Viruses, Viroids, and Prions

Transcription

1 PowerPoint Lecture Presentations prepared by Bradley W. Christian, McLennan Community College C H A P T E R 13 Viruses, Viroids, and Prions

2 General Characteristics of Viruses Obligatory intracellular parasites Require living host cells to multiply Contain DNA or RNA Contain a protein coat No ribosomes No ATP-generating mechanism

3 Table 13.1 Viruses and Bacteria Compared

4 Host Range The spectrum of host cells a virus can infect Most viruses infect only specific types of cells in one host Determined by specific host attachment sites and cellular factors Bacteriophages viruses that infect bacteria Range from 20 nm to 1000 nm in length

5 Figure 13.1 Virus sizes. Bacteriophage M nm Bacteriophages f2, MS2 Poliovirus 24 nm 30 nm Ebola virus 970 nm Rhinovirus 30 nm Adenovirus Rabies virus Prion Bacteriophage T4 90 nm nm nm 225 nm E. coli bacterium nm 300 nm Chlamydia bacterium elementary body Human red blood cell 10,000 nm in diameter Tobacco mosaic virus Viroid nm nm Plasma membrane of red blood cell 10 nm thick Vaccinia virus nm

6 Viral Structure Virion complete, fully developed viral particle Nucleic acid DNA or RNA can be single- or doublestranded; linear or circular Capsid protein coat made of capsomeres (subunits) Envelope lipid, protein, and carbohydrate coating on some viruses Spikes projections from outer surface

7 Figure 13.2 Morphology of a nonenveloped polyhedral virus. Nucleic acid Capsomere Capsid

8 Figure 13.3 Morphology of an enveloped helical virus. Nucleic acid Capsomere Spikes Envelope

9 General Morphology Helical viruses hollow, cylindrical capsid Polyhedral viruses many-sided Enveloped viruses Complex viruses complicated structures

10 Figure 13.4 Morphology of a helical virus. Nucleic acid Capsomere Capsid Ebola virus

11 Figure 13.5 Morphology of complex viruses. 65 nm Capsid (head) DNA Sheath Tail fiber Pin Baseplate A T-even bacteriophage Orthopoxvirus Small pox

12 Taxonomy of Viruses Genus names end in -virus Family names end in -viridae Order names end in -ales Viral species: a group of viruses sharing the same genetic information and ecological niche (host) Descriptive common names are used for species Subspecies are designated by a number

13 Growing Bacteriophages in the Laboratory Viruses must be grown in living cells Bacteriophages are grown in bacteria Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar Each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU)

14 Figure 13.6 Viral plaques formed by bacteriophages. Plaques

15 Growing Animal Viruses in the Laboratory In living animals In embryonated eggs Virus injected into the egg Viral growth is signaled by changes or death of the embryo

16 Figure 13.7 Inoculation of an embryonated egg. Air sac Shell Amniotic cavity Chorioallantoic membrane Chorioallantoic membrane inoculation Amniotic inoculation Yolk sac Allantoic inoculation Shell membrane Albumin Allantoic cavity Yolk sac inoculation

17 Growing Animal Viruses in the Laboratory In cell cultures Tissues are treated with enzymes to separate cells Virally infected cells are detected via their deterioration, known as the cytopathic effect (CPE) Continuous cell lines are used

18 Figure 13.8 Cell cultures. Normal cells Transformed cells A tissue is treated with enzymes to separate the cells. Cells are suspended in culture medium. Normal cells or primary cells grow in a monolayer across the glass or plastic container. Transformed cells or continuous cell cultures do not grow in a monolayer.

19 Figure 13.9 The cytopathic effect of viruses.

20 Viral Identification Cytopathic effects Serological tests Western blotting reaction of the virus with antibodies Nucleic acids RFLPs (Restriction fragment length polymorphisms) PCR

21 Viral Multiplication For a virus to multiply: It must invade a host cell It must take over the host's metabolic machinery One-step growth curve

22 Figure A viral one-step growth curve.

23 Multiplication of Bacteriophages Lytic cycle Phage causes lysis and death of the host cell Lysogenic cycle Phage DNA is incorporated in the host DNA Phage conversion Specialized transduction

