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Viruses

Viruses. Chapter 19. Viral Structure. Very small (smaller than ribosomes !) Genetic material is double or single-stranded DNA or double or single-stranded RNA All have a capsid – protein shell that surrounds the DNA or RNA

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Viruses

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  1. Viruses Chapter 19

  2. Viral Structure • Very small (smaller than ribosomes!) • Genetic material is double or single-stranded DNA or double or single-stranded RNA • All have a capsid – protein shell that surrounds the DNA or RNA • Some viruses also have viral envelopes which surround the capsid and aid in viral infection of host cells

  3. Viral Structure

  4. Viruses are Host Specific • Viruses can only infect a limited variety of hosts (certain organisms and particular cells)

  5. Viral Infection Cycles • Viruses inject DNA into host cell, then either the lytic or lysogenic cycle begins, depending on certain conditions in the host cell

  6. Lytic Cycle • Virus takes over the cell organelles to make new copies of viral DNA and protein coats (capsids). Viral parts self-assemble, cell lyses, and viruses are released to infect other cells • Shorter cycle

  7. Lysogenic Cycle • Viral DNA becomes incorporated into the host cell’s DNA • viral DNA is called a Prophage • The prophage is replicated with the host cell’s DNA • The cell can enter the lytic cycle at any time (HIV can hide out in the lysogenic cycle for years which is why we don’t see the effects for 5-10 years)

  8. Lytic & Lysogenic Cycles

  9. Classes of Animal Viruses • Classified by genome type: • dsDNA, ssDNA, dsRNA, ssRNA • These can be a template for mRNA or DNA synthesis

  10. ssRNA is template for mRNA • Influenza, ebola, measles, mumps, rabies • Use RNA to make mRNA, then makes capsid and envelope proteins, assembles new viruses with copies of RNA genome

  11. ssRNA is template for DNA • These are called retroviruses because they use reverse transcriptase to transcribe DNA from viral RNA • Viral DNA called a provirus is then incorporated into host chromosomal DNA in the nucleus and stays in the host genome • Host transcribes viral DNA into mRNA, makes viral proteins which are assembled into new viruses • Ex: HIV, RNA tumor viruses like leukemia

  12. HIV Reproductive Cycle

  13. Viroids • Extremely small RNA molecules that infect plants, causing errors in regulatory systems that control growth

  14. Prions • Misfolded, infectious proteins that cause misfolding in normal proteins in animals • Destroy brain cells, causing brain degeneration • No cure for prion diseases – the process by which they cause infection is not understood • Mad Cow Disease (in cows) and CreutzfeldtJakob Disease (in humans)

  15. Vaccines • Harmless variants of pathogens (like viruses) that can stimulate the immune system to produce memory cells for the pathogen (in case of actual pathogen exposure at a later time) • Vaccines are generally very effective, but they are usually only preventative (HPV) • Viral infections cannot usually be cured by medicine • Antibiotics do NOT help because viruses are NOT living cells • Some anti-viral drugs interfere with nucleic acid synthesis and enzymes used for viral assembly – interferons like Tamiflu

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