Prions and viroids Misfolded prion For decades scientists have been fascinated by a peculiar group of fatal brain diseases. These diseases have the un usual property that it is years and often decades after infec tion before the disease is detected in infected individuals The brains of infected individuals develop numerous small cavities as neurons die, producing a marked spongy appear- ance Called transmissible spongiform encephalopathies TSEs), these diseases include scrapie in sheep, "mad cow disease in cattle, and kuru and Creutzfeldt-Jakob disease in TSEs can be transmitted by injecting infected brain tis sue into a recipient animals brain. TSEs can also spread via tissue transplants and, apparently, food. Kuru was common in the Fore people of Papua New Guinea, when they prac- ticed ritual cannibalism, literally eating the brains of in fected individuals. Mad cow disease spread widely among the cattle herds of England in the 1990s because cows were fed bone meal prepared from cattle carcasses to increase the protein content of their diet. Like the Fore, the british cattle were literally eating the tissue of cattle that had died FIGURE 33.11 of the disease tein to misfold simply by contacting them. When priorty ion How prions arise. Misfolded prions seem to cause normal p misfolded in different ways(blue)contact normal prion protein A Heretical St (purple), the normal prion protein misfolds in the same way In the 1960s, British researchers T. Alper and J. griffith noted that infectious TSE preparations remained infectious even after exposed to radiation that would destroy DNA or Prusiner injected prions of a different abnormal conforma RNA. They suggested that the infectious agent was a pro- tion into several different hosts, these hosts developed tein.Perhaps, they speculated, the protein usually preferred ons with the same abnormal conformations as the parent one folding pattern, but could sometimes misfold, and then prions(figure 33. 11). In another important experiment, catalyze other proteins to do the same, the misfolding Charles Weissmann showed that mice genetically engi- spreading like a chain reaction. This heretical suggestion neered to lack Prusiner's prion protein are immune to TSE was not accepted by the scientific community, as it violates infection. However, if brain tissue with the prion protein is a key tenant of molecular biology: only DNA or RNA act afted into the mice, the grafted tissue--but not the rest as hereditary material, transmitting information from one of the brain-can then be infected with TSE. In 1997 generation to the next. Prusiner was awarded the Nobel Prize in Physiology or Medicine for his work on prions. Prusiner's Prions Viroids In the early 1970s, physician Stanley Prusiner, moved by the death of a patient from Creutzfeldt-Jakob disease, Viroids are tiny, naked molecules of RNA, only a few hun began to study TSEs. Prusiner became fascinated with dred nucleotides long, that are important infectious disease Alper and Griffith's hypothesis. Try as he might, Prusiner agents in plants. A recent viroid outbreak killed over ten could find no evidence of nucleic acids or viruses in the in- million coconut palms in the Philippines. It is not clear fectious TSE preparations, and concluded, as Alper and how viroids cause disease. One clue is that viroid nu Griffith had, that the infectious agent was a protein, which cleotide sequences resemble the sequences of introns in a 1982 paper he named a prion, for "proteinaceous in within ribosomal RNA genes. These sequences are capable fectious particle.” of catalyzing excision from DNA--perhaps the viroids are Prusiner went on to isolate a distinctive prion protein, catalyzing the destruction of chromosomal integrity and for two decades continued to amass evidence that pri- ons play a key role in triggering TSEs. The scientific com Prions are infectious proteins that some scientists munity resisted Prusiner's renegade conclusions, but even- believe are responsible for serious brain diseases. In ally experiments done in Prusiner's and other plants, naked RNA molecules called viroids can also transmit disease laboratories began to convince many. For example, when Chapter 33 Viruses 677
