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Viruses

Viruses. Ch 18, pt 1. What is a Virus?. Obligate, intracellular parasites Made of: Capsid - Protein coat Genetic material inside: DNA – virus RNA - retrovirus. Viruses and hosts. Each virus only infects certain host species – the host range

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Viruses

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  1. Viruses Ch 18, pt 1

  2. What is a Virus? • Obligate, intracellular parasites • Made of: • Capsid - Protein coat • Genetic material inside: • DNA – virus • RNA - retrovirus

  3. Viruses and hosts • Each virus only infects certain host species – the host range • Recognition - Proteins on outside of virus match with receptors on surface of host cell • Bacteriophage: • http://www.youtube.com/watch?v=kdz9VGH8dwY

  4. Viral reproduction – uses host cell

  5. Lytic reproduction (phage) • Phage injects DNA into cell • Host cell replicates DNA, and and transcribes the viral DNA into mRNA, which gets translated into viral proteins • Viral capsid and nucleic acids self assemble into new viruses – breaks open cell as they release

  6. Lysogenic infection (phage)

  7. Animal Viruses • Viral envelopes – surround some animal virus capsids • An outer membrane, made of glycoproteins • Glycoproteins recognize & bind to receptor molecules on surface of host cell • ER makes viral envelope glycoproteins, which cluster in certain area on cell membrane

  8. When viruses leave cell, they exit in these areas and wrap selves in the cell membrane with the glycoproteins (so envelope is derived from host cell membrane)

  9. Provirus • When animal virus DNA is integrated into the cell’s genome • Herpes viruses – envelopes are derived from nuclear membrane, act as proviruses • Remains in animal throughout animal’s life • Can have recurring infections, esp. due to stress

  10. RNA genomes in viruses (vs. DNA) • Different types of ssRNA (single strand RNA) genomes • ssRNA – directly acts as mRNA • ssRNA- template for mRNA – • RNA gets transcribed to mRNA (enzyme for this is in virus) • ssRNA – template for DNA synthesis • Most complex - Retrovirus

  11. Retrovirus • Includes enzyme reverse transcriptase – transcribes DNA from RNA template • DNA is incorporated as a provirus in chromosome of animal cell • mRNA from the new DNA can be the genome for new retroviruses, or used to translate viral proteins • HIV is a retrovirus

  12. HIV life cycle In step 2, reverse trancriptase synthesizes a DNA strand complementary to the RNA strand http://www.hhmi.org/biointeractive/hiv-life-cycle

  13. Preventing viral disease in animals Antibiotics do not work – only kill bacteria Vaccines- killed or weakened form of pathogen that stimulates the immune system to produce defenses against the pathogen - can prevent certain viral illnesses Some medications interfere with enzymes that the viruses code for

  14. Emerging viruses • What contributes to this? • 1) Mutations of viruses • RNA viruses – lots of mutations (no proofreading & corrections) • i.e. flu • 2) Dissemination of virus from small, isolated human population • i.e. HIV • 3) Spread of existing viruses from other animals • Virus may mutate as it passes from host species to host species • Swine flu, avian flu

  15. Evolution and viruses • Viruses most likely came after first cells – they depend on cells • Hypothesis: viruses originated from fragments of nucleic acids that could move from one cell to another • evidence: viral genomes are very similar to host cell genomes • Possible source of viral genomes: • Plasmids – small circular DNA molecules found in bacteria & yeast • Transposons – DNA segments that can move from one location to another in the cell’s genome

  16. Possible source of viral genomes: • Plasmids – small circular DNA molecules found in bacteria & yeast • Transposons – DNA segments that can move from one location to another in the cell’s genome

  17. Flu virus • http://www.youtube.com/watch?v=Rpj0emEGShQ

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