Characterizing and Classifying Viruses, Viroids, and Prions

Transcription

1 PowerPoint Lecture Presentations prepared by Mindy Miller-Kittrell, North Carolina State University C H A P T E R 13 Characterizing and Classifying Viruses, Viroids, and Prions SLOs What are the Characteristics of Viruses (pp ) How to Classify Viruses (pp ) How do Viruses Replicate (pp ) The Role of Viruses in Cancer (pp ) How to Culture Viruses in the Laboratory (pp ) Other Parasitic Particles: Viroids and Prions (pp ) Summary of Textbook Characteristics of Viruses (pp ) Classification of Viruses (pp ) Viral Replication (pp ) The Role of Viruses in Cancer (pp ) Culturing Viruses in the Laboratory (pp ) Are Viruses Alive? (p. 398) Other Parasitic Particles: Viroids and Prions (pp ) 1

2 Characteristics of Viruses Viruses Minuscule, acellular, infectious agent Why acellular? - like a chemical Cannot carry out any metabolic pathway Neither grow nor respond to the environment Cannot reproduce independently Recruit the cell's metabolic pathways to increase their numbers No cytoplasmic membrane, cytosol, organelles Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope. Crystals TMV Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope. Outside the cell = virion Capsid (sectioned to show interior) receptors Outermost layer provides protection and recognition sites for host cells nucleocapsid envelope = phospholipid membrane Nucleic acid (viral genome) 2

3 Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope. Capsid (sectioned to show interior) Nucleic acid (viral genome) RNA or DNA Never both Primary way scientists categorize and classify viruses single stranded/double stranded Linear and segmented Single and circular Figure 13.4 Sizes of selected virions. E. coli (bacterium) (1000 nm x 3000 nm) Red blood cell (10,000 nm in diameter) Poliovirus (30 nm) Bacterial ribosomes (25 nm) Bacteriophage T4 (50 nm x 225 nm) Bacteriophage MS2 (24 nm) Smallpox virus (200 nm x 300 nm) Tobacco mosaic virus (15 nm x 300 nm) Figure 13.2 The relative sizes of genomes. Partial genome of E. coli Viral genome Much smaller than genomes of cells 3

4 Characteristics of Viruses Hosts of Viruses Most viruses infect only particular host's cells Due to affinity of viral surface proteins for complementary proteins on host cell surface May be so specific they only infect particular kind of cell in a particular host Specific HIV helper T cells (WBC) Generalists infect many kinds of cells in many different hosts - eg West Nile virus most bird, several mammals Figure 13.3 Some examples of plant, bacterial, and human hosts of viral infections. Characteristics of Viruses Capsid Morphology Capsids 2 major functions Provide protection for viral nucleic acid Means of attachment to host's cells Composed of proteinaceous subunits called capsomeres Capsomere may be made of single or multiple types of proteins 4

5 Figure 13.5 The shapes of virions help classify them (not sizes) 3 shapes TMV TMV 1) helical 2) polyhedral 3) complex Figure 13.6 The complex shape of bacteriophage T4. complex Head Tail fibers Tail Base plate 5

6 Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope. Outside the cell = virion Capsid (sectioned to show interior) Outermost layer provides protection and recognition sites for host cells nucleocapsid The Viral Envelope Nucleic acid (viral genome) Figure 13.7 Enveloped virion. Envelope does not perform endocytosis or active transport Envelope proteins and glycoproteins often play role in host recognition Without envelope = nonenveloped or naked virion SARS virus Glycoproteins Helical capsid Matrix protein Envelope Enveloped virus with helical capsid Characteristics of Viruses The Viral Envelope Acquired from host cell during viral replication or release Envelope is portion of membrane system of host Composed of phospholipid bilayer and proteins Some proteins are virally coded glycoproteins (spikes) Envelope proteins and glycoproteins often play role in host recognition 6

