Presentation on theme: "Viruses, Viroids, and Prions"— Presentation transcript:
1Viruses, Viroids, and Prions Chapter 13Viruses, Viroids, and Prions
2Differentiate between a virus and a bacterium. Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals OR exceptionally simple living microbes.
3Viruses Viruses contain DNA or RNA And a protein coat Some are enclosed by an envelope (lipids, proteins, and carbohydrates)Some viruses have spikesMost viruses infect only specific types of cells in one hostHost range is determined by specific host attachment sites and cellular factorsObligatory intracellular parasites, causing synthesis of specialized elements that transfer viral nucleic acid to other cells.
4Viruses (range from 20 to 1000 nm) nm = 10-9 mFigure 13.1
5Nonenveloped Polyhedral Viruses Describe the chemical composition and physical structure of an enveloped and a nonenveloped virus.Virion = complete, fully developed viral particle of nucleic acid surrounded by a coatFigure 13.2a, b
6Enveloped Helical Virus Viruses contain either DNA or RNA, but never both. Nucleic acid may be single or double stranded, linear or circular, or divided into several separate molecules.
7Helical Viruses Helical viruses look like long or coiled threads. Their capsids are hollow cylinders surrounding the DNA/RNA.Figure 13.4a, b
8Complex Viruses Capsid – protein coat surrounding nucleic acid Composed of capsomeres, single or multiple proteinsFigure 13.5a
9Viral TaxonomyDefine viral species.Classification based on type of nucleic acid, replication, and morphology.Family names end in -viridaeGenus names end in -virusViral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for speciesSubspecies are designated by a number
10Viral Taxonomy Herpesviridae Retroviridae Herpesvirus Lentivirus Give an example of a family, genus, and common name for a virus.HerpesviridaeHerpesvirusHuman herpes virus 1, HHV 2, HHV 3RetroviridaeLentivirusHuman Immunodeficiency Virus 1, HIV 2
15Growing Viruses Viruses must be grown in living cells. Describe how bacteriophages are cultured.Viruses must be grown in living cells.Bacteriophages form plaques (clearings) on a lawn of bacteria.Easiest to growFigure 13.6
16Growing Viruses Viruses must be grown in living host cells. Describe how animal viruses are cultured.Viruses must be grown in living host cells.Animal viruses may be grown in living animals or in embryonated eggs.Figure 13.7
17Growing VirusesAnimal and plants viruses may be grown in cell culture.Continuous cell lines may be maintained indefinitely.Cytopathic effects due to viral growthFigure 13.8
18Cytopathic effect of viruses Uninfected mouse cells form monolayer (left). Infected cells 24 hours later pile up and round up (right).Figure 13.9
19Virus Identification Serological tests List three techniques that are used to identify viruses.Serological testsDetect antibodies against viruses in a patientUse antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blotNucleic acidsRFLPs – restriction fragment length polymorphismsPCR – polymerase chain reaction (used to identify West Nile virus in U.S. in 1999)
20Multiplication of Bacteriophages (Lytic Cycle) Attachment Phage attaches by tail fibers to host cellPenetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cellBiosynthesis Production of phage DNA and proteinsMaturation Assembly of phage particlesRelease Phage lysozyme breaks cell wall
21Lytic cycle of T-even bacteriophage Bacterial cell wallBacterial chromosomeCapsidDNACapsidSheathTail fiber1Attachment: Phage attaches to host cell.TailBase platePinCell wallPlasma membrane2Penetration: Phage penetrates host cell and injects its DNA.Sheath contractedTail core3Merozoites released into bloodstream from liver may infect new red blood cellsDescribe the lytic cycle of T-even bacteriophages.Figure
22Lytic cycle of T-even bacteriophage Burst time is generally about 20 – 40 minutes after phage absorption. Burst size ranges from 50 to 200 new phage cells.TailDNA4Maturation: Viral components are assembled into virions.Capsid5Release: Host cell lyses and new virions are released.Tail fibersFigure
23One-step Growth Curve for bacteriophage During biosynthesis and maturation, separate components of DNA and protein may be detected in the host cell.Figure 13.11
24Lytic cycle Phage causes lysis and death of host cell Lysogenic cycle Prophage DNA incorporated in host DNA
25The Lysogenic Cycle – bacteriophage lambda in E.coli Describe the lysogenic cycle of bacteriophage lambda.Figure 13.12
26Specialized Transduction gal geneProphageBacterial DNA1Prophage exists in galactose-using host (containing the gal gene).Galactose-positive donor cellgal gene2Phage genome excises, carrying with it the adjacent gal gene from the host.3Phage matures and cell lyses, releasing phage carrying gal gene.gal gene4Phage infects a cell that cannot utilize galactose (lacking gal gene).Galactose-negative recipient cell5Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA.6Lysogenic cell can now metabolize galactose.Galactose-positive recombinant cellFigure 13.13
28Multiplication of Animal viruses Attachment Viruses attaches to cell membranePenetration By endocytosis or fusionUncoating By viral or host enzymesBiosynthesis Production of nucleic acid and proteinsMaturation Nucleic acid and capsid proteins assembleRelease By budding (enveloped viruses) or rupture
29Attachment, Penetration, and Uncoating Entry of herpes simplex virus into an animal cell.Figure 13.14
30Compare and contrast the multiplication cycle of DNA- and RNA-containing animal viruses.