Prions and Viroids For decades scientists have been fascinated by a peculiar group of fatal brain diseases. These diseases have the unusual property that it is years and often decades after infection before the disease is detected in infected individuals. The brains of infected individuals develop numerous small cavities as neurons die, producing a marked spongy appearance. Called transmissible spongiform encephalopathies (TSEs), these diseases include scrapie in sheep, “mad cow” disease in cattle, and kuru and Creutzfeldt-Jakob disease in humans. TSEs can be transmitted by injecting infected brain tissue into a recipient animal’s brain. TSEs can also spread via tissue transplants and, apparently, food. Kuru was common in the Fore people of Papua New Guinea, when they practiced ritual cannibalism, literally eating the brains of infected individuals. Mad cow disease spread widely among the cattle herds of England in the 1990s because cows were fed bone meal prepared from cattle carcasses to increase the protein content of their diet. Like the Fore, the British cattle were literally eating the tissue of cattle that had died of the disease. A Heretical Suggestion In the 1960s, British researchers T. Alper and J. Griffith noted that infectious TSE preparations remained infectious even after exposed to radiation that would destroy DNA or RNA. They suggested that the infectious agent was a protein. Perhaps, they speculated, the protein usually preferred one folding pattern, but could sometimes misfold, and then catalyze other proteins to do the same, the misfolding spreading like a chain reaction. This heretical suggestion was not accepted by the scientific community, as it violates a key tenant of molecular biology: only DNA or RNA act as hereditary material, transmitting information from one generation to the next. Prusiner’s Prions In the early 1970s, physician Stanley Prusiner, moved by the death of a patient from Creutzfeldt-Jakob disease, began to study TSEs. Prusiner became fascinated with Alper and Griffith’s hypothesis. Try as he might, Prusiner could find no evidence of nucleic acids or viruses in the infectious TSE preparations, and concluded, as Alper and Griffith had, that the infectious agent was a protein, which in a 1982 paper he named a prion, for “proteinaceous infectious particle.” Prusiner went on to isolate a distinctive prion protein, and for two decades continued to amass evidence that prions play a key role in triggering TSEs. The scientific community resisted Prusiner’s renegade conclusions, but eventually experiments done in Prusiner’s and other laboratories began to convince many. For example, when Prusiner injected prions of a different abnormal conformation into several different hosts, these hosts developed prions with the same abnormal conformations as the parent prions (figure 33.11). In another important experiment, Charles Weissmann showed that mice genetically engineered to lack Prusiner’s prion protein are immune to TSE infection. However, if brain tissue with the prion protein is grafted into the mice, the grafted tissue—but not the rest of the brain—can then be infected with TSE. In 1997, Prusiner was awarded the Nobel Prize in Physiology or Medicine for his work on prions. Viroids Viroids are tiny, naked molecules of RNA, only a few hundred nucleotides long, that are important infectious disease agents in plants. A recent viroid outbreak killed over ten million coconut palms in the Philippines. It is not clear how viroids cause disease. One clue is that viroid nucleotide sequences resemble the sequences of introns within ribosomal RNA genes. These sequences are capable of catalyzing excision from DNA—perhaps the viroids are catalyzing the destruction of chromosomal integrity. Prions are infectious proteins that some scientists believe are responsible for serious brain diseases. In plants, naked RNA molecules called viroids can also transmit disease. Chapter 33 Viruses 677 Misfolded prion proteins Normal prion proteins Neuron FIGURE 33.11 How prions arise. Misfolded prions seem to cause normal prion protein to misfold simply by contacting them. When prions misfolded in different ways (blue) contact normal prion protein (purple), the normal prion protein misfolds in the same way
Chapter 33 www.mhhe.com/raven6e www.biocourse.com Summary Questions Media resources 33.1 Viruses are strands of nucleic acid encased within a protein coat ·Ⅴ iruses are fragments of dna or rna surrounded 1. Why are viruses not Characteristics of by protein that are able to replicate within cells by considered to be living using the genetic machinery of those cells organisms: The simplest viruses use the enzymes of the host cell 2. How did early scientists come for both protein synthesis and gene replication; the more complex ones contain up to 200 genes and are infectious agents associated with hoof-and-mouth disease in cattle capable of synthesizing many structural proteins an were not bacter 3. What is the approximate size Viruses are basically either helical or isometric. Most inge of viruses and type of symetric viruses are icosahedral In shape