7 Classification of Viruses (pp ) 1) Shapes 2) Genetic material - primary ICTV (1966) = International Committee on Taxonomy of Viruses No kingdoms/divisions/classes 3 orders Many familes DNA or RNA (primary) Viral Replication Dependent on hosts' organelles and enzymes to produce new virions 2 modes lytic and lysogenic 2 DNA viruses T4 and lambda Lytic replication Viral replication usually results in death and lysis of host cell Five stages of lytic replication cycle Attachment Entry Synthesis Assembly Release 7

8 Viral Replication Dependent on hosts' organelles and enzymes to produce new virions 2 modes lytic and lysogenic 2 DNA viruses T4 and lambda Lytic replication Viral replication usually results in death and lysis of host cell Five stages of lytic replication cycle Attachment Entry Synthesis Assembly Release Figure 13.8 The lytic replication cycle in bacteriophages. Attachment Bacteriophage genome Entry Tail sheath Lysozyme within capsid To enter Outer membrane Peptidoglycan Cytoplasmic membrane Bacterial chromosome Transduction Reason? Lysozyme to exit 1 Attachment 2 Entry Lytic replication cycle of bacteriophage Phage DNA 3 Bacterial chromosome degraded 6 Release 4 Synthesis 5 Assembly Phage proteins Assembly Base Tail Sheath DNA Capsid Mature head Tail fibers Mature virion Figure 13.9 Pattern of virion abundance in lytic cycle. Burst size = number of virions released per host cell 8

9 Figure Viral plaques in a lawn of bacterial growth on the surface of an agar plate. Bacterial lawn Viral plaques Viral Replication Lysogeny/ lysogenic replication cycle Modified replication cycle Infected host cells grow and reproduce normally for generations before they lyse Lysogenic also called Temperate phages Prophages inactive phages makes bacteria resistant can change bacteria into a pathogen (hence phage therapy is dangerous) Results when phages carry genes that alter phenotype of a bacterium toxins Release from lysogeny is induction inductive agents same as those which damage DNA UV/ Xray and carcinogens Figure Bacteriophage lambda. 9

10 Figure The lysogenic replication cycle in lambda bacteriophages. 1 Attachment Lambda phage 2 Entry 3 Prophage in chromosome Random collision Chemical attraction Receptor specificity Lytic cycle Lysogeny 6 Synthesis 8 Release 7 Assembly 4 Replication of chromosome and virus; cell division 5 Induction Further replications and cell divisions Viral Replication Replication of Animal Viruses Same basic replication pathway as bacteriophages Differences result from Presence of envelope around some viruses Eukaryotic nature of animal cells Lack of cell wall in animal cells Viral Replication Replication of Animal Viruses Attachment of animal viruses Chemical attraction between viral protein and cell receptor Animal viruses do not have tails or tail fibers Have glycoprotein spikes or other attachment molecules that mediate attachment 10

11 Figure Three mechanisms of entry of animal viruses. 1 Phage genome inside capsid 2 Capsid 3 Cytoplasmic membrane of host engulfs virus (endocytosis) Receptors on cytoplasmic membrane Viral genome 4 Direct penetration Viral glycoproteins Envelope Viral glycoproteins remain in cytoplasmic Viral genome Uncoating capsid 3 membrane Endocytosis 4 Receptors on cytoplasmic membrane of host Viral genome Uncoating capsid Membrane fusion Viral Replication Replication of Animal Viruses Synthesis of DNA viruses of animals Each type of animal virus requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses often replicate in the cytoplasm Viral Replication Replication of Animal Viruses dsdna viruses (some exceptions) Similar to replication of cellular DNA Viral genome replicated in the nucleus Viral proteins are made in the cytoplasm ssdna viruses (eg Parvoviruses) Cells do not use ssdna Host enzymes produce DNA strand complementary to viral genome to form dsdna molecule dsdna used for viral replication and transcription 11