24 Viral Replication: Virulent Bacteriophages PLAY Animation: Viral Replication: Virulent Bacteriophages

25 Viral Replication: Temperate Bacteriophages PLAY Animation: Viral Replication: Temperate Bacteriophages

26 T-Even Bacteriophages: The Lytic Cycle Attachment: phage attaches by the tail fibers to the host cell Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell Biosynthesis: production of phage DNA and proteins Maturation: assembly of phage particles Release: phage lysozyme breaks the cell wall

27 Figure The lytic cycle of a T-even bacteriophage. Bacterial cell wall Bacterial chromosome Capsid DNA Capsid (head) Attachment: Phage attaches to host cell. Sheath Tail fiber Baseplate Tail Pin Cell wall Plasma membrane Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Biosynthesis: Phage DNA directs synthesis of viral components by the host cell. Tail core Tail DNA Maturation: Viral components are assembled into virions. Capsid Release: Host cell lyses, and new virions are released. Tail fibers

28 Bacteriophage Lambda (λ): The Lysogenic Cycle Lysogeny: phage remains latent Phage DNA incorporates into host cell DNA Inserted phage DNA is known as a prophage When the host cell replicates its chromosome, it also replicates prophage DNA Results in phage conversion the host cell exhibits new properties

29 Figure The lysogenic cycle of bacteriophage λ in E. coli. Phage DNA (double-stranded) Phage attaches to host cell and injects DNA. Bacterial chromosome Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle. Many cell divisions Lytic cycle Lysogenic cycle Cell lyses, releasing phage virions. Phage DNA circularizes and enters lytic cycle or lysogenic cycle. OR Prophage Lysogenic bacterium reproduces normally. New phage DNA and proteins are synthesized and assembled into virions. Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage.

30 Bacteriophage Lambda (λ): The Lysogenic Cycle Specialized transduction Specific bacterial genes transferred to another bacterium via a phage Changes genetic properties of the bacteria

31 Figure Specialized transduction. Prophage gal gene Bacterial DNA Prophage exists in galactose-using host (containing the gal gene). Galactosepositive donor cell gal gene Phage genome excises, carrying with it the adjacent gal gene from the host. gal gene Phage matures and cell lyses, releasing phage carrying gal gene. Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactosenegative recipient cell Along with the prophage, the bacterial gal gene becomes integrated into the new host's DNA. Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell

32 Transduction: Generalized Transduction PLAY Animation: Transduction: Generalized Transduction

33 Transduction: Specialized Transduction PLAY Animation: Transduction: Specialized Transduction

34 Table 13.3 Bacteriophage and Animal Viral Multiplication Compared

35 Multiplication of Animal Viruses Attachment: viruses attach to the cell membrane Entry by receptor-mediated endocytosis or fusion Uncoating by viral or host enzymes Biosynthesis: production of nucleic acid and proteins Maturation: nucleic acid and capsid proteins assemble Release by budding (enveloped viruses) or rupture

36 Figure The entry of viruses into host cells. Host plasma membrane proteins at site of receptor-mediated endocytosis Fusion of viral envelope and plasma membrane Entry of pig retrovirus by receptor-mediated endocytosis. Entry of herpesvirus by fusion.

37 Figure Budding of an enveloped virus. Viral capsid Host cell plasma membrane Viral protein Bud Bud Envelope Release by budding Lentivirus

38 Viral Replication: Overview PLAY Animation: Viral Replication: Overview

39 Viral Replication: Animal Viruses PLAY Animation: Viral Replication: Animal Viruses

40 The Biosynthesis of DNA Viruses DNA viruses replicate their DNA in the nucleus of the host using viral enzymes Synthesize capsid in the cytoplasm using host cell enzymes

41 Figure Replication of a DNA-Containing Animal Virus. RELEASE Virions are released. MATURATION Virions mature. Papovavirus DNA Capsid ATTACHMENT Virion attaches to host cell. Nucleus Cytoplasm Host cell A papovavirus is a typical DNA-containing virus that attacks animal cells. ENTRY and UNCOATING Virion enters cell, and its DNA is uncoated. Capsid proteins Viral DNA BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made. mrna Capsid proteins Late translation; capsid proteins are synthesized. A portion of viral DNA is transcribed, producing mrna that encodes "early" viral proteins. KEY CONCEPTS Viral replication in animals generally follows these steps: attachment, entry, uncoating, biosynthesis of nucleic acids and proteins, maturation, and release. Knowledge of viral replication phases is important for drug development strategies, and for understanding disease pathology.