microscope is generally required to visualize viruses 33.2 Bacterial viruses exhibit two sorts of reproductive cycles. Virulent bacteriophages infect bacterial cells by ae 4. What is a bacteriophage? Life Cycle of viruses injecting their viral DNA or RNA into the cell, where How does a T4 phage infect a it directs the production of new virus particles host cell ultimately lysing the cell cell, insert their DNA into the cell genome, where P Temperate bacteriophages, upon entering a bacter they may remain integrated into the bacterial genome as a prophage for many generations 33.3 HIV is a complex animal virus. AIDS, a viral infection that destroys the immune 5. What specific type of human Bioethics Case Study: system, is caused by HIV(human immunodeficiency cell does the AIDS AIDS Vaccine virus). After docking on a specific protein called CD4, How does it recognize this On Science articles. HIV enters the cell and replicates, destroying the cell. specific kind of cell rug Therapy for Considerable progress has been made in the 6. How do many animal viruses AIDS treatment of AIDS, particularly with drugs such as penetrate the host cell? How Curing AIDs Just Got protease inhibitors that block cleavage of Hiv does a plant virus infect its hos How does a bacterial virus infect HIV Delivery Protein polyproteins into functional segments. its host? 33. 4 Nonliving infectious agents are responsible for many human diseases. Viruses are responsible for many serious human 7. Why is it so much more Scientists on Science diseases. Some of the most serious. like AIDs and difficult to treat a viral infection Ebola, have only recently transferred to humans from than a bacterial one? Is this · Book review:Tbe ome other animal host different from treating bacterial Coming Plague by Proteins called prions may transmit serious brain On Science articles: diseases from one individual to another 8. What is a prion? How does it integrate into living systems Prions and bloo 678 Part IX Viruses and Simple organism
678 Part IX Viruses and Simple Organisms Chapter 33 Summary Questions Media Resources 33.1 Viruses are strands of nucleic acid encased within a protein coat. • Viruses are fragments of DNA or RNA surrounded by protein that are able to replicate within cells by using the genetic machinery of those cells. • The simplest viruses use the enzymes of the host cell for both protein synthesis and gene replication; the more complex ones contain up to 200 genes and are capable of synthesizing many structural proteins and enzymes. • Viruses are basically either helical or isometric. Most isometric viruses are icosahedral in shape. 1. Why are viruses not considered to be living organisms? 2. How did early scientists come to the conclusion that the infectious agents associated with hoof-and-mouth disease in cattle were not bacteria? 3. What is the approximate size range of viruses and type of microscope is generally required to visualize viruses? • Virulent bacteriophages infect bacterial cells by injecting their viral DNA or RNA into the cell, where it directs the production of new virus particles, ultimately lysing the cell. • Temperate bacteriophages, upon entering a bacterial cell, insert their DNA into the cell genome, where they may remain integrated into the bacterial genome as a prophage for many generations. 4. What is a bacteriophage? How does a T4 phage infect a host cell? 33.2 Bacterial viruses exhibit two sorts of reproductive cycles. • AIDS, a viral infection that destroys the immune system, is caused by HIV (human immunodeficiency virus). After docking on a specific protein called CD4, HIV enters the cell and replicates, destroying the cell. • Considerable progress has been made in the treatment of AIDS, particularly with drugs such as protease inhibitors that block cleavage of HIV polyproteins into functional segments. 5. What specific type of human cell does the AIDS virus infect? How does it recognize this specific kind of cell? 6. How do many animal viruses penetrate the host cell? How does a plant virus infect its host? How does a bacterial virus infect its host? 33.3 HIV is a complex animal virus. • Viruses are responsible for many serious human diseases. Some of the most serious, like AIDS and Ebola, have only recently transferred to humans from some other animal host. • Proteins called prions may transmit serious brain diseases from one individual to another. 7. Why is it so much more difficult to treat a viral infection than a bacterial one? Is this different from treating bacterial infections? 8. What is a prion? How does it integrate into living systems? 33.4 Nonliving infectious agents are responsible for many human diseases. www.mhhe.com/raven6e www.biocourse.com • Characteristics of Viruses • Life Cycle of Viruses • Bioethics Case Study: AIDS Vaccine On Science Articles: • HIV’s Waiting Game • Drug Therapy for AIDS • Curing AIDS Just Got Harder • HIV Delivery Protein • Scientists on Science: Prions • Book Review: The Coming Plague by Garrett On Science Articles: • Smallpox: Tomorrow’s Nightmare? • Smallpox Questions • Mad Cows and Prions • Prions and Blood Supply • Hepatitis C • Increasing Mad Cow Diseases
