshown in FIGURE 18.1—are clearly living things, since they have each of the traitsof life. But are viruses living things? Like living cells, viruses respond to theirenvironment. Viruses have genes and can reproduce. Unlike cells, however,viruses cannot reproduce on their own. Instead, they need living cells to helpthem reproduce and make proteins. Viruses are also much smaller than mostcells, as you can see in FIGURE 18.2. While viruses have key traits similar to livingcells, they also have many differences. In fact, viruses are not even given a placein the Linnaean system of biological classification.A viroid has even less in common with living things than do viruses.Viroids are infectious particles that cause disease in plants. Viroids are made ofsingle-stranded RNA without a protein coat. They are passed through seeds orpollen. Viroids have had a major economic impact on agriculture because theycan stunt the growth of plants.544 Unit 6: Classification and Diversity
Page 4 and 5: FIGURE 18.2 Relative Sizes of Cells
Page 6 and 7: 18.2Viral Structure andReproduction
Page 8 and 9: In some viruses, capsids form a 20-
Page 10 and 11: Lytic InfectionA lytic infection (L
Page 12 and 13: Influenza Winter usually causes con
Page 14 and 15: 18.4Bacteria and ArchaeaKEY CONCEPT
Page 16 and 17: FIGURE 18.13 Prokaryote StructureTh
Page 18 and 19: 18.5Beneficial Rolesof ProkaryotesK
Page 20 and 21: Recall from Chapter 13 that nitroge
Page 22 and 23: 18.6Bacterial Diseasesand Antibioti
Page 24 and 25: MAIN IDEABacteria can evolve resist
Page 26 and 27: INVESTIGATIONModeling VirusesThe st
Page 28 and 29: Chapter AssessmentChapter Vocabular
Page 30: For more test practice,go to ClassZ

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