1. Bellwork Question: How does the common cold spread
Bellwork Question: How does the common cold spread? What precautions can you take to stop yourself getting ill?

2. Viruses
Section 20.1

3. Why were viral diseases hard to identify?
Tobacco mosaic disease caused a large number of issues in the late 19th Century
Dimitri Ivanovski inferred that cause of disease came from host plants, but could not pin down exactly what caused it
Martinus Bejerink proposed that tiny particles were the cause – he called them viruses
Crystals of the virus isolated by Wendell Stanley in 1935

4. Are Viruses alive?
No!
Virus – nonliving particle made of proteins, nucleic acid and sometimes lipids
They can only reproduce by infecting living cells

5. Different types of viruses

6. Common links between viruses
All viruses have a protein coat called a capsid
Number of genes can be highly varied between viruses ( a few to hundreds)

7. How do Viruses work? Viruses bind to receptor proteins on host cells
Cell is tricked to taking on the virus
Most will only infect a specific kind of cell
Plant viruses – plant cells
Animal viruses – animal cells
Bacteria viruses – bacteriophages

8. What happens after a virus infects a cell?
Viruses use their genetic information to make multiple copies of themselves
Some viruses will replicate immediately
Others will not – remain inactive
Two types of infection
Lytic infection
Lysogenic infection

9. What happens during a lytic infection?
Virus enters cell
Copies of virus produced within the cell
The cell bursts (lyse)
Example – bacteriophage T4

10. What happens during a lysogenic infection?
Viral nucleic acid is inserted into the host cell’s DNA
Copied along with the host DNA without damaging the host
Embedded bacterial DNA is called a prophage
Can remain dormant for many generations
An environmental trigger can cause it to ‘activate’
Examples – radiation, heat, specific chemicals
Removes itself from the host cells DNA and directs the synthesis of new virus particles
Become and active lytic infection

11. Comparing lytic and lysogenic infections

12. Comparing lytic and lysogenic infections

13. RNA viruses Most viruses contain RNA rather than DNA (70%)
Common example – common cold!
Capsids settle on cell (normally in nose), brought inside, whereby a viral protein makes new copies of viral RNA
Hosts ribosomes mistake viral RNA for mRNA, and translate it into capsids and other viral proteins
New capsids assemble around viral RNA copies, and within hours, hundreds of new viruses are released

14. What is HIV?
Acquired immune deficiency syndrome is caused by an RNA virus called human immunodeficiency virus
HIV virus belongs to a group of viruses called retroviruses
RNA from virus is copied into DNA
This is inserted into the DNA of the host cell
Very similar to lysogenic infections of bacteria
Can remain dormant for a long time
Once active, it begins to destroy the very system of the body that would normally fight infections
How is this different from how the virus that causes common colds works?

15. Viruses and Cells
Viruses are parasites – their existence relies on other organisms

16. Writing assignment (well kind of…)
Using your notes and the textbook, compare the structure of a virus to the structure of both a prokaryotic cell and a Eukaryotic cell. Display the information in a graphic organizer – chart, venn diagram, spider diagram etc…
Explain in a paragraph why viruses are not considered living things, paying special attention to the different modes of reproduction between cells and viruses.

17. Key summary questions? Are viruses alive?
What structures do all viruses contain?
What are the two main types of viral infections? Explain the different mechanisms
Give examples of diseases caused by viruses?
How are viruses and cells different?

18. Summary Viruses are parasites, they are not alive!
All viruses are very small and contain a protein coat, called a capsid
There are two types of viral infections – Lytic and Lyosgenic
Most viruses contain RNA
Common Cold and HIV examples of RNA viruses
With a common cold, viral RNA does not enter nucleus
With HIV virus, genetic material enters nucleus
Disease can remain dormant for large periods of time

19. Bellwork: Why are nitrogen fixing bacteria so important
Bellwork: Why are nitrogen fixing bacteria so important? How does this relate to crop rotation?