34 Bacteria Concept Outline 34.1 Bacteria are the smallest and most numerous The Prevalence of Bacteria. The simplest of organisms, bacteria are thought to be the most ancient. They are the most abundant living organisms. Bacteria lack the high degree of internal compartmentalization characteristic of 34.2 Bacterial cell structure is more complex than commonly supposed. The Bacterial Surface. Some bacteria have a secondary membranelike covering outside of their cell wall. The Cell Interior While bacteria lack extensive intern compartments, they may have complex internal 34.3 Bacteria exhibit considerable diversity in both structure and metabolism Bacterial Diversity. There are at least 16 phyla of FIGURE 34.1 bacteria, although many more remain to be discovered A colony of bacteria. With their enormous adaptability and Bacterial Variation. Mutation and recombination metabolic versatility, bacteria are found in every habitat on earth, generate enormous variation within bacterial populations carrying out many of the vital processes of ecosystems, including Bacterial Metabolism. Bacteria obtain carbon atoms and photosynthesis, nitrogen fixation, and decomposition. ergy from a wide array of sources. Some can thrive in the absence of other organisms, while others must obtain their energy and carbon atoms from other organisms he simplest organisms living on earth today are bacte 34.4 Bacteria are responsible for many diseases but ria, and biologists think they closely resemble the first organisms to evolve on earth. Too small to see with the un- also make important contributions to ecosystems. aided eye, bacteria are the most abundant of all organisms Human Bacterial Diseases. Many serio ous human (figure 34. 1)and are the only ones characterized by diseases are caused by bacteria, some of them responsible prokaryotic cellular organization. Life on earth could not for millions of deaths each year exist without bacteria because bacteria make possible many nce of bacter of the essential functions of ecosystems, including the cap- impact on the worlds ecology, and play a major role in ture of nitrogen from the atmosphere, decomposition of modern medicine and agriculture organic matter, and, in many aquatic communities, photo- synthesis. Indeed, bacterial photosynthesis is thought to have been the source for much of the oxygen in the earths atmosphere. Bacterial research continues to provide extra understanding of bacteria is thus essential 679
679 34 Bacteria Concept Outline 34.1 Bacteria are the smallest and most numerous organisms. The Prevalence of Bacteria. The simplest of organisms, bacteria are thought to be the most ancient. They are the most abundant living organisms. Bacteria lack the high degree of internal compartmentalization characteristic of eukaryotes. 34.2 Bacterial cell structure is more complex than commonly supposed. The Bacterial Surface. Some bacteria have a secondary membranelike covering outside of their cell wall. The Cell Interior. While bacteria lack extensive internal compartments, they may have complex internal membranes. 34.3 Bacteria exhibit considerable diversity in both structure and metabolism. Bacterial Diversity. There are at least 16 phyla of bacteria, although many more remain to be discovered. Bacterial Variation. Mutation and recombination generate enormous variation within bacterial populations. Bacterial Metabolism. Bacteria obtain carbon atoms and energy from a wide array of sources. Some can thrive in the absence of other organisms, while others must obtain their energy and carbon atoms from other organisms. 