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Viruses Chapter 19 Viruses Chapter 19 What you must know: The components of a virus. The differences between lytic and lysogenic cycles. How viruses can introduce genetic variation into host organisms. Mechanisms that introduce genetic variation into viral populations. Bacteria vs. Viruses Bacteria Virus Prokaryotic cell Not a living cell (genes Most are free-living (some packaged in protein shell) parasitic) Intracellular parasite Relatively large size 1/1000 size of bacteria Antibiotics used to kill Vaccines used to prevent bacteria viral infection Antiviral treatment Viruses Very small (<ribosomes) Components = nucleic acid + capsid ◦ Nucleic acid: DNA or RNA (double or single-stranded) ◦ Capsid: protein shell ◦ Some viruses also have viral envelopes that surround capsid Derived from host cell membranes and viral proteins Viruses Limited host range ◦ Entry = attach to host cell membrane receptors through capsid proteins or glycoproteins on viral envelope (animal) ◦ Eg. human cold virus (rhinovirus) upper respiratory tract (mouth & nose) Reproduce quickly within host cells Can mutate easily ◦ RNA viruses: no error-checking mechanisms Simplified viral replicative cycle Entry: Injection endocytosis, fusion VIDEO: T4 PHAGE INFECTION Bacteriophage Virus that infects bacterial cells Viral Reproduction - phages Lytic Cycle: ◦ Use host machinery to replicate, assemble, and release copies of virus ◦ Virulent phages: Cells die through lysis or apoptosis Lysogenic (Latent) Cycle: ◦ DNA incorporated into host DNA and replicated along with it ◦ Bacteriophage DNA = prophage ◦ Animal virus DNA = provirus ◦ UV radiation, chemicals: lysogenic lytic cycle ◦ Temperate Phage: uses both methods of replication Lytic Cycle of T4 Phage Lytic Cycle vs. Lysogenic Cycle Animal viruses have a membranous envelope Host membrane forms around exiting virus Difficult for host immune system to Viral RNA polymerase detect virus VIDEO: HOW DENGUE VIRUS ENTERS A CELL VIDEO: HIV LIFE CYCLE Retrovirus RNA virus that uses reverse transcriptase (RNA DNA) Newly made viral DNA inserted into chromosome of host (provirus) ◦ *provirus never leaves the host genome Host transcribes provirus to make new virus parts Example: HIV (Human Immunodeficiency Virus) HIV = Retrovirus HIV ◦ Origin: Chimpanzee virus ◦ Infects white blood cells (helper T) ◦ HIV+: provirus (DNA inserted), latent ◦ AIDS: <200 WBC count, opportunistic infections Other Human Viruses Herpes virus Smallpox Herpes Simplex Virus 1 (HSV-1) Herpes Simplex Virus 2 (HSV-2) Eradicated in 1979 due to worldwide vaccination campaigns Emerging Viruses = mutation of existing viruses Pandemic: global epidemic Current Outbreaks Zika Virus ◦ Spread by Aedes mosquitoes (Aedes aegypti) ◦ Major outbreak in Brazil and Latin America ◦ Linked to birth defects (microcephaly) Dengue Fever Chikungunya ZikaVirus (as of Dec. 2015) Drugs for Prevention/Treatment Vaccine: weakened virus or part of pathogen that triggers immune system response to prevent infection ◦ Ex. HPV, MMR, HepA, Flu shot Antiviral Drugs: block viral replication after infection ◦ Ex. Tamiflu (influenza), AZT (HIV) Viroids Small, circular RNA molecules that infect plants Cause errors in regulatory systems that control plant growth Eg. coconut palms in Philippines Prions Misfolded, infectious proteins that cause misfolding of normal proteins Eg. scrapie (sheep), mad cow disease (BSE), Creutzfeldt-Jakob disease (humans), kuru (humans – New Guinea) Diseases caused by prions Prions act slowly – incubation period of at least 10 years before symptoms develop Prions are virtually indestructible (cannot be denatured by heating) No known cure for prion diseases Kuru in New Guinea Prion Neurodegenerative Diseases Alzheimer’s Disease Parkinson’s Disease Prions Video AMOEBA SISTERS: VIRUSES: VIRUS REPLICATION AND THE MYSTERIOUS COMMON COLD TED-ED: HOW WE CONQUERED THE DEADLY SMALLPOX VIRUS.
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