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Eukaryotes and Viruses Chapters 12 and 13 Viral Characteristics and Structure Why Viruses aren’t Alive General Characteristics of Viruses • • • • • Name derives from the Latin for “poison” Obligatory intracellular parasites Referred to as filterable Contain a single type of nucleic material The nucleic material is covered in a protein coat. • Use the synthesis machinery of the host to multiply. Why are they not Alive? • No independent metabolism or reproduction • No single phylogenetic origin • No cellular structure • No ribosomes • Though they DO evolve and reproduce. Host Range • Viruses have a specific subset of cell types they will infect, referred to as Host Range. • Most viruses can only infect a single species • Some viruses can cross species barriers • Numerous factors influence host range • Viruses that infect bacteria are referred to as bacteriophage or simply phage. Viral Particle Size Poxviridae Lentiviruses Staphylococcus Bacteria 1 μm in diameter Picornaviridae Bacteriophage Viral Structure • A Virion is a complete, infectious viral particle and is composed of… – Nucleic Acid – Capsid and Envelope Nucleic Acid • Only a single type of nucleic acid (RNA or DNA) is present in any species of virus. • Unlike cellular life, viral nucleic acid can be either single or double-stranded (again only a single type per species) • Size of the genetic structure can range from a few thousand base pairs to a quarter of a million Capsid and Envelope • Capsids are regular repeating protein structures composed of capsomeres. • Some viral species also have a hostderived envelope surrounding the capsid • Some viral species have protein/ carbohydrate “spikes” rising from the surface that can be used for identification Viral Morphology Viral Taxonomy How do you do a taxonomy of something that isn’t alive? Viral Taxonomy • Without a shared phylogeny there is no use for the higher taxons (Domain, Kingdom, Phylum, and Class) • Typically viral species are referred to by Order, Family, Genus and a descriptive common name (in place of a species epithet) • Based on – – – – Nucleic Acid type Strategy of replication Morphology Host range Viral Replication No, not 1 becomes 2, more like 1 becomes 1000. Growing Bacteriophage Growing Animal Viruses Viral Identification • Polyphasic Identification – Morphology – Detection of Antibodies – Western Blotting of known viral proteins – Nucleic Methodologies • PCR • RFLP • RNA PCR Lytic Bacteriophage Cycle Attachment Release Penetration Biosynthesis Maturation Lysogenic Bacteriophage Cycle Lytic Cycle Animal Unenveloped DNA Viral Replication DNA vs. RNA Viral Replication • There are various types of RNA viruses. • Replication of the Genetic Material can be simple or a multistep process. – +RNA, direct translation and replication by viral protein – -RNA, indirect translation and replication by viral protein – dsRNA, direct translation and relication by viral protein – Retroviruses, conversion of RNA to DNA, integration and then production by host. Comparison Bacteriophage • Attachment to Cell Wall proteins. • Viral DNA is injected into cell • No removal of capsid required • Biosynthesis in cytoplasm • Lysogeny • Host cell lysed for release Animal • Attachment to plasma membrane proteins and glycoproteins. • Capsid enters cells • Capsid removed by enzymes • Biosynthesis in nucleus or cytoplasm • Latency • Enveloped viruses bud and nonenveloped rupture. Viruses and Cancer One of many factors Oncogenic Viruses • Some viruses and known to help trigger cancers, called oncogenic viruses. • These viruses affect oncogenes, natural parts of our genetic structure that can cause cancer. • The process of becoming cancerous is termed transformation. • Oncogenic Viruses integrate into the host genetic material. Known Oncogenic Viruses Type of Virus Viral Species Associated Cancer DNA Human Papillomavirus Cervical Cancer Epstein-Barr Virus Burkitt’s Lymphoma Hepatitis B Virus Liver Cancer Kaposi Sarcoma-Associated Hepervirus Kaposi Sarcoma Merkel Cell Polyomavirus Merkel Cell Carcinoma Human T-Lymphotropic Virus 1 Leukemia Hepatitis C Virus Liver Cancer RNA Latency, Persistence and the Prions Acute, Latent, and Persistent Viral Infections • Acute Infections are those that cause immediate proliferation. • Latent infections can occur by itself or after an acute infection, where the viral load remains undetected for a long period of time before reemerging quickly. • Persistent Infections are ones where the viral load build over a long period of time. Graph of Acute, Latent and Persistent Viral Infections Prions • Prions are infectious protein particles • Prions are altered forms of a normal protein in the host that can catalyze the alteration of other “normal” protein particles to the “prion” state • They cause neurological degradation and death with no known treatment. • Since each prion protein is infectious, they are extremely resistant to control measures. Prion Reaction C PrP + Sc PrP 2 Sc PrP