34.4 Bacteria are responsible for many diseases but also make important contributions to ecosystems. Human Bacterial Diseases. Many serious human diseases are caused by bacteria, some of them responsible for millions of deaths each year. Importance of Bacteria. Bacteria have had a profound impact on the world’s ecology, and play a major role in modern medicine and agriculture. The simplest organisms living on earth today are bacteria, and biologists think they closely resemble the first organisms to evolve on earth. Too small to see with the unaided eye, bacteria are the most abundant of all organisms (figure 34.1) and are the only ones characterized by prokaryotic cellular organization. Life on earth could not exist without bacteria because bacteria make possible many of the essential functions of ecosystems, including the capture of nitrogen from the atmosphere, decomposition of organic matter, and, in many aquatic communities, photosynthesis. Indeed, bacterial photosynthesis is thought to have been the source for much of the oxygen in the earth’s atmosphere. Bacterial research continues to provide extraordinary insights into genetics, ecology, and disease. An understanding of bacteria is thus essential. FIGURE 34.1 A colony of bacteria. With their enormous adaptability and metabolic versatility, bacteria are found in every habitat on earth, carrying out many of the vital processes of ecosystems, including photosynthesis, nitrogen fixation, and decomposition
34.1 Bacteria are the smallest and most numerous organis The Prevalence of bacteria Bacteria are the oldest, structurally simplest, and the most abundant forms of life on earth. They are also the only organisms with prokaryotic cellular organization. Represented in the oldest rocks from which fossils have been obtained.3.5 to 3. 8 billion ars old, bacteria were abundant for over 2 billion vears before eukaryotes appeared in the world(see figure 4.11). Early photosynthetic bacteria (cyanobacteria)altered the earth's at- (a) mosphere with the production of oxy- FIGURE 34.2 gen which lead to extreme bacterial The diversity of bacteria. (a) Pseudomonas aeruginosa, a rod-shaped, flagellated bacterium and eukaryotic diversity. Bacteria play (bacillus). Pseudomonas includes the bacteria that cause many of the most serious plant a vital role both in productivity and in diseases. (6)Streptococus. The spherical individual bacteria(cocci)adhere in chains ycling the substances essential to all members of this genus (4,000X).(@) Spirillum volitans, one of the spirilla. This. the other life-forms. Bacteria are the only bacterium, which occurs in stagnant fresh water, has a tuft of flagella at each end (500X) organisms capable of fixing atmos- pheric nItrogen About 5000 different kinds of bac- teria are currently recognized, but there are doubtless Bacterial Form many thousands more awaiting proper identification(figure 34.2). Every place microbiologists look, new species are Bacteria are mostly simple in form and exhibit one of three basic structures: bacillus (plural, bacilli) straight and rod- about bacteria. In the 1970s and 80s a new type of bac- shaped, coccus(plural, cocci)spherical-shaped, and spiril- terium was analyzed that eventually lead to the classifica tion of a new prokary otic cell type, the archeabacteria (or spirochetes. Spirilla bacteria generally do not form associa- Archaea). Even when viewed with an electron microscope, tions with other cells and swim singly through their envi- the structural differences between different bacteria are ronments. They have a complex structure within their cell minor compared to other groups of organisms. Because the membranes that allow them to spin their corkscrew-shaped structural differences are so slight, bacteria are classified bodies which propels them along. Some rod-shaped and based primarily upon their metabolic and genetic charac- pherical bacteria form colonies, adhering end-to-end after they have divided, forming chains(see figure 34. 2). Some teristics. Bacteria can be characterized properly only when bacterial colonies change into stalked structures,grow they are grown on a defined medium because the charac- long, branched filaments, or form erect structures that re- teristics of these organisms often change, depending on lease spores, single-celled bodies that grow into new bacte- their growth conditions Bacteria are ubiquitous on Earth, and live everywhere rial individuals. Some filamentous bacteria are capable of eukaryotes do. Many of the other more extreme environ- gliding motion, often combined with rotation around a ments in which bacteria are found would be lethal to any longitudinal axis. Biologists have not yet determ other form of life. Bacteria live in hot springs that would mechanism by which they move cook other organisms, hypersaline environments that would dehydrate other cells, and in atmospheres rich in Prokaryotes versus Eukaryotes toxic gases like methane or hydrogen sulfide that would kill Prokaryotes-eubacteria and archaea-differ from eukary- most other organisms. These harsh environments may be similar to the conditions present on the early Earth, when otes in numerous important features. These differences life first began. It is likely that bacteria evolved to dwell in he of the most fundamental disting these harsh conditions early on and have retained the abil- separate any groups of rganisnns ity to exploit these areas as the rest of the atmosphere has 1. Multicellularity. All prokaryotes are fundamentally changed single-celled. In some types, individual cells adhere to 680 Part IX Viruses and Simple organism
