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Viruses Warm Up What is a virus? How do you think (or do you remember from the video) a virus replicates? Homework: Virus Replication WS due Friday What is a virus? A virus is a particle (not a cell) made of nucleic acid (DNA or RNA) and a protein coat Viruses don’t require food or water Viruses can only replicate in a host cell by using the host cell’s “machinery” How does its size compare to that of various cells? Viruses are smaller than prokaryotic cells (like bacteria) and significantly smaller than eukaryotic cells (like human cells) http://www.cellsalive.com/howbig.htm Is a virus a living organism? Most scientists describe viruses as non-living Don’t consist of cells Use “machinery” of other cells to carry out necessary cell functions Characteristics of Life Virus Cell Growth Homeostasis Metabolism Mutation Nucleic acid Reproduction No No No Yes DNA/RNA Only in host Yes Yes Yes Yes DNA Independently Structure of a Virus A virus has three main parts: Nucleic acid Capsid (protein coat) Glycoproteins (proteins on surface used for attachment) Some viruses also have an envelope (protein layer outside of the capsid) Bacteriophages also have tail fibers Come in a variety of shapes Virus Structure Examples of Virus Shapes Making more copies The purpose of a virus is to infect cells so that it can force cells to make more copies of itself. How does the virus make this happen? 1. 2. 3. Virus glycoproteins recognize and attach to receptor sites on host cell (lock and key) Virus enters cell • Injection or • “enveloped” by cell Virus takes control of host cell machinery Virus “Life” Cycles Up to this point, all viruses would have followed the same steps as they entered the cell. Once inside a cell, a virus may follow one of two “life” cycles: Lytic Lysogenic Lytic Cycle Virus has entered host cell: 3. Replicates itself immediately • Takes over the ribosomes to make the necessary proteins 4. Different proteins are assembled into whole viruses 5. When enough copies are made, an enzyme is produced that “lyses” the host cell, releasing copies to infect more cells Lysogenic Cycle Virus enters cell (same as before) Virus inserts its DNA into the host cell’s chromosome (latent phase) Virus may remain part of host cell chromosome for a short while or for many years (considered “temperate” HIV is a lysogenic virus. Why is this a problem? Eventually, trigger causes virus to remove itself from the host cell chromosome and enter lytic cycle (considered “virulent”) Lysogenic Cycle http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html Retroviruses Most viruses contain DNA. This makes it easy for them to instruct their host cell in what to do because the host cell recognizes DNA instructions. Retroviruses contain RNA instead of DNA. Host cell won’t recognize RNA instructions, so virus must make a DNA copy of its RNA first Reverse transcriptase (an enzyme) copies RNA instructions into DNA HIV is an example of a retrovirus http://www.whfreeman.com/kuby/content/anm/kb03an01.htm Reverse Transcriptase RNA virus DNA (single strand) A T U A G Reverse transcriptase C C G C G A T DNA (double strand) Single DNA strand used as template for complementary DNA strand T A A T C G G C G C T A Particles Similar to Viruses Viroids Contain RNA No capsid Infect plants Prions Clumped proteins Mad cow disease, Creutzfeldt-Jakob disease Preventing Viruses Vaccines exist for certain viruses Vaccines contain a pathogen (like a virus) in a form that will trigger the immune system, but not make the person sick Immune system will “remember” the pathogen if the person is ever exposed again and will be able to fight it quickly Treating Viral Infections Very few treatments are available for viral infections. Why? Viruses mutate very frequently- “old” vaccines won’t work on “new” mutation Some antiviral drugs exist that interfere with protein production