y:口 Haploid□pbi Haploid Gametes cells Reproductive Spore-forming dividuals Zygote /Diploid Gametophytes ophite Zygote Syngamy Gametes (b)Gametic meiosis (c)Sporic meiosis FIGURE 32.14 Diagrams of the three major kinds of life cycles in eukaryotes. (a) Zygotic meiosis, (b)gametic meiosis, and(o) sporic meiosis Eukaryotic Life Cycles gametes that fuse to form the zygote zygote is Eukaryotes are characterized by three major types of life the first cell of the multicellular dipl This kind of life cycle is characterized by alternation of cycles(figure 32. 14): generations and has sporic meiosis 1. In the simplest cycle, found in algae, the zygote is the only diploid cell. Such a life cycle is said to be charac- table 32.2 The characteristics of the six kingdoms are outlined in terized by zygotic meiosis, because the zygote im- mediately undergoes meIosIs 2. In most animals, the gametes are the only haploid cells. Animals exhibit gametic meiosis, meiosis pro- Eukaryotic cells acquired mitochondria and chloroplasts ducing gametes which fuse, giving rise to a zygote by endosymbiosis, mitochondria being derived from 3. Plants show a regular alternation of generations be purple bacteria and chloroplasts from cyanobacteria tween a multicellular haploid phase and a multicell The complex differentiation that we associate with lular diploid phase. The diploid phase undergoes advanced life-forms depends on multicellularity and meiosis producing haploid spores that give rise to sexuality, which must have been highly advantageous to the haploid phase, and the haploid phase produces have evolved independently so often Table 32 2 Characteristics of the Six Kingdoms Nuclear Kingdom Cell Tyo Envelope Mitochondria Chloroplasts Cell wall Archaebacteria Prokary Absent None(photosynthetic Noncellulose(polysaccharide and Eubacteria membranes in some types) plus amino acids Protista Eukaryotic Present or absent Present(some forms) Present in some forms various types Eukaryotic Present Present or absen Chitin and other noncellulose polysaccharides Present Cellulose and other polysaccharides Eukaryotic Absent Absent 662 Part IX Viruses and Simple organism
Eukaryotic Life Cycles Eukaryotes are characterized by three major types of life cycles (figure 32.14): 1. In the simplest cycle, found in algae, the zygote is the only diploid cell. Such a life cycle is said to be characterized by zygotic meiosis, because the zygote immediately undergoes meiosis. 2. In most animals, the gametes are the only haploid cells. Animals exhibit gametic meiosis, meiosis producing gametes which fuse, giving rise to a zygote. 3. Plants show a regular alternation of generations between a multicellular haploid phase and a multicellular diploid phase. The diploid phase undergoes meiosis producing haploid spores that give rise to the haploid phase, and the haploid phase produces gametes that fuse to form the zygote. The zygote is the first cell of the multicellular diploid phase. This kind of life cycle is characterized by alternation of generations and has sporic meiosis. The characteristics of the six kingdoms are outlined in table 32.2. Eukaryotic cells acquired mitochondria and chloroplasts by endosymbiosis, mitochondria being derived from purple bacteria and chloroplasts from cyanobacteria. The complex differentiation that we associate with advanced life-forms depends on multicellularity and sexuality, which must have been highly advantageous to have evolved independently so often. 662 Part IX Viruses and Simple Organisms Table 32.2 Characteristics of the Six Kingdoms Nuclear Kingdom Cell Type Envelope Mitochondria Chloroplasts Cell Wall Archaebacteria and Eubacteria Protista Fungi Plantae Animalia Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic Absent Present Present Present Present Absent Present or absent Present or absent Present Present None (photosynthetic membranes in some types) Present (some forms) Absent Present Absent Noncellulose (polysaccharide plus amino acids) Present in some forms, various types Chitin and other noncellulose polysaccharides Cellulose and other polysaccharides Absent 2n Zygote n + + – – Haploid individuals – + Meiosis Syngamy Haploid cells Gametes (a) Zygotic meiosis Key: Haploid Diploid + – – – + Syngamy Gametes Gametes (b) Gametic meiosis (c) Sporic meiosis n 2n Reproductive 2n cell 2n Diploid individual Zygote Meiosis n + + – – – + Meiosis Syngamy Spores Gametes n Gametophytes (haploid) 2n Spore-forming 2n cell 2n Sporophyte (diploid) Zygote + FIGURE 32.14 Diagrams of the three major kinds of life cycles in eukaryotes. (a) Zygotic meiosis, (b) gametic meiosis, and (c) sporic meiosis