Bacterial Form Bacteria are mostly simple in form and exhibit one of three basic structures: bacillus (plural, bacilli) straight and rodshaped, coccus (plural, cocci) spherical-shaped, and spirillus (plural, spirilla) long and helical-shaped, also called spirochetes. Spirilla bacteria generally do not form associations with other cells and swim singly through their environments. They have a complex structure within their cell membranes that allow them to spin their corkscrew-shaped bodies which propels them along. Some rod-shaped and spherical bacteria form colonies, adhering end-to-end after they have divided, forming chains (see figure 34.2). Some bacterial colonies change into stalked structures, grow long, branched filaments, or form erect structures that release spores, single-celled bodies that grow into new bacterial individuals. Some filamentous bacteria are capable of gliding motion, often combined with rotation around a longitudinal axis. Biologists have not yet determined the mechanism by which they move. Prokaryotes versus Eukaryotes Prokaryotes—eubacteria and archaea—differ from eukaryotes in numerous important features. These differences represent some of the most fundamental distinctions that separate any groups of organisms. 1. Multicellularity. All prokaryotes are fundamentally single-celled. In some types, individual cells adhere to 680 Part IX Viruses and Simple Organisms The Prevalence of Bacteria Bacteria are the oldest, structurally simplest, and the most abundant forms of life on earth. They are also the only organisms with prokaryotic cellular organization. Represented in the oldest rocks from which fossils have been obtained, 3.5 to 3.8 billion years old, bacteria were abundant for over 2 billion years before eukaryotes appeared in the world (see figure 4.11). Early photosynthetic bacteria (cyanobacteria) altered the earth’s atmosphere with the production of oxygen which lead to extreme bacterial and eukaryotic diversity. Bacteria play a vital role both in productivity and in cycling the substances essential to all other life-forms. Bacteria are the only organisms capable of fixing atmospheric nitrogen. About 5000 different kinds of bacteria are currently recognized, but there are doubtless many thousands more awaiting proper identification (figure 34.2). Every place microbiologists look, new species are being discovered, in some cases altering the way we think about bacteria. In the 1970s and 80s a new type of bacterium was analyzed that eventually lead to the classification of a new prokaryotic cell type, the archeabacteria (or Archaea). Even when viewed with an electron microscope, the structural differences between different bacteria are minor compared to other groups of organisms. Because the structural differences are so slight, bacteria are classified based primarily upon their metabolic and genetic characteristics. Bacteria can be characterized properly only when they are grown on a defined medium because the characteristics of these organisms often change, depending on their growth conditions. Bacteria are ubiquitous on Earth, and live everywhere eukaryotes do. Many of the other more extreme environments in which bacteria are found would be lethal to any other form of life. Bacteria live in hot springs that would cook other organisms, hypersaline environments that would dehydrate other cells, and in atmospheres rich in toxic gases like methane or hydrogen sulfide that would kill most other organisms. These harsh environments may be similar to the conditions present on the early Earth, when life first began. It is likely that bacteria evolved to dwell in these harsh conditions early on and have retained the ability to exploit these areas as the rest of the atmosphere has changed. 34.1 Bacteria are the smallest and most numerous organisms. (a) (b) (c) FIGURE 34.2 The diversity of bacteria. (a) Pseudomonas aeruginosa, a rod-shaped, flagellated bacterium (bacillus). Pseudomonas includes the bacteria that cause many of the most serious plant diseases. (b) Streptococcus. The spherical individual bacteria (cocci) adhere in chains in the members of this genus (34,000). (c) Spirillum volutans, one of the spirilla. This large bacterium, which occurs in stagnant fresh water, has a tuft of flagella at each end (500).
