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1. Review What do viruses depend on for their reproduction 2. Review Describe each of the two paths viruses may follow once they have entered a cell 3.

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Presentation on theme: "1. Review What do viruses depend on for their reproduction 2. Review Describe each of the two paths viruses may follow once they have entered a cell 3."— Presentation transcript:

1 1. Review What do viruses depend on for their reproduction 2. Review Describe each of the two paths viruses may follow once they have entered a cell 3. Compare and Contrast Compare the structure of a virus to the structure of both a prokaryotic cell and a eukaryotic cell. Use a graphic organizer of your choice to organize the information

2 CH 20 VIRUSES AND PROKARYOTES 20.1 Viruses

3 Viruses  Nonliving particle made of proteins, nucleic acids, and sometimes lipids  Can reproduce only by infecting living cells  Wide variety of size and structure  Very small  Need electron microscope to see.

4  Capsid  Protein coat surrounding a virus  Proteins on their surface membrane that trick receptor cells on host cell into accepting its genetic material  Contain a few to a few hundred genes.

5  Most viruses infect only a very specific kind of cell  Bacteriophages  Viruses that infect bacteria  Plant viruses infect plant cells  Most animal viruses infect only certain related species of animals.

6 Lytic Infections  Virus enters a bacterial cell  Makes copies of itself  Causes the cell to burst, or lyse  Bacteriophage T4.

7  The virus injects its DNA into the cell.

8  Cell then begins to make messenger RNA (mRNA) from the viral genes.

9  Viral mRNA is translated into viral proteins that chop up the cell’s DNA.

10  Host cell’s metabolic system makes copies of viral nucleic acid and capsid proteins.

11  Viral nucleic acid and capsid proteins are then assembled into new virus particles.

12  Host cell lyses, releasing hundreds of virus particles that go on to infect other cells.

13  Lytic virus is similar to an outlaw in the Wild West  Outlaw eliminates the town’s existing authority.

14  Outlaw demands to be outfitted with new equipment from the local townspeople.

15  Outlaw forms a gang that leaves the town to attack new communities.

16 Lysogenic Infection  Host cell is not immediately taken over.

17  Viral nucleic acid is inserted into the host cell’s DNA  Prohage  Bacteriophage DNA that becomes embedded in the bacterial host’s DNA.

18  Viral DNA is then copied along with the host DNA without damaging the host.

19  Viral DNA multiplies as the host cells multiply  Each generation of daughter cells derived from the original host cell is infected.

20  Prophage may remain part of the DNA of the host cell for many generations.

21  Influences from the environment (radiation, heat, etc) trigger the prophage to become active.

22  Removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.

23  Cold viruses attack with a very simple, fast-acting infection  Capsid settles on a cell, typically in nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.

24  Host cell’s ribosomes mistake the viral RNA for it’s own and translates it into capsids and other viral proteins.

25  New capsids assemble around the viral RNA copies  Host cell releases hundreds of new virus particles to infect other cells within 8 hours.

26 HIV  Human immunodeficiency virus (HIV)  Acquired immune deficiency syndrome (AIDS)  Retrovirus  Genetic information is copied from RNA to DNA.

27  Enter cell and makes a DNA copy of its RNA.

28  Copy inserts itself into the DNA of the host cell.

29  HIV is similar to a lysogenic infection  Viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.

30  Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts  After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.

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