1. Viruses Evolution Objectives:
Explain why a virus is not considered to be “living”.
Be able to describe and identify the four basic virus structures.
Describe the lifecycle of a virus.
Explain what lysogeny is and how the lysogenic cycle differs from the lytic cycle.
Explain what Retroviruses are.
Describe how vaccinations work.
Evolution

2. What Are Viruses?
Microscopic, non-living particles that reproduce within a living cell
viruses have no cellular structures, no cytoplasm, no organelles, and no cell membrane

3. Are Viruses Living?
viruses exhibit some characteristics of living things and some of non-living things:
do not respire
do not respond to stimulus
do not grow
DO reproduce
DO evolve
they are little more than
mobile genes that parasitize
cells

4. Origin of Viruses?
must have evolved after the first cells came into existence
fragments of nucleic acid that escaped cells that then became parasites on similar cells
each type of virus is probably more related to its host cell than to other viruses

5. Virus Structure viruses consist of two parts:
outer protein coat = capsid
nucleic acid – double stranded DNA or single
stranded RNA; linear or circular

6. Classifying Viruses
viruses are often classified according to their shape:
cylindrical
polyhedral
circular
complex

7. Viral Replication viruses must reproduce inside a living cell
each kind of virus is specific to a specific type of cell
E.g.: influenza viruses only affect the lungs, hepatitis viruses affect liver cells

8. Lytic Cycle
during reproduction, the virus kills the host cell which results in the victim showing symptoms of disease (E.g. Rhinovirus – the common cold)
1. Attachment
virus attaches to host cell membrane at a specific receptor site (hijacks the cell)

9. Lytic Cycle
2. Entry
virus releases enzyme to break down cell wall/cell membrane or the cell membrane folds to bring the virus in
nucleic acid passes into host; leaves empty capsid outside

10. Lytic Cycle

11. Lytic Cycle 3. Replication
viral nucleic acid is copied (a lot!) using DNA and/or RNA of host
protein for capsid is also produced

12. Lytic Cycle

13. Lytic Cycle
4. Assembly
viral nucleic acids and proteins are assembled to complete the virus particle

14. Lytic Cycle

15. Lytic Cycle
5. Release
new viruses release an enzyme to break cell membrane of host and are released

16. Lytic Cycle

17. Lysogenic Cycle
some viruses may remain inactive for many generations (herpes, HIV)
these viruses are known as proviruses

18. Lysogenic Cycle 1. Attachment
nucleic acid of virus attaches to DNA of host
2. Replication
host cell replicates and replicates viral DNA along with it – viral DNA stays within the host DNA so no new viruses are made
this can go on for many years with no effect on the host, but now viral DNA is in many cells
3. Activation
virus DNA begins lytic cycle, cells die
the trigger for this is still unknown

19. Lysogenic Cycle

20. Retroviruses
viruses that have RNA (not DNA) are termed retroviruses (70% of viruses are retroviruses)
RNA viruses have higher mutation rates than DNA viruses (single stranded)
HIV virus is a provirus that also happens to be a retrovirus
HIV Virus
Source:

21. Vaccinations
RNA vaccines are mixtures containing weakened forms or parts of a dangerous virus
exposure triggers an immune response without causing infection
this results in a form of “chemical memory” that allows the immune system to react quickly if exposed to the virus again
Eradication - reduction of an infectious disease's prevalence in the global host population to zero
the virus has ceased to exist on Earth – small pox (humans) and Rinderpest (cows) are the only two so far, many more are being worked on with Polio being the front runner (Rotary International)

22. Viruses for Cancer Treatment
1.How are viruses being used to treat disease?

23. Viroids and Prions What are viroids and prions?
Why are these particles problematic?

24. Consolidate the Concepts
Read pages 54 – 59 (and of course, annotate your notes)
Do questions on page 59 #1-11