FIGURE 34.3 Approaches multicellularity in bacteria the gliding bacteria. The rod-shaped individuals move together, forming the composite spore-bearing structures shown ere. Millions of spores, which are basically individual FIGURE 34.4 Flagella in the common intestinal bacterium, Escbericbia coli. teria, are eleased from The long strands are flagella, while the shorter hairlike outgrowths these structures are called pili. each other within a matrix and form filaments. ho nisms are far less regular than those of eukaryotes and ever the cells retain their individuality Cyanobacte do not involve the equal participation of the individuals ria, in particular, are likely to form such associations between which the genetic material is transferred but their cytoplasm is not directly interconnected, as 5. Internal compartmentalization. In eukaryotes, often is the case in multicellular eukaryotes. The ac the enzymes for cellular respiration are packaged in tivities of a bacterial colony are less integrated and mitochondria. In bacteria, the corresponding en coordinated than those in multicellular eukaryotes. A zymes are not packaged separately but are bound to primitive form of colonial organization occurs in he cell membranes(see chapters 5 and 9). The cyto- gliding bacteria, which move together and form plasm of bacteria, unlike that of eukaryotes, contain nated multicellular forms are rare among bacteri spore-bearing structures(figure 34.3). Such coor no internal compartments or cytoskeleton and no or ganelles except ribosomes 2. Cell size. As new species of bacteria are discovered 6. Flagella. Bacterial flagella are simple in structure, we are finding that the size of prokaryotic cells varies composed of a single fiber of the protein flagellin tremendously, by as much as five orders of magni- (figure 34.4; see also chapter 5). Eukaryotic flagella tude. Most prokaryotic cells are only 1 micrometer or and cilia are complex and have a 9+2 structure of less in diameter. Most eukaryotic cells are well over microtubules(see figure 5.27). Bacterial flagella also 10 times that size function differently, spinning like propellers, while 3. Chromosomes. Eukaryotic cells have a membrane- eukaryotic flagella have a whiplike motion bound nucleus containing chromosomes made up of 7. Metabolic diversity. Only one kind of photosyn- both nucleic acids and proteins. Bacteria do not have thesis occurs in eukaryotes, and it involves the release membrane-bound nuclei, nor do the have chromo of oxygen. Photosynthetic bacteria have several dif- somes of the kind present in eukaryotes, in which ferent patterns of anaerobic and aerobic photosynthe- DNA forms a structural complex with proteins. I of end products such as stead. their naked circular dna is localized in a zone sulfur, sulfate, and oxygen(see chapter 10). Prokary of the cytoplasm called the nucleoid. otic cells can also be chemoautotrophic, using the en 4. Cell division and genetic recombination. Cell divi- ergy stored in chemical bonds of inorganic molecules sion in eukaryotes takes place by mitosis and involves D synthesize carbohydrates; eukaryotes are not capa les made up of microtubules. Cell division in bac ble of this metabolic process teria takes place mainly by binary fission(see chapter 11). True sexual reproduction occurs only in eukaryotes Bacteria are the oldest and most abundant organisms on and involves syngamy and meiosis, with an alternation earth. Bacteria, or prokaryotes, differ from eukaryotes of diploid and haploid forms. Despite their lack of sex- in a wide variety of characteristics, a degree of ual reproduction, bacteria do have mechanisms that difference as great as any that separates any groups of lead to the transfer of genetic material. These mecha organisms. Chapter 34 Bacteria 681