Viruses: A Special Case Viruses pose e a caen to biologists as they do not posses the fundamental characteristics of living organisms. Viruses appear to be fragments of nucleic acids originally derived from the genome of a living cell. Unlike all living organ isms, viruses are acellular--that is, they are not cells and de not consist of cells. They do not have a metabolism; in other words, viruses do not carry out photosynthesis, cellu lar respiration, or fermentation. The one characteristic of life that they do display is reproduction, which they do by hijacking the metabolism of living cells Viruses thus present a special classification problem. Be they are gically place them in any of the kingdoms. Viruses are really just com- plicated associations of molecules, bits of nucleic acids usu ally surrounded by a protein coat. But, despite their sim licity, viruses are able to invade cells and direct the genetic machinery of these cells to manufacture more of the mole- cules that make up the virus(figure 32. 15). Viruses can in- fect organisms at all taxonomic levels Viruses are not organisms and are not classified in the kingdoms of life. FIGURE 32.15 Viruses are cell parasites. In this micrograph, several T4 bacteriophages(viruses) are attacking an Escherichia coli bacterium. Some of the viruses have already entered the cell and are reproducing within it. Table 32.2 Characteristics of the Six Kingdoms Means of genetic Recombination Mode of Nervous if Present Nutrition Motility Multicellular System Conjugation, transduction, Autotrophic(chemo- Bacterial flagella, gliding Absent None ton synthetic, photosyn thetic)or heterotrophic Fertilization and meiosis Photosynthetic or het- 9+2 cilia and flagella bsent in most forms Primitive mechanisms tile fibrils for conducting stimuli tion of both Fertilization and meiosis Absorption Nonmotile Present in most forms Nor Fertilization and meiosis syntheti None in most forms Present in all forms None chlorophylls a and b 9+2 cilia and flagella in gametes of some forms Fertilization and meiosis eston 9+2 cilia and flagella Present in all forms Present, often complex contractile fibril Chapter 32 How We Classify Organisms 663
Viruses: A Special Case Viruses pose a challenge to biologists as they do not possess the fundamental characteristics of living organisms. Viruses appear to be fragments of nucleic acids originally derived from the genome of a living cell. Unlike all living organisms, viruses are acellular—that is, they are not cells and do not consist of cells. They do not have a metabolism; in other words, viruses do not carry out photosynthesis, cellular respiration, or fermentation. The one characteristic of life that they do display is reproduction, which they do by hijacking the metabolism of living cells. Viruses thus present a special classification problem. Because they are not organisms, we cannot logically place them in any of the kingdoms. Viruses are really just complicated associations of molecules, bits of nucleic acids usually surrounded by a protein coat. But, despite their simplicity, viruses are able to invade cells and direct the genetic machinery of these cells to manufacture more of the molecules that make up the virus (figure 32.15). Viruses can infect organisms at all taxonomic levels. Viruses are not organisms and are not classified in the kingdoms of life. Chapter 32 How We Classify Organisms 663 Table 32.2 Characteristics of the Six Kingdoms Means of Genetic Recombination, Mode of Nervous if Present Nutrition Motility Multicellularity System Conjugation, transduction, transformation Fertilization and meiosis Fertilization and meiosis Fertilization and meiosis Fertilization and meiosis Autotrophic (chemosynthetic, photosynthetic) or heterotrophic Photosynthetic or heterotrophic, or combination of both Absorption Photosynthetic chlorophylls a and b Digestion Bacterial flagella, gliding or nonmotile 9 + 2 cilia and flagella; amoeboid, contractile fibrils Nonmotile None in most forms, 9 + 2 cilia and flagella in gametes of some forms 9 + 2 cilia and flagella, contractile fibrils Absent Absent in most forms Present in most forms Present in all forms Present in all forms None Primitive mechanisms for conducting stimuli in some forms None None Present, often complex FIGURE 32.15 Viruses are cell parasites. In this micrograph, several T4 bacteriophages (viruses) are attacking an Escherichia coli bacterium. Some of the viruses have already entered the cell and are reproducing within it
