1. Lesson Overview
20.1 Viruses

2. THINK ABOUT IT
Farmers began to lose their tobacco crop to a plant disease.
What would you do next?
How would you deal with the invisible?

3. Discovery of Viruses
In 1892, Dmitri Ivanovski--found in the liquid extracted from infected plants.
In 1897, Martinus Beijerinck- named tiny particles in the juice viruses-“poison.”
In 1935, Wendell Stanley- found crystals of tobacco mosaic virus. Since living organisms do not crystallize- viruses were not alive

4. Discovery of Viruses
A virus is a nonliving particle made of
Proteins
nucleic acids
sometimes lipids.
Viruses can reproduce only by infecting living cells.

5. Structure and Composition
Viruses differ widely in terms of size & structure.
Most viruses are so small they can be seen only with the aid of a powerful electron microscope.

6. Structure and Composition
The protein coat surrounding a virus- capsid.
May have an additional membrane that surrounds the capsid (flu virus)
Contain a few genes to hundreds of genes

7. Structure and Composition
Most viruses’ proteins on the surface membrane binds to proteins on the host cell.
The proteins “trick” the cell to take the virus or some genetic material
Once inside, genes are expressed & may destroy the cell.

8. Structure and Composition
Most viruses infect only a very specific kind of cell.
Plant viruses infect plant cells
Animal viruses infect only certain related species of animals
Viruses that infect bacteria -bacteriophages.
RABIES!

9. Lytic Infections Lytic infection virus enters a bacterial cell
makes copies of itself
causes cell to burst- lyse.
EX.) T4
DNA core inside
protein capsid

10. Lytic Infections 1.) Attachment
2.) Injection: The virus injects its DNA into host cell.
3.) Synthesis: Virus DNA uses host DNA to make more viruses
4.) Assembly: Host cell creates more of the viruses
5.) Releases: Finally the host cell lyses (ruptures)
hundreds of viruses
that go on & infect
other cells.

11. Lytic Infections
A lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host.
First, the outlaw eliminates the town’s existing authority.
(In a lytic infection, the host cell’s DNA is chopped up)

12. Lytic Infections
Next, the outlaw demands to be outfitted with new equipment from the local townspeople.
(In a lytic infection, the viruses use the host cell to make viral DNA & viral proteins.)

13. Lytic Infections
Finally, the outlaw forms a gang that leaves the town to attack new communities.
(In a lytic infection, the host cell bursts, releasing hundreds of virus particles.)

14. Lysogenic Infection
Some Viruses cause a lysogenic infection.
dormant state
Prophase-DNA that is embedded in the host’s DNA
The prophage may remain part of the DNA of the host cell for many generations.

15. Lysogenic Infection
Influences from the environment—radiation, heat, etc—trigger the prophage to become active.
becomes an active lytic infection.

16. A Closer Look at Two RNA Viruses
About 70 % of viruses contain RNA rather than DNA.
In humans, RNA viruses cause a wide range of infections
mild colds to severe HIV.
Certain kinds of cancer also begin with an infection by viral RNA.
HPV
Common
Cold

17. The Common Cold
Cold viruses attack with a very simple, fast-acting infection.
A capsid settles on a cell
typically in the nose
brought inside
Virus makes many new copies of the viral RNA.

18. The Common Cold
The host cell’s ribosomes mistake the viral RNA for its own & makes other virus proteins.
The new capsids assemble & within 8 hours, the host cell releases 100s of new virus particles to infect other cells.

19. HIV
Acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV).
HIV belongs to a group of RNA viruses-retroviruses.
The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.

20. HIV
When a retrovirus infects a cell, it makes a DNA copy of its RNA.
The copy inserts itself into the DNA of the host cell.

21. HIV
Retroviral infections are similar to lysogenic infections of bacteria.
like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles
HIV damages the host’s immune system.

22. Viruses and Cells
All viruses are parasites.
Parasites depend upon other living organisms for their existence +/-
Viruses infect living cells in order to grow & reproduce
taking advantage of the nutrients & cellular machinery.

23. Viruses and Cells
Viruses have many of the characteristics of living things.
After infecting living cells, viruses can reproduce, regulate gene expression, & even evolve.
Swine Flu Rotavirus

24. Viruses and Cells
Some of the main differences between cells and viruses are summarized in this chart.