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Viruses and Prions and Bacteria, OH MY! Genetics of Viruses and Bacteria Introduction to the Genetics of Viruses Work on the tobacco mosaic virus (TMV) provided the first evidence for the presence of viruses, a pathogen smaller than bacteria. Structure of viruses Smallest viruses are 20 nm in diameter Largest can barely be seen by a light microscope Not a cell! Is it living? We can’t decide! Infectious particles consisting of at least a nucleic acid (RNA or DNA) enclosed in a protein coat Structure of Viruses Viral Genomes Can be : Double Stranded Single Stranded DNA X X RNA X X Structure of Viruses Capsids and Envelopes Capsid= protein shell enclosing the viral genome Rod-shaped, polyhedral, helical, or more complex Large number of protein subunits (but generally similar) The most complex are found in phages (bacteriophages) Viral envelopes Derived from host cell Help virus infect host Structure of Viruses Viral envelopes General Viral Reproductive Cycles Obligate intracellular parasites: Viruses lack metabolic enzymes, ribosomes, and other cellular machinery Each cell has a host range Can only reproduce in a host Limitations of what the virus can infect Due to the lock-and-key specificity of the virus and the cell’s receptor molecules Genome replication DNADNA RNA RNA RNADNARNA General Viral Reproductive Cycles Virus injects its genome into the host cell Viral genome reprograms the cell to perform reproductive work for the virus using the cell’s resources Assembly of viral proteins and genomes is often spontaneous after synthesis Viral progeny exit the cell Reproductive Cycles of Phages Lytic Cycle vs. OR Lysogenic Cycle Reproductive Cycles of Phages Lytic Cycle Phage infects a healthy cell Bacteria copies virus Progeny exit bacteria by lysing it Viruses in this cycle are considered virulent Bacterial defenses: Natural selection favors bacterial mutants with unrecognizable receptor sites Although, natural selection also favors phage mutants Phage DNA often recognized and dismantled by the cell’s restriction enzymes Reproductive Cycles of Phages Lysogenic Cycle Viral genome replicated without destroying host Phages that use both types of cycles are called temperate phages Viral DNA is incorporated into bacterial genome Bacteria reproduces normally, but also copies viral genome Viral DNA called a prophage at this point Prophages genes can regulate host cell Environmental signal triggers the switch from lysogenic cycle to lytic cycle Lytic and Lysogenic cycles Reproductive Cycles of Animal Viruses Many animal viruses with RNA genomes have a viral envelopes derived from the plasma membranes Enveloped viruses can exit the cell without destroying the cell: contrasts with lytic cycle of phages Some animal viruses with DNA genomes have viral envelopes derived from nuclear membrane Reproductive Cycles of Animal Viruses Attachment Entry Uncoating RNA and protein synthesis Assembly and exit Reproductive Cycles of Animal Viruses RNA Viruses Contains a viral enzyme that can be used for RNARNA synthesis In different viruses, original RNA genome can: Be used as mRNA Be used as a template for mRNA Reproductive Cycles of Animal Viruses Most complicated RNA animal viruses: Retroviruses (An example is HIV) Contain reverse transcriptase RNADNA New viral DNA integrates into cell DNA and stays in the cell’s DNA as a provirus (never leaves) Cell produces viral RNA to function both as mRNA and as genomic RNA Evolution of Viruses Not likely to have originated prior to the first cells Why? Because they are obligate intracellular parasites Believed to have originated from fragments of cellular nucleic acids High mutation rate in many viruses Vaccines Harmless variations of viruses used to elicit an initial immune response by the body The main way to fight viruses Why can’t we treat viral diseases? Drugs that would attack the virus would have to also attack the host cell Emerging Viruses Viruses that seem to appear suddenly Mutations of pre-existing viruses Jump to a new type of host RNA viruses particularly susceptible because they lack proofreading in replication e.g. Hantavirus Ebola, HIV, SARS, influenza Viruses and Cancer Some viruses have been shown to cause cancer e.g. Hepatitis B and liver cancer Viruses can trigger cancerous genetic conditions Oncogenes- a gene found in viruses or as a part of the normal genome that is involved in triggering cancerous conditions Proto-oncogenes- a gene that has be potential to cause cancer pending alteration into an oncogene Viroids and Prions Viroids Naked circular RNA molecules that infect plants Prions Infectious proteins e.g. scrapie in sheep, mad cow, Creutzfeldt-Jakob disease Slow acting No cures Cannot replicate itself, but can convert normal proteins