Characterizing and Classifying Viruses, Viroids, and Prions

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1 PowerPoint Lecture Presentations prepared by Mindy Miller-Kittrell, North Carolina State University Modified by Ossi Turunen, Aalto University C H A P T E R 13 Characterizing and Classifying Viruses, Viroids, and Prions 1

2 Glycoprotein gp120 binds to a receptor on T-cells and macrophages. Rudiviruses infect hyperthermophilic archaea of the kingdom Crenarchaeota - Wikipedia Rudivirus Complex symmetry (poxvirus) Icosahedral Symmetry 2

3 Types of nonenveloped viruses 3

4 Characteristics of Viruses Viruses Minuscule, acellular, infectious agents having either DNA or RNA Cause infections of humans, animals, plants, and bacteria Cause most of the diseases that plague the industrialized world 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 Have extracellular and intracellular state Characteristics of Viruses Extracellular state Called virion Protein coat (capsid) surrounding nucleic acid Nucleic acid and capsid, also called nucleocapsid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular state Capsid removed Virus exists as nucleic acid 4

5 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) Characteristics of Viruses Genetic Material of Viruses Show more variety in their genomes than do cells Primary way scientists categorize and classify viruses May be DNA or RNA, but never both Can be dsdna, ssdna, dsrna, ssrna May be linear and segmented or single and circular Much smaller than genomes of cells 5

6 Figure 13.2 The relative sizes of genomes. Partial genome of E. coli Viral genome 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 infect only a particular kind of cell in a particular host Generalists infect many kinds of cells in many different hosts 6

7 Characteristics of Viruses Hosts of Viruses All types of organisms are susceptible to viral attack A bacteriophage (phage) is a virus that infects bacteria Plant viruses infect many food crops Introduced through abrasions of the cell wall or by plant parasites Fungal viruses are not well studied Appear to have no extracellular state Figure 13.3 Some examples of plant, bacterial, and human hosts of viral infections. 7

8 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) Characteristics of Viruses Capsid Morphology Capsids 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 8

9 Figure 13.5 The shapes of virions. Figure 13.6 The complex shape of bacteriophage T4. Head Tail fibers Tail Base plate 9

10 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 Figure 13.7 Enveloped virion. Glycoproteins Helical capsid Matrix protein Envelope Enveloped virus with helical capsid 10

11 Influenza virus hemagglutinin (HA) contains antigenic sites recognized by the host immune system, cleavage sites cleaved by host proteases, receptor binding sites attaching to sialyl receptors on the target cell, and fusion peptides mediating membrane fusion

12 New flu drug stops virus in its tracks February 21,

13 Characteristics of Viruses Tell Me Why Why are naked icosahedral viruses able to crystallize? Classification of Viruses International Committee on Taxonomy of Viruses determines virus classification Viruses classified by nucleic acid, presence of envelope, shape, and size Relationship among viruses not well understood No kingdoms, divisions, or classes have been defined 13

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15 Classification of Viruses Tell Me Why What characteristics of the genomes of parvoviruses and of reoviruses make them very different from cells? Viral Replication Dependent on hosts' organelles and enzymes to produce new virions Lytic replication Viral replication usually results in death and lysis of host cell Five stages of lytic replication cycle Attachment Entry Synthesis Assembly Release 15

16 Figure 13.8 The lytic replication cycle in bacteriophages. Figure 13.9 Pattern of virion abundance in lytic cycle. 16

17 Viral Replication Lysogeny Modified replication cycle Infected host cells grow and reproduce normally for generations before they lyse Temperate phages Prophages inactive phages Lysogenic conversion Results when phages carry genes that alter phenotype of a bacterium Figure Bacteriophage lambda. 17

18 Figure The lysogenic replication cycle in bacteriophages. 1 Attachment Lambda phage 2 Entry 3 Prophage in chromosome 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 18

19 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 Viral Replication Replication of Animal Viruses Entry and uncoating of animal viruses At least three different mechanisms by which animal viruses enter a cell Direct penetration Membrane fusion Endocytosis Viruses that enter cell with capsid intact are uncoated 19

20 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 1 Direct penetration 2 Viral glycoproteins Envelope 3 Viral glycoproteins remain in cytoplasmic membrane Viral genome Uncoating capsid Endocytosis 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 Must consider How mrna is synthesized What serves as template for nucleic acid replication 20