each other within a matrix and form filaments, however the cells retain their individuality. Cyanobacteria, in particular, are likely to form such associations but their cytoplasm is not directly interconnected, as often is the case in multicellular eukaryotes. The activities of a bacterial colony are less integrated and coordinated than those in multicellular eukaryotes. A primitive form of colonial organization occurs in gliding bacteria, which move together and form spore-bearing structures (figure 34.3). Such coordinated multicellular forms are rare among bacteria. 2. Cell size. As new species of bacteria are discovered, we are finding that the size of prokaryotic cells varies tremendously, by as much as five orders of magnitude. Most prokaryotic cells are only 1 micrometer or less in diameter. Most eukaryotic cells are well over 10 times that size. 3. Chromosomes. Eukaryotic cells have a membranebound nucleus containing chromosomes made up of both nucleic acids and proteins. Bacteria do not have membrane-bound nuclei, nor do they have chromosomes of the kind present in eukaryotes, in which DNA forms a structural complex with proteins. Instead, their naked circular DNA is localized in a zone of the cytoplasm called the nucleoid. 4. Cell division and genetic recombination. Cell division in eukaryotes takes place by mitosis and involves spindles made up of microtubules. Cell division in bacteria takes place mainly by binary fission (see chapter 11). True sexual reproduction occurs only in eukaryotes and involves syngamy and meiosis, with an alternation of diploid and haploid forms. Despite their lack of sexual reproduction, bacteria do have mechanisms that lead to the transfer of genetic material. These mechanisms are far less regular than those of eukaryotes and do not involve the equal participation of the individuals between which the genetic material is transferred. 5. Internal compartmentalization. In eukaryotes, the enzymes for cellular respiration are packaged in mitochondria. In bacteria, the corresponding enzymes are not packaged separately but are bound to the cell membranes (see chapters 5 and 9). The cytoplasm of bacteria, unlike that of eukaryotes, contains no internal compartments or cytoskeleton and no organelles except ribosomes. 6. Flagella. Bacterial flagella are simple in structure, composed of a single fiber of the protein flagellin (figure 34.4; see also chapter 5). Eukaryotic flagella and cilia are complex and have a 9 + 2 structure of microtubules (see figure 5.27). Bacterial flagella also function differently, spinning like propellers, while eukaryotic flagella have a whiplike motion. 7. Metabolic diversity. Only one kind of photosynthesis occurs in eukaryotes, and it involves the release of oxygen. Photosynthetic bacteria have several different patterns of anaerobic and aerobic photosynthesis, involving the formation of end products such as sulfur, sulfate, and oxygen (see chapter 10). Prokaryotic cells can also be chemoautotrophic, using the energy stored in chemical bonds of inorganic molecules to synthesize carbohydrates; eukaryotes are not capable of this metabolic process. Bacteria are the oldest and most abundant organisms on earth. Bacteria, or prokaryotes, differ from eukaryotes in a wide variety of characteristics, a degree of difference as great as any that separates any groups of organisms. Chapter 34 Bacteria 681 FIGURE 34.3 Approaches to multicellularity in bacteria. Chondromyces crocatus, one of the gliding bacteria. The rod-shaped individuals move together, forming the composite spore-bearing structures shown here. Millions of spores, which are basically individual bacteria, are released from these structures. FIGURE 34.4 Flagella in the common intestinal bacterium, Escherichia coli. The long strands are flagella, while the shorter hairlike outgrowths are called pili