42 The Biosynthesis of DNA Viruses Adenoviridae Double-stranded DNA, nonenveloped Respiratory infections in humans Tumors in animals

43 Figure 13.16a DNA-containing animal viruses. Capsomere Mastadenovirus

44 The Biosynthesis of DNA Viruses Poxviridae Double-stranded DNA, enveloped Cause skin lesions Vaccinia and smallpox viruses (Orthopoxvirus)

45 The Biosynthesis of DNA Viruses Herpesviridae Double-stranded DNA, enveloped HHV-1 and HHV-2 Simplexvirus; cause cold sores HHV-3 Varicellovirus; causes chickenpox HHV-4 Lymphocryptovirus; causes mononucleosis HHV-5 Cytomegalovirus HHV-6 and HHV-7 Roseolovirus HHV-8 Rhadinovirus; causes Kaposi's sarcoma

46 Figure 13.16b DNA-containing animal viruses. Capsomeres Simplexvirus

47 The Biosynthesis of DNA Viruses Papovaviridae Double-stranded DNA, nonenveloped Papillomavirus Causes warts Can transform cells and cause cancer

48 The Biosynthesis of DNA Viruses Hepadnaviridae Double-stranded DNA, enveloped Hepatitis B virus Use reverse transcriptase to make DNA from RNA

49 The Biosynthesis of RNA Viruses Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase ssrna; + (sense) strand Viral RNA serves as mrna for protein synthesis ssrna; (antisense) strand Viral RNA is transcribed to a + strand to serve as mrna for protein synthesis dsrna double-stranded RNA

50 Figure 13.17a Pathways of multiplication used by various RNA-containing viruses. Attachment RNA Capsid Host cell Nucleus Cytoplasm Maturation and release Entry and uncoating Translation and synthesis of viral proteins RNA replication by viral RNA dependent RNA polymerase KEY + or sense strand of viral genome Capsid protein strand is transcribed from + viral genome. Viral genome (RNA) Uncoating releases viral RNA and proteins. Viral protein or antisense strand of viral genome ss = single-stranded ds = double-stranded + strand mrna is transcribed from the strand. ssrna; + or sense strand; Picornaviridae

51 Figure 13.17b Pathways of multiplication used by various RNA-containing viruses. Attachment RNA Capsid Host cell Nucleus Cytoplasm Maturation and release Entry and uncoating KEY + or sense strand of viral genome Translation and synthesis of viral proteins Capsid protein RNA replication by viral RNA dependent RNA polymerase The + strand (mrna) must first be transcribed from the viral genome before proteins can be synthesized. Viral genome (RNA) Uncoating releases viral RNA and proteins. Viral protein or antisense strand of viral genome ss = single-stranded ds = double-stranded strands are incorporated into capsid Additional strands are transcribed from mrna. ssrna; or antisense strand; Rhabdoviridae

52 Figure 13.17c Pathways of multiplication used by various RNA-containing viruses. Attachment RNA Capsid Host cell Nucleus Cytoplasm KEY + or sense strand of viral genome Maturation and release Translation and synthesis of viral proteins RNA polymerase initiates production of strands. The mrna and strands form the dsrna that is incorporated as new viral genome. Entry and uncoating RNA replication by viral RNA dependent RNA polymerase mrna is produced inside the capsid and released into the cytoplasm of the host. Viral genome (RNA) Uncoating releases viral RNA and proteins. Viral protein or antisense strand of viral genome ss = single-stranded ds = double-stranded Capsid proteins and RNAdependent RNA polymerase dsrna; + or sense strand with or antisense strand; Reoviridae

53 The Biosynthesis of RNA Viruses Picornaviridae Single-stranded RNA, + strand, nonenveloped Enterovirus Poliovirus and coxsackievirus Rhinovirus Common cold Hepatitis A virus

54 The Biosynthesis of RNA Viruses Togaviridae Single-stranded RNA, + strand, enveloped Alphavirus Transmitted by arthropods; includes chikungunya Rubivirus Rubella

55 The Biosynthesis of RNA Viruses Rhabdoviridae Single-stranded RNA, strand, one RNA strand Lyssavirus Rabies Numerous animal diseases