21 Viral Replication Replication of Animal Viruses Synthesis of DNA viruses of animals dsdna viruses Similar to replication of cellular DNA Viral genome replicated in the nucleus Viral proteins are made in the cytoplasm Some exceptions Poxvirus replication occurs in the cytoplasm Hepatitis B viruses replicate DNA from an RNA intermediary Viral Replication Replication of Animal Viruses Synthesis of DNA viruses of animals ssdna viruses Cells do not use ssdna Parvoviruses have ssdna genomes Host enzymes produce DNA strand complementary to viral genome to form dsdna molecule dsdna used for viral replication and transcription 21

22 Figure Synthesis of proteins and genomes in animal RNA viruses. +ssrna virus Receptors on cytoplasmic membrane of host ssrna virus dsrna virus +ssrna ssrna dsrna Transcription by viral RNA polymerase Transcription by RNA-dependent RNA transcriptase 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 Positive-sense ssrna virus Negative-sense ssrna virus Double-stranded RNA virus Viral Replication Replication of Animal Viruses Synthesis of RNA viruses of animals Retroviruses Do not use their genomes as mrna Use DNA intermediary transcribed by viral reverse transcriptase as template to produce viral genomes 22

23 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 are often released by budding Enables some viruses to cause persistent infections Naked viruses are released by exocytosis or lysis 23

24 Figure 13.14: The process of budding in enveloped viruses. Enveloped virion 5 Budding of enveloped virus Viral capsid Viral glycoproteins Cytoplasmic membrane of host Figure Pattern of virion abundance in persistent infections. 24

25 Viral Replication Replication of Animal Viruses Latency of animal viruses 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 Incorporation of provirus into host DNA is permanent 25

26 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 are "turned off," or genes inhibiting division are "turned on" Neoplasia Uncontrolled cell division in multicellular animal Mass of neoplastic cells is tumor Benign versus malignant tumors Malignant tumors also called cancers Metastasis occurs when tumors spread Figure The oncogene theory of the induction of cancer in humans. 26

27 The Role of Viruses in Cancer Environmental factors that contribute to the activation of oncogenes Ultraviolet light Radiation Carcinogens Viruses 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 27

28 The Role of Viruses in Cancer Tell Me Why Why are DNA viruses more likely to cause neoplasias than are RNA viruses? Culturing Viruses in the Laboratory Viruses cannot grow in standard microbiological media Are cultured inside host cells Three types of media for culturing viruses Media consisting of mature organisms Embryonated eggs Cell cultures 28

29 Culturing Viruses in the Laboratory Culturing Viruses in Mature Organisms Culturing viruses in bacteria Phages are 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 29

30 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 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 are prepared in chicken cultures 30

31 Figure Inoculation sites for the culture of viruses in embryonated chicken eggs. Air sac Injection into chorioallantoic membrane Injection into chorioallantois Injection into embryo Injection into amnion Injection into yolk sac Culturing Viruses in the Laboratory Culturing Viruses in Cell (Tissue) Culture Cells are 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 31

32 Figure An example of cell culture. Are Viruses Alive? Some microbiologists 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 32

33 Are Viruses Alive? Tell Me Why Why are viruses seemingly alive and yet not alive? 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 because of hydrogen bonding Viroidlike agents affect some fungi 33

34 Figure The RNA strand of the small potato spindle tuber viroid (PSTV). Genome of bacteriophage T7 PSTV Figure One effect of viroids on plants. 34

35 Other Parasitic Particles: Viroids and Prions Characteristics of Prions Proteinaceous infectious agents Cellular PrP Made by all mammals Normal, functional structure has α-helices Prion PrP Disease-causing form has β-sheets Prion PrP causes cellular PrP to refold into prion PrP Figure The two stable, three-dimensional forms of prion protein (PrP). α-helices β-pleated sheet Cellular PrP Prion PrP 35

36 Other Parasitic Particles: Viroids and Prions Characteristics of Prions Prion diseases Spongiform encephalopathies Large vacuoles form in brain Characteristic spongy appearance BSE, vcjd, kuru Transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissues Prions are destroyed by incineration or autoclaving in concentrated sodium hydroxide Figure A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases. Vacuole 36

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