31Multiplication of Papovarius, a DNA-containing Virus Papovavirus1Virion attaches to host cell7Virions are releasedHost cellDNACapsidDNA2Virion penetrates cell and its DNA is uncoated6Virions matureCytoplasmCapsid proteinsmRNA5Late translation; capsid proteins are synthesized3Early transcription and translation; enzymes are synthesized4Late transcription; DNA is replicatedFigure 13.15
36Multiplication & Inheritance in a Retrovirus Reverse transcriptaseCapsidDNAVirusTwo identical + stands of RNA1Retrovirus penetrates host cell.Host cellDNA of one of the host cell’s chromosomes5Mature retrovirus leaves host cell, acquiring an envelope as it buds out.Reverse transcriptase2Viral RNAVirion penetrates cell and its DNA is uncoatedIdentical strands of RNA4Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins.Viral proteinsRNA3The new viral DNA is transported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA.ProvirusFigure 13.19
37Release of an enveloped virus by budding Most enveloped viruses take part of host’s plasma membrane for their envelope.Figure 13.20
38CancerDefine oncogene and transformed cell.Activated oncogenes transform normal cells into cancerous cells. (malignant transformation)Transformed cells have increased growth, loss of contact inhibition, tumor specific transplant and T antigens, chromosome abnormalities, can produce tumors when injected into susceptible animals.Several DNA viruses and retroviruses are oncogenic.The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.
39Oncogenic Viruses Oncogenic DNA Viruses Adenoviridae Heresviridae Discuss the relationship of DNA- and RNA-containing viruses to cancer.Oncogenic DNA VirusesAdenoviridaeHeresviridaePoxviridaePapovaviridaeHepadnaviridaeOncogenic RNA virusesRetroviridaeViral RNA is transcribed to DNA which can integrate into host DNAHTLV 1HTLV 2Retroviruses carry reverse transcriptase which allows RNA to DNA, permitting oncogenic properties
40Latent Viral Infections Provide an example of a latent viral infection.Latent Viral InfectionsVirus remains in asymptomatic host cell for long periodsCold sores, shinglesPersistent Viral InfectionsDisease processes occurs over a long period, generally fatalSubacute sclerosing panencephalitis (measles virus)
41Differentiate between persistant viral infections and latent viral infections. Persistent viral infections are caused by conventional viruses, occur over a long period, generally fatal.
42Prions Infectious proteins first discovered in 1980’s Discuss how a protein can be infectious.Infectious proteins first discovered in 1980’sInherited and transmissible by ingestion, transplant, & surgical instrumentsSpongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow diseasePrPC, normal cellular prion protein, on cell surfacePrPSc, scrapie protein, accumulate in brain cells forming plaques
43PrionsHow a protein can be infectious: if an abnormal prion protein enters cell, it changes a normal prion to PrPSc, which changes another normal PrP (accumulation of abnormal PrPSc)PrPScPrPc1234LysosomeEndosome5678Figure 13.21
44Some Plant Viruses Name a virus that causes a plant disease. Table 13.6
45Linear and circular potato spindle tuber viroid Differentiate between virus, viroid, and prion.Plant VirusesPlant viruses enter through wounds or via insectsViroidsViroids are infectious RNA; potato spindle tuber diseasePrion = infectious proteinFigure 13.22
46Virus Families Single-stranded DNA, nonenveloped viruses Parvoviridae Human parvovirusFifth diseaseAnemia in immunocompromised patients
47Double-stranded DNA, nonenveloped viruses MastadenovirusRespiratory infections in humansTumors in animals
48Double-stranded DNA, nonenveloped viruses Papillomavirus (human wart virus)PolyomavirusCause tumors, some cause cancer