备容mmem Chapter 32 www.mhhe.com/raven6e www.biocourse.com Summary Questions Media resources 32.1 Biologists name organisms in a systematic way Biologists give every species a two-part(binomial) 1. What was the polynomial Hierarchies name that consists of the name of its genus plus a system? Why didn't this system distinctive specific epithet become the standard for naming particular species? In the hierarchical system of classification used in biology, genera are grouped into families, families 2. From the most specific to the Book Reviews: into orders, orders into classes, classes into phyla, and most general, what are the Ship Fever by Barrett names of the in the phyla into kingdoms hierarchical taxonomic system? There are perhaps 10 million species of plants, Which two are given special animals, fungi, and eukaryotic microorganisms, but consideration in the way in names. About 15% of the total number of species are are these d are printed? What only about 1.5 million of them have been assigned which they marine; the remainder are mostly terrestrial 32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms Taxonomists may use different approaches to classify 3. What types of features are ohasized in a cladistic Cladistic systems of classification arrange organisms classification system? What according to evolutionary relatedness based on the the resulting relationship of organisms that are classified in presence of shared, derived traits. this manner? raditional taxonomy classifies organisms based on 4. What does it mean when large amounts of information, giving due weight to haracters are weighted the evolutionary significance of certain characters 32.3 All living organisms are grouped into one of a few major categories A fundamental division among organisms is between 5. Is there a greater fundamental Art activity: prokaryotes, which lack a true nucleus, and difference between plants and eukaryotes, which have a true nucleus and several animals or between prokaryotes Classification membrane-bound organelles. and eukaryotes? Explain Prokaryotes, or bacteria, are assigned to two quite 6. From which of the four different kingdoms, Archaebacteria and Eubacteria ukaryotic kingdoms have the Three domains ·Ph The eukaryotic kingdoms are more closely related than are the two kingdoms of prokaryotes. Many 7. What is the apparent origin of distinctive evolutionary lines of unicellular eukaryotes all eukaryotes? Book Review. exist, most are in the Protista kingdom bowin Way leg 8. What defines if a collection of by Flannel Three of the major evolutionary lines of eukaryotic cells is truly multicellular?Did organisms that consist principally or entirely of multicellularity arise once or multicellular organisms are recognized as separate many times in the evolutionary kingdoms: Plantae, Animalia, and Fungi process? What advantages de True multicellularity and sexuality are found only multicellular organisms have over unicellular ones? among eukaryotes. Multicellularity confers the advantages of functional specialization. Sexuality 9. What are the three major permits genetic variation among descendants types of life cycles in eukaryotes Describe the major events of Viruses are not organisms and are not included in the each. classification of organisms. They are self-replicating portions of the genomes of organisms 664 Part IX Viruses and Simple organism
664 Part IX Viruses and Simple Organisms • A fundamental division among organisms is between prokaryotes, which lack a true nucleus, and eukaryotes, which have a true nucleus and several membrane-bound organelles. • Prokaryotes, or bacteria, are assigned to two quite different kingdoms, Archaebacteria and Eubacteria. • The eukaryotic kingdoms are more closely related than are the two kingdoms of prokaryotes. Many distinctive evolutionary lines of unicellular eukaryotes exist, most are in the Protista kingdom. • Three of the major evolutionary lines of eukaryotic organisms that consist principally or entirely of multicellular organisms are recognized as separate kingdoms: Plantae, Animalia, and Fungi. • True multicellularity and sexuality are found only among eukaryotes. Multicellularity confers the advantages of functional specialization. Sexuality permits genetic variation among descendants. • Viruses are not organisms and are not included in the classification of organisms. They are self-replicating portions of the genomes of organisms. 5. Is there a greater fundamental difference between plants and animals or between prokaryotes and eukaryotes? Explain. 6. From which of the four eukaryotic kingdoms have the other three evolved? 7. What is the apparent origin of the organelles found in almost all eukaryotes? 8. What defines if a collection of cells is truly multicellular? Did multicellularity arise once or many times in the evolutionary process? What advantages do multicellular organisms have over unicellular ones? 9. What are the three major types of life cycles in eukaryotes? Describe the major events of each. 32.3 All living organisms are grouped into one of a few major categories. Chapter 32 Summary Questions Media Resources 32.1 Biologists name organisms in a systematic way. • Biologists give every species a two-part (binomial) name that consists of the name of its genus plus a distinctive specific epithet. • In the hierarchical system of classification used in biology, genera are grouped into families, families into orders, orders into classes, classes into phyla, and phyla into kingdoms. • There are perhaps 10 million species of plants, animals, fungi, and eukaryotic microorganisms, but only about 1.5 million of them have been assigned names. About 15% of the total number of species are marine; the remainder are mostly terrestrial. 