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  1. 1. Virus
  2. 2. Virus <ul><li>Viruses are obligate intracellular parasites and can be seen at magnifications provided by the electron microscope. </li></ul>
  3. 3. Virus <ul><li>All viruses essentially consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid. </li></ul><ul><li>Capsid is made of protein subunits called capsomeres . </li></ul>
  4. 4. Virus <ul><li>Viruses cannot grow or replicate on their own (inactive particles). </li></ul><ul><li>Viruses only reproduce inside a living host cell using its raw materials & enzymes. </li></ul>Tobacco Mosaic Virus Leaf infected with TMV
  5. 5. Virus <ul><li>They lack ribosomes & enzymes needed for protein synthesis or metabolism. </li></ul><ul><li>Viruses are extremely small particles ranging from 20 - 400 nanometers on average </li></ul>
  6. 6. Structure of Virus <ul><li>Nucleocapsid includes the viral nucleic acid & its capsid. </li></ul>
  7. 7. Structure of Virus <ul><li>Some viruses have lipid rich covering around capsid called the envelope. </li></ul><ul><li>Envelope usually is formed from host cell membrane. </li></ul><ul><li>Envelope may have spikes to help chemically recognize & attach to the host cell. </li></ul>
  8. 8. Structure of Virus <ul><li>Structurally complete, mature and infectious virus is called the ‘virion’. </li></ul>
  9. 9. Viroids <ul><li>Smallest particle able to replicate. </li></ul><ul><li>Made of a short, single strand of RNA with no capsid. </li></ul><ul><li>Cause disease in plants. </li></ul>
  10. 10. Prions <ul><li>No nucleic acid or capsids. </li></ul><ul><li>Made of protein particles that have folded incorrectly. </li></ul><ul><li>Attacks the central nervous system. </li></ul><ul><li>Cause animal diseases in cows (Mad Cow disease), sheep, & humans. </li></ul>
  11. 11. Bacteriophage <ul><li>Bacteriophage is one of the most complex viruses. </li></ul><ul><li>They attack bacterial cells. </li></ul>
  12. 12. Bacteriophage <ul><li>The virus has a head, tail, base plate, & tail fibers. </li></ul><ul><li>A long DNA molecule is found inside the head. </li></ul><ul><li>Tail is used to inject the viral DNA into the host cell. </li></ul><ul><li>Tail fibers are used to attach to the host. </li></ul>
  13. 13. Bacteriophage Replication
  14. 14. Bacteriophage Replication <ul><li>In the lytic cycle, the host cell bursts, releasing new phage particles. </li></ul><ul><li>Some phage can also undergo a lysogenic cycle, in which their DNA is inserted into the host chromosome, where it replicates for generations. </li></ul><ul><li>When conditions are appropriate, the phage DNA exits the host chromosome and enters a lytic cycle. </li></ul>
  15. 15. <ul><li>Some viruses have promoters for host RNA polymerase, which they use to transcribe their own genes. </li></ul>
  16. 16. <ul><li>Some RNA viruses use their RNA to make mRNA to code for enzymes and replicate their genomes without using DNA. </li></ul>
  17. 18. <ul><li>Retroviruses, such as HIV, have RNA genomes that they reproduce through a complementary DNA intermediate. </li></ul>