Transcript Viruses
Viruses Obligate Intracellular Parasites: • Require living host cell in order to replicate. Very species-specific. • Genome of a single type of nucleic acid (RNA or DNA; ss or ds) • None have a complete mechanism for nucleic acid synthesis. • They have no mechanism for expression of their genes. • Must invade and take over host cell’s functions & resources. • A protein coat and additional specialized structures and/or enzymes facilitate transfer of viral nucleic acid to the host. Viral Structure Growing (Cultivating) Bacteriophage Life Cycles Scale? A complete virus particle is called a virion. Viral Structure (all have a nucleocapsid core) Polyhedral Helical Protein coat (capsid) is made of capsomere proteins, whose slight mutation may result in new strains of that virus. Capsid protects the nucleic acid genome within. Capsomere proteins bind some viruses to specific sites on their host cell’s surface. Complex Virus Bacteriophage (virus of a bacterium). Tail fibers and pins attach virus to specific sites on the cell surface of a bacterium. (e.g. T-Even Phage) Enveloped Viruses Only some eukaryotic virus types are enveloped. Envelope is made of plasma membrane from the host cell with viral proteins added into the membrane prior to “budding” out from the host cell. Bacteriophage • All naked •Characterized shape and nucleic acid type. • Few are as complex as T-even viruses (dsDNA). • Some viruses are specific to a particular strain of a bacterium. Bacteriophage Lytic Life Cycle Phage lysozyme is involved. Scaffolding proteins help assembly of the capsid and then removed; other viral proteins are involved in final maturation. Release involves phage endolysin and holin to form holes in membrane and peptidoglycan Time Scale of Lytic Cycle Eclipse Period (= can’t see intact phage particles in cell) Virulent phage only have a lytic life cycle. Lysogenic Life Cycle Temperate Phage (e.g. Lambda) Lytic or Lysogenic? • Immediate expression by phage of a lytic cycle repressor gene (cI) and lytic cycle activator gene (cro). • Most often repressor is produced faster and “blocks” transcription of lytic genes and cro activator. • Integrase (int gene product) facilitates integration at att sites. • Repressor will also block transcription of other invading phage; bacterium with prophage is “immune”. • Loss of repressor by degradation allows cro gene product to activate excision and/or lytic cycle; excisionase (xis). Host cell stress is associated with the control. Specialized Transduction: self-study BONUS • Error in excision take gal gene. • A rare event among many normal excisions. • Replicated like normal lambda phage. • Low-frequency of tranducing lysate (LFT-lysate) NOTE: Integration in this fashion requires a normal prophage in the chromosome; “helper phage”. Excision and the lytic cycle for this bacterial population will form a high-frequency tranducing lysate (HFT-lysate) The role of a normal helper phage in integration of defective lambda. Growing (Cultivating) Viruses Bacteriophage: grow on bacterial “lawns”; will form plaques (clearing zone). Growing (Cultivating) Viruses • Eukaryotic Viruses: grow in tissue culture; plaques form or infected (transformed) cells may look abnormal and clump. Enveloped eukaryotic viruses grow in embryonated chicken egg membranes. * If you have an egg allergy, be careful of vaccines for enveloped viruses that were cultivated in eggs. Why? Purification by Ultracentrifugation