1. Viruses

2. If you've ever had the flu or chicken pox or even the common cold, then you have been a perfect host for a virus.
Viruses cannot survive on their own. They require a host cell for survival.
Viruses are non-living particles that contain no cellular structures. Their structure is simple. They contain genetic material, either DNA or RNA, and a protein coat that surrounds the genetic information.

3. RNA makes up 70% of viruses and they mutate often.
Viruses are smaller than prokaryotic cells and therefore are microscopic.
Viruses are significant to living things and to the diversity of organisms.

4. Viral Structure Viruses can take on a number of different shapes:
- Polyhedral
- Spherical
- Cylindrical
- Polyhedral head with protein tail & fibres

6. Viral Reproduction A virus particle cannot reproduce on its own.
Therefore the virus must penetrate a host cell in order to produce more viral particles.
The viral genome only codes for a few proteins to construct its capsule and produce more DNA or RNA.
Once the genetic information has been duplicated and new protein coats synthesized, assembly of the new formed viruses take place.
Now the newly formed viruses leave the host cell to infect more cells.

7. Reproductive Cycle
There are two types of reproductive cycles: - Lytic cycle or
- Lysogenic cycle.
Each one is explained in point form and the image that follows illustrates the process.

8. Lytic Cycle
Virus attaches to a specific site on plasma membrane of host cell.
Nucleic acid or virus enters host cell or the host cell membrane surrounds the virus and forms a vacuole.
Nucleic acid of virus then breaks out and is released into the cell.
Replication host cell replicates viral DNA/RNA.
Assembly viruses with nucleic acid and new protein coat are created.
Host cell releases new viruses.

9. Lysogenic Cycle Viral DNA combines and becomes part of host DNA.
Host cell DNA copies its own DNA and the DNA of the virus.
Copying may occur for many generations without affecting the organism. Viral DNA is inactive.
At some time, possibly due to an environmental stimulus, the viral DNA becomes active.
New viruses are then made.

11. Effect of Viruses
You probably remember going to the doctor when you were young and getting shots.
Perhaps you even visited a health centre to receive a booster shot or a flu shot.
These vaccinations provide protection against specific viruses that cause mumps, chicken pox, meningitis and the flu.
However, you are probably aware that some viruses, like HIV, do not have an effective vaccine.

12. Did you get the flu shot last year and two weeks later were struck down with a fever, head ache, and coughing?

13. The difficulty with destroying viruses is that their genetic information mutates often.
You may have gotten a flu shot for one type of virus, but instead the virus changed its shape or its marker, and your body no longer recognizes it.
Viruses evolve to stay active and avoid being destroyed by antibodies and vaccines.

15. Not all viruses attack animal cells.
Plant viruses only attack plant cells.
The tobacco mosaic virus attacks tobacco plants.
Most plant viruses contain RNA and can devastate a farmer's crop.
Plant viruses are easily spread by insects and birds.
They can also spread through farming and gardening tools.

16. Practice Questions – True or False
Viruses are larger than prokaryotic and smaller than eukaryotic cells.
Viral DNA or RNA mutates often.
Viruses belong in the Protist kingdom because they have a nucleus.
Vaccinations are only effective against viruses.
All viruses spread by common contact, like shaking hands.

17. Answers
False. While it is true that viruses are smaller than eukaryotic cells they are also smaller than prokaryotic cells.
True. The genetic information changes often in viruses
False. Viruses are not classified in any kingdom. Scientists do not consider viruses to be living things
True. Vaccinations help to build up immunity against specific types of viruses. Vaccines do not exist for all types of viruses.
False. Some viruses can be spread through casual contact. However some viruses like HIV need direct entry into the blood supply or body fluids.

18. Short Answer Questions
6. Is there evidence that suggests that viruses existed prior to other organisms appearing on Earth? Explain.
7. List the characteristics that all viruses share.

19. Answers
6. Scientists believe that viruses originated from fragmented DNA from other living organisms. Therefore the evidence suggests that viruses evolved from other living things. 7. All viruses contain a protein coat and genetic information either DNA or RNA. They require host cells to survive and are parasitic. They reproduce using the resources of the host cell.