1. What was the polynomial system? Why didn’t this system become the standard for naming particular species? 2. From the most specific to the most general, what are the names of the groups in the hierarchical taxonomic system? Which two are given special consideration in the way in which they are printed? What are these distinctions? • Taxonomists may use different approaches to classify organisms. • Cladistic systems of classification arrange organisms according to evolutionary relatedness based on the presence of shared, derived traits. • Traditional taxonomy classifies organisms based on large amounts of information, giving due weight to the evolutionary significance of certain characters. 3. What types of features are emphasized in a cladistic classification system? What is the resulting relationship of organisms that are classified in this manner? 4. What does it mean when characters are weighted? 32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms. www.mhhe.com/raven6e www.biocourse.com • Hierarchies • Book Reviews: Ship Fever by Barrett • Art Activity: Organism Classification • Kingdoms • Three Domains • Phylogeny • Book Review: Thowim Way Leg by Flannery
3 Viruses Concept Outline 33.1 Viruses are strands of nucleic acid encased within a protein coat. The Discovery of Viruses. The first virus to be isolated proved to consist of two chemicals, one a protein and the The Nature of Viruses. Viruses occur in all organisms Able to reproduce only within living cells, viruses are not hemselves alive 33.2 Bacterial viruses exhibit two sorts of reproductive Bacteriophages. Some bacterial viruses, called bacteriophages, rupture the cells they infect, while other integrate themselves into the bacterial chromosome to become a stable part of the bacterial genome Cell transformation and phage Conversion. Integrated bacteriophages sometimes modify the host 33.3 HIV is a complex animal virus. FIGURE 33.1 Influenza viruses. A virus has been referred to as "a piece of bad AIDS. The animal virus HIV infects certain key cells of news wrapped up in a protein. " How can something as"simple"as the immune system, destroying the ability of the body a virus have such a profound effect on living organisms?(30,000X) defend itself from cancer and disease. The hiv infection cycle is typically a lytic cycle, in which the HIV RNA first directs the production of a corresponding DNA, and this DNA then directs the production of progeny virus e start our exploration of the diversity of life with viruses. Viruses are genetic elements enclosed in The Future of HIV Treatment. Combination therapies protein and are not considered to be organisms, as they and chemokines offer promising avenues of AIDS therapy cannot reproduce independently. Because of their disease producing potential, viruses are important biological enti 33.4 Nonliving infectious agents are responsible for ties. The virus particles you see in figure 33 1 produce the ses important disease influenza. Other viruses cause AIDS, Disease viruses. Some of the most serious viral diseases polio, flu, and some can lead to cancer. Many scientists have only recently infected human populations, the result have attempted to unravel the nature of viral genes and of transfer from other hosts how they work. For more than four decades, viral studies Prions and Viroids. In some instances, pr have been thoroughly intertwined with those of genetics "naked" RNA molecules can also transmit diseases and molecular biology. In the future, it is expected that viruses will be one of the principal tools used to experimen tally carry genes from one organism to another. Already viruses are being employed in the treatment of human ge netic diseases
665 33 Viruses Concept Outline 33.1 Viruses are strands of nucleic acid encased within a protein coat. The Discovery of Viruses. The first virus to be isolated proved to consist of two chemicals, one a protein and the other a nucleic acid. The Nature of Viruses. Viruses occur in all organisms. Able to reproduce only within living cells, viruses are not themselves alive. 33.2 Bacterial viruses exhibit two sorts of reproductive cycles. Bacteriophages. Some bacterial viruses, called bacteriophages, rupture the cells they infect, while others integrate themselves into the bacterial chromosome to become a stable part of the bacterial genome. Cell Transformation and Phage Conversion. Integrated bacteriophages sometimes modify the host bacterium they infect. 