1. VIRUSES
What are they &
Where do they come from?

2. What Are Viruses? They are acellular (not cells)
They aren’t alive (or are they?)
They can’t grow or
Reproduce or
Metabolize outside of a host cell
But that is true for some bacteria also
They can be crystallized

3. What Are Viruses?
They are smaller than a bacterium.

4. What Are Viruses? They are nucleic acids (DNA or RNA)
Wrapped in a protein coat (capsid)
Some have a surrounding envelope stolen from the host cell
They are intracellular parasites

5. What Are Viruses? They are nucleic acids (DNA or RNA)
Wrapped in a protein coat (capsid)
Some have a surrounding envelope stolen from the host cell
They are intracellular parasites; they can’t grow or reproduce outside the host
They are much smaller and more numerous than bacteria

6. What Are Viruses? Viruses cause disease
Polio, measles, herpes, flu, small pox, AIDS, Ebola
All organisms are subject to viral infection
Antibiotics have no effect on viruses
Viruses are being tested to attack cancer cells

7. Healthy people are full of viruses
Test: The number of distinct viruses in stool samples ranged from 52—2773 !
Nasal swabs:
10,000 vial sequences in children with fever
Without fever = 1,000

8. What Do Viruses Look Like?

9. Characteristics Used to Classify Viruses
Type of nucleic acid (DNA or RNA)
Single or double stranded
Shape of virus
Number of units (capsomeres) making up the capsid
Naked or with envelope (envelope is part of the host membrane around virus)
Size of virus particle (=virion)
Site of reproduction (host cytoplasm or nucleus)
Biochemical steps involved to produce mRNA

11. Hepatitis B
Herpes
Phages
HIV
Ebola
Gastroenteritis in infants
Rubella

12. Lytic Life Cycle ssDNA dsDNA mRNA Attachment Penetration Replication
Assembly
Release

13. Lytic Cycle Again—20 minutes--> 300 virions

14. Phage therapy uses viruses to attack some bacterial infections
For every bacterium in your body there are ~100 bacteriophages.
Estimated 10 billion phages packed into each gram of human stool.
Phage therapy uses viruses to attack some bacterial infections

15. Lysogenic Life Cycle Integration Attachment Penetration Lytic Cycle
Replication
Lytic Cycle

16. Lysogenic Life Cycle Again

17. Four Major Ways Virus Enters Host Cells
Injection e.g. bacteriophage

18. Four Major Ways Virus Enters Host Cells
Endocytosis (envelope viruses; e.g. ebola)

19. Four Major Ways Virus Enters Host Cells
Fusion (envelope viruses -
e.g. herpes, HIV)

20. Four Major Ways Virus Enters Host Cells
Wound Entry—Plant viruses generally passed by insects as they eat plants.

21. Two Major Ways Viruses Escape Host Cells
Budding
Bursting
LYSIS

22. Budding off HIV
Virions in host cell

23. Ebola virus

24. Hepatitis B
Herpes
Phages
HIV
Ebola
Gastroenteritis in infants
Rubella

25. Complete Virus Life Cycle
-ssRNA
ebola

28. Human Immunodeficiency Virus
AIDS
HIV
Human Immunodeficiency Virus

29. HIV attacks cells of the immune system
HIV infects Macrophages & T cells and destroys them. Then other infections can invade and kill the victim.

31. Hepatitis B
Herpes
Phages
HIV
Ebola
Gastroenteritis in infants
Rubella

32. Retro Viruses
RNA viruses
make DNA
e.g. HIV
+ssRNA/DNA hybrid

33. Life cycle
of HIV
Retro-virus
RNADNA

34. VIRUSES PASS GENES
Viruses are the most common biological agents in the sea
1010 per liter in surface water
5—25 times the number of bacteria!
Viruses cause major mortality in the sea microbes. Produce cellular debris. Probably eaten by bacteria
Transformation DNA Via water Bacteria

35. VIRUSES PASS GENES Recombination
Different kinds of viruses infecting the same host can exchange genes occasionally.
e.g flu epidemic bird + human virus
e.g. H1N1 virus is combination of
2 pig + bird + human virus

36. VIRUSES PASS GENES Between Species
Transduction = virus carries host DNA along with its own to another host cell.
Host Species 1 Virus carries sp.1 genes Host Species 2
(Bacteria sp. 1) (Bacteria sp. 2)
Recombination occurs in bacterial DNA
We use viruses to inject new genes into plants & animals—genetic engineering

37. Probably Multiple Origins
ORIGIN OF VIRUSES
Viruses-first Hypothesis
Viruses evolved from protein and nucleic acid at the same time as the first cells were evolving.
Degeneracy Hypothesis
Viruses were once small cells that parasitized larger cells. Over time they lost those genes not required.
Escaped-genes Hypothesis
Viruses evolved from bits of naked RNA or DNA that escaped from cells or as plasmids or transposons.
Probably Multiple Origins