12 Figure types of animal RNA viruses. 4) Retrovirus a type of positive ssrna Positive-sense ssrna virus Receptors on cytoplasmic membrane of host +ssrna virus Negative-sense ssrna virus ssrna virus Double-stranded RNA virus dsrna virus +ssrna Transcription by viral RNA unique polymerase ssrna Transcription by RNA-dependent RNA transcriptase dsrna Unwinding Complementary ssrna to act as template Further transcription Copies of +ssrna Translation of viral proteins, genome acts as mrna Further transcription Copies of ssrna Complementary +ssrna to act as template and as mrna Translation of viral proteins ssrna Transcription by viral RNA polymerase to make complementary RNA strands +ssrna acts as mrna Translation of viral proteins Assembly Assembly Assembly Check Table 13.3 for comparison of strategies Viral Replication 4) positive sense RNA - Retroviruses Do not use their genomes as mrna Use DNA intermediary transcribed by viral reverse transcriptase as template to produce viral genomes RNA (genome) to DNA to RNA Viral Replication Replication of Animal Viruses Assembly and release of animal viruses Most DNA viruses assemble in nucleus Most RNA viruses develop solely in cytoplasm Number of viruses produced depends on type of virus and size and initial health of host cell Enveloped viruses cause persistent infections Naked viruses are released by exocytosis or lysis 12

13 Figure 13.14: The process of budding in enveloped viruses. Budding Causes Persistent infections No envelope? 2 Enveloped virion Budding of enveloped virus Viral capsid Viral glycoproteins Cytoplasmic membrane of host (also NER,ER) Figure Pattern of virion abundance in persistent infections. bacteriophage Viral Replication Replication of Animal Viruses Latency of animal viruses (chicken pox/herpes virus) When animal viruses remain dormant in host cells Viruses are called latent viruses or proviruses May be prolonged for years with no viral activity Some latent viruses do not become incorporated into host chromosome difference with lysogenic Incorporation of provirus into host DNA is permanent 13

14 The Role of Viruses in Cancer Cell division is under strict genetic control Genes dictate that some cells can no longer divide at all Cells that can divide are prevented from unlimited division Genes for cell division "turned off" or genes inhibiting division "turned on" Neoplasia neoplastic - tumor Uncontrolled cell division in multicellular animal Mass of neoplastic cells is tumor Benign vs. malignant tumors Malignant tumors also called cancers Metastasis occurs when tumors spread The Role of Viruses in Cancer Environmental factors that contribute to the activation of oncogenes Ultraviolet light Radiation Carcinogens Viruses 14

15 The Role of Viruses in Cancer Viruses cause 20 25% of human cancers Some carry copies of oncogenes as part of their genomes Some promote oncogenes already present in host Some interfere with tumor repression Specific viruses are known to cause ~15% of human cancers Burkitt's lymphoma Hodgkin's disease Kaposi's sarcoma Cervical cancer Figure The oncogene theory of the induction of cancer in humans. Culturing Viruses in the Laboratory Viruses cannot grow in standard microbiological media Cultured inside host cells Three types of media for culturing viruses Media consisting of mature organisms Embryonated eggs Cell cultures 15

16 Culturing Viruses in the Laboratory Culturing Viruses in Mature Organisms Culturing viruses in bacteria Phages grown in bacteria in liquid cultures or on agar plates Lysis of bacteria produces plaques Allows estimation of phage numbers by plaque assay Figure Viral plaques in a lawn of bacterial growth on the surface of an agar plate. Bacterial lawn Viral plaques Culturing Viruses in the Laboratory Culturing Viruses in Mature Organisms Culturing viruses in plants and animals Numerous plants and animals have been used to culture viruses Laboratory animals can be difficult and expensive to maintain Ethical concerns 16

17 Culturing Viruses in the Laboratory Culturing Viruses in Embryonated Chicken Eggs Inexpensive Among the largest of cells Free of contaminating microbes Contain a nourishing yolk Fertilized chicken eggs are often used Embryonic tissues provide ideal site for growing viruses Some vaccines prepared in chicken cultures Figure Inoculation sites for the culture of viruses in embryonated chicken eggs. Air sac Injection into chorioallantois Injection into chorioallantoic membrane Injection into embryo Injection into amnion Injection into yolk sac Culturing Viruses in the Laboratory Culturing Viruses in Cell (Tissue) Culture Cells isolated from an organism and grown on a medium or in a broth Cell cultures sometimes inaccurately called tissue cultures Two types of cell cultures Diploid cell cultures Continuous cell cultures 17