33.3 HIV is a complex animal virus. AIDS. The animal virus HIV infects certain key cells of the immune system, destroying the ability of the body to defend itself from cancer and disease. The HIV infection cycle is typically a lytic cycle, in which the HIV RNA first directs the production of a corresponding DNA, and this DNA then directs the production of progeny virus particles. The Future of HIV Treatment. Combination therapies and chemokines offer promising avenues of AIDS therapy. 33.4 Nonliving infectious agents are responsible for many human diseases. Disease Viruses. Some of the most serious viral diseases have only recently infected human populations, the result of transfer from other hosts. Prions and Viroids. In some instances, proteins and “naked” RNA molecules can also transmit diseases. We start our exploration of the diversity of life with viruses. Viruses are genetic elements enclosed in protein and are not considered to be organisms, as they cannot reproduce independently. Because of their diseaseproducing potential, viruses are important biological entities. The virus particles you see in figure 33.1 produce the important disease influenza. Other viruses cause AIDS, polio, flu, and some can lead to cancer. Many scientists have attempted to unravel the nature of viral genes and how they work. For more than four decades, viral studies have been thoroughly intertwined with those of genetics and molecular biology. In the future, it is expected that viruses will be one of the principal tools used to experimentally carry genes from one organism to another. Already, viruses are being employed in the treatment of human genetic diseases. FIGURE 33.1 Influenza viruses. A virus has been referred to as “a piece of bad news wrapped up in a protein.” How can something as “simple” as a virus have such a profound effect on living organisms? (30,000)
33.1 Viruses are strands of nucleic acid encased within a protein coat The Discovery of viruses The border between the living and the nonliving is very lear to a biologist. Living organisms are cellular and able to grow and reproduce independently, guided by informa tion encoded within DNA. The simplest creatures living or earth today that satisfy these criteria are bacteria. Even simpler than bacteria are viruses. As you will learn in this Influenza section, viruses are so simple that they do not satisfy the T4 bacteriophage criteria for“ living. Viruses possess only a portion of the properties of ganisms. Viruses are literally"parasitic"chemicals ments of DNA or RNa wrapped in a protein coat. They cannot reproduce on their own, and for this reason they are not considered alive by biologists. They can, however, re produce within cells, often with disastrous results to the host organism. Earlier theories that viruses represent a kind of halfway point between life and nonlife have largely been HIV-1 Herpes simplex abandoned. Instead viruses are now viewed as detached hes of organisms due to the high de- Tobacco mosaic gree of similarity found among some viral and eukaryotic enes Viruses vary greatly in appearance and size. The smallest are only about 17 nanometers in diameter, and the largest are up to 1000 nanometers(1 micrometer)in their greatest Adenovirus Poliovirus dimension(figure 33. 2). The large est viruses are vIsI- ble with a light microscope, but viral morphology is best revealed using the electron microscope. Viruses are so small that they are comparable to molecules in size; a hy- drogen atom is about 0. 1 nanometer in diameter, and a Ebola virus large protein molecule is several hundred nanometers in its test dimension FIGURE 33.2 Biologists first began to suspect the existence of viral d ral diversity. A sample of the extensive diversity and small size viruses near the end of the nineteenth century. European viruses is depicted At the scale these viruses are shown, a human scientists attempting to isolate the infectious agent re hair would be nearly 8 meters thick. sponsible for hoof-and-mouth disease in cattle concluded that it was smaller than a bacterium. Investigating the agent further, the scientists found that it could not multi- ply in solution-it could only reproduce itself within liv- ing host cells that it infected. The infecting agents were lar but rather chemical. Each particle of TMv virus is in called viruses fact a mixture of two chemicals: RNA and protein. The The true nature of viruses was discovered in 1933 TMV virus has the structure of a twinkie. a tube made of when the biologist Wendell Stanley prepared an extract of an RNA core surrounded by a coat of protein. Later work a plant virus called tobacco mosaic virus (TMV) and at ers were able to separate the rNa from the protein and tempted to purify it. To his great surprise, the purified purify and store each