56 The Biosynthesis of RNA Viruses Reoviridae Double-stranded RNA, nonenveloped Reovirus (respiratory enteric orphan) Rotavirus (mild respiratory infections and gastroenteritis)

57 Biosynthesis of RNA Viruses That Use DNA Single-stranded RNA, produce DNA Use reverse transcriptase to produce DNA from the viral genome Viral DNA integrates into the host chromosome as a provirus Retroviridae Lentivirus (HIV) Oncoviruses

58 Figure Multiplication and inheritance processes of the Retroviridae. Reverse transcriptase Capsid Envelope Host cell Virus Two identical + strands of RNA Retrovirus enters by fusion between attachment spikes and the host cell receptors. Mature retrovirus leaves the host cell, acquiring an envelope and attachment spikes as it buds out. DNA of one of the host cell's chromosomes Viral enzymes Uncoating releases the two viral RNA strands and the viral enzymes reverse transcriptase, integrase, and protease. Viral DNA Viral RNA Reverse transcriptase copies viral RNA to produce doublestranded DNA. Viral proteins are processed by viral protease; some of the viral proteins are moved to the host plasma membrane. Provirus Viral proteins RNA Identical strands of RNA The new viral DNA is transported into the host cell's nucleus, where it's integrated into a host cell chromosome as a provirus by viral integrase. The provirus may be replicated when the host cell replicates. Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that encodes the retrovirus capsid, enzymes, and envelope proteins.

59 Table 13.2 Families of Viruses That Affect Humans (1 of 4)

60 Table 13.2 Families of Viruses That Affect Humans (2 of 4)

61 Table 13.2 Families of Viruses That Affect Humans (3 of 4)

62 Table 13.2 Families of Viruses That Affect Humans (4 of 4)

63 Viruses and Cancer Several types of cancer are caused by viruses May develop long after a viral infection Cancers caused by viruses are not contagious Sarcoma: cancer of connective tissue Adenocarcinomas: cancers of glandular epithelial tissue

64 The Transformation of Normal Cells into Tumor Cells Oncogenes transform normal cells into cancerous cells Oncogenic viruses become integrated into the host cell's DNA and induce tumors A transformed cell harbors a tumor-specific transplant antigen (TSTA) on the surface and a T antigen in the nucleus

65 DNA Oncogenic Viruses Adenoviridae Herpesviridae Epstein-Barr virus Poxviridae Papovaviridae Human papillomavirus Hepadnaviridae Hepatitis B virus

66 RNA Oncogenic Viruses Retroviridae Viral RNA is transcribed to DNA (using reverse transcriptase), which can integrate into host DNA HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma

67 Latent Viral Infections and Persistent Viral Infections Latent virus remains in asymptomatic host cell for long periods May reactivate due to changes in immunity Cold sores, shingles A persistent viral infection occurs gradually over a long period; is generally fatal Subacute sclerosing panencephalitis (measles virus)

68 Figure Latent and persistent viral infections. Acute infection Latent infection Persistent infection

69 Table 13.5 Examples of Latent and Persistent Viral Infections in Humans

70 Prions Proteinaceous infectious particles Inherited and transmissible by ingestion, transplant, and surgical instruments Spongiform encephalopathies "Mad cow disease" Creutzfeldt-Jakob disease (CJD) Gerstmann-Sträussler-Scheinker syndrome Fatal familial insomnia Sheep scrapie

71 Prions PrP C : normal cellular prion protein, on the cell surface PrP Sc : scrapie protein; accumulates in brain cells, forming plaques

72 Prions: Overview PLAY Animation: Prions: Overview

73 Prions: Characteristics PLAY Animation: Prions: Characteristics

74 Prions: Diseases PLAY Animation: Prions: Diseases

75 Figure How a protein can be infectious. PrP Sc PrP c PrP c produced by cells is secreted to the cell surface. PrP Sc may be acquired or produced by an altered PrP c gene. PrP Sc reacts with PrP c on the cell surface. PrP Sc converts the PrP c to PrP Sc. Lysosome Endosome The new PrP Sc converts more PrP c. The new PrP Sc is taken in, possibly by receptormediated endocytosis. PrP Sc accumulates in endosomes. PrP Sc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death.

76 Plant Viruses and Viroids Plant viruses: enter through wounds or via insects Plant cells are generally protected from disease by an impermeable cell wall Viroids: short pieces of naked RNA Cause potato spindle tuber disease