18 Figure An example of cell culture. Are Viruses Alive? Some consider them complex pathogenic chemicals Others consider them to be the least complex living entities Use sophisticated methods to invade cells Have the ability to take control of their host cell Are able to replicate themselves Other Parasitic Particles: Viroids and Prions Characteristics of Viroids Extremely small, circular pieces of RNA that are infectious and pathogenic in plants Similar to RNA viruses, but lack capsid May appear linear due to hydrogen bonding Infectious genetic material = virus? 18

19 Figure The RNA strand of the small potato spindle tuber viroid (PSTV). Genome of bacteriophage T7 PSTV Figure One effect of viroids on plants. PSTV = small potato spindle tuber viroid Other Parasitic Particles: Viroids and Prions Infectious nongenetic material Characteristics of Prions Proteinaceous infectious agents Cellular PrP Made by all mammals Normal, functional structure has -helices Prion PrP Disease-causing form has -sheets Prion PrP causes cellular PrP to refold into prion PrP 19

20 Figure The two stable, three-dimensional forms of prion protein (PrP). -helices -pleated sheet Cellular PrP Prion PrP Figure A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases. Vacuole Other Parasitic Particles: Viroids and Prions Characteristics of Prions Prion diseases Spongiform encephalopathies Large vacuoles form in brain Characteristic spongy appearance BSE, vcjd, kuru, CWD Transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissues Prions destroyed by incineration or autoclaving in concentrated sodium hydroxide 20

21 4) Which of the following is a characteristic by which viruses are classified? A) type of nucleic acid B) type of life cycle C) number of chromosomes D) type of host E) size 5) Host specificity of a virus is due to A) particular genes that it shares with the infected cell. B) the presence of an envelope. C) differences in size between the virus and the host cell. D) the presence or absence of a cell wall on the host cell. E) interactions between viral and cellular surface molecules. 6) Which of the following is primarily responsible for the shape of a virion? A) the type of nucleic acid B) the number of segments of the viral genome C) the source of the envelope D) the capsid E) the specific host protein the virus targets 1) In what ways do viruses differ from other pathogens? A) Viruses lack genetic material. B) Viruses lack cytoplasm and organelles. C) Viruses have no protein structure. D) Viruses are composed of protein only. E) Viruses are composed of both protein and lipid. 2) The outermost layer of a virion fulfills which of the following functions of the virus? A) protection B) recognition C) replication D) both protection and recognition E) both recognition and replication 21

22 3) A(n) is a virus that infects bacterial cells. A) virion B) prion C) bacteriophage D) nucleocapsid E) envelope 8) Which of the following infectious particles do NOT have protein in their structure? A) bacteriophages B) prions C) animal viruses D) viroids E) both prions and viroids 9) Which of the following may occur in a lysogenic infection, but not a latent one? A) The inserted viral DNA may leave the host DNA. B) The viral DNA integrates (inserts) into the host cell DNA. C) The provirus/prophage directs the synthesis of viral proteins. D) The prophage/provirus alters gene expression in the host cell. E) The prophage/provirus is replicated with the host DNA. 12) Which of the following places stages of a lytic replication cycle in order, from earliest to latest stages? I. Synthesis II. Assembly III. Attachment IV. Release V. Entry A) III, V, I, II, IV B) III, II, V, I, IV C) V, III, II, IV, I D) I, III, V, II, IV E) I, II, III, V, IV 22

23 13) Which of the following is associated with the attachment of a bacteriophage to a bacterial cell? A) random collisions B) chemical attractions C) receptor specificity D) both chemical attractions and receptor specificity E) random collisions, chemical attractions, and receptor specificity 14) The enzyme lysozyme is critical for which of the stages of a bacteriophage T4 infection cycle? A) entry B) assembly C) entry and release D) synthesis E) attachment 18) Which of the following agents is capable of inducing conversion of a prophage to the lytic cycle? A) UV light B) X-rays C) presence of +ssrna D) both UV light and X-rays E) both UV light and the presence of +ssrna 23