chemical. Then, when they reassem- TMV preparation precipitated(that is, separated from so- bled the two components, the reconstructed TMV particles lution)in the form of crystals. This was surprising because were fully able to infect healthy tobacco plants and precipitation is something that only chemicals do-the clearly were the virus itself, not merely chemicals derri o TMV virus was acting like a chemical off the shelf rather from it. Further experiments carried out on other viru than an organism. Stanley concluded that TMv is best re- yielded similar results garded as just that--chemical matter rather than a living Within a few years, scientists disassembled the TMV Viruses are chemical assemblies that can infect cells and virus and found that Stanley was right. TMV was not cellu replicate within them. They are not alive 666 Part IX Viruses and Simple organism
lar but rather chemical. Each particle of TMV virus is in fact a mixture of two chemicals: RNA and protein. The TMV virus has the structure of a Twinkie, a tube made of an RNA core surrounded by a coat of protein. Later workers were able to separate the RNA from the protein and purify and store each chemical. Then, when they reassembled the two components, the reconstructed TMV particles were fully able to infect healthy tobacco plants and so clearly were the virus itself, not merely chemicals derived from it. Further experiments carried out on other viruses yielded similar results. Viruses are chemical assemblies that can infect cells and replicate within them. They are not alive. 666 Part IX Viruses and Simple Organisms The Discovery of Viruses The border between the living and the nonliving is very clear to a biologist. Living organisms are cellular and able to grow and reproduce independently, guided by information encoded within DNA. The simplest creatures living on earth today that satisfy these criteria are bacteria. Even simpler than bacteria are viruses. As you will learn in this section, viruses are so simple that they do not satisfy the criteria for “living.” Viruses possess only a portion of the properties of organisms. Viruses are literally “parasitic” chemicals, segments of DNA or RNA wrapped in a protein coat. They cannot reproduce on their own, and for this reason they are not considered alive by biologists. They can, however, reproduce within cells, often with disastrous results to the host organism. Earlier theories that viruses represent a kind of halfway point between life and nonlife have largely been abandoned. Instead, viruses are now viewed as detached fragments of the genomes of organisms due to the high degree of similarity found among some viral and eukaryotic genes. Viruses vary greatly in appearance and size. The smallest are only about 17 nanometers in diameter, and the largest are up to 1000 nanometers (1 micrometer) in their greatest dimension (figure 33.2). The largest viruses are barely visible with a light microscope, but viral morphology is best revealed using the electron microscope. Viruses are so small that they are comparable to molecules in size; a hydrogen atom is about 0.1 nanometer in diameter, and a large protein molecule is several hundred nanometers in its greatest dimension. Biologists first began to suspect the existence of viruses near the end of the nineteenth century. European scientists attempting to isolate the infectious agent responsible for hoof-and-mouth disease in cattle concluded that it was smaller than a bacterium. Investigating the agent further, the scientists found that it could not multiply in solution—it could only reproduce itself within living host cells that it infected. The infecting agents were called viruses. The true nature of viruses was discovered in 1933, when the biologist Wendell Stanley prepared an extract of a plant virus called tobacco mosaic virus (TMV) and attempted to purify it. To his great surprise, the purified TMV preparation precipitated (that is, separated from solution) in the form of crystals. This was surprising because precipitation is something that only chemicals do—the TMV virus was acting like a chemical off the shelf rather than an organism. Stanley concluded that TMV is best regarded as just that—chemical matter rather than a living organism. Within a few years, scientists disassembled the TMV virus and found that Stanley was right. TMV was not cellu- 33.1 Viruses are strands of nucleic acid encased within a protein coat. Vaccinia virus (cowpox) Influenza virus T4 bacteriophage HIV-1 (AIDS) Tobacco mosaic virus (TMV) Herpes simplex virus Rhinovirus (common cold) Adenovirus (respiratory virus) Poliovirus (polio) Ebola virus 100 nm FIGURE 33.2 Viral diversity. A sample of the extensive diversity and small size viruses is depicted. At the scale these viruses are shown, a human hair would be nearly 8 meters thick