1. Viruses
Runaway Genes?

2. Not Usually Considered Alive Because…
They aren’t made of cells (order)
They can’t reproduce on their own
Don’t grow or develop

3. What Are Viruses? We’re not sure exactly
Pieces of nucleic acid wrapped in a protein (and sometimes lipids)
No ribosomes or organelles

4. Viral Genomes Can be DNA or RNA Can be single or double stranded
Usually short and circular

5. Examples of Viruses Cold Influenza (flu) HIV West Nile Virus
Chicken pox
Small pox
Ebola
Herpes
HPV
Rabies
Polio
Meningitis
Measles
Mumps
Tobacco mosaic virus
Also affect plants, bacteria and archaea – extremely widespread

6. Viruses Depend on a Host
Cannot live independently
Have no machinery for reproduction or metabolism

7. Viral Reproduction 2 “phases” of viral reproduction: Lytic Lysogenic
Use the hosts to replicate
RNA viruses use transcription
DNA viruses use DNA replication
2 “phases” of viral reproduction:
Lytic
Lysogenic

8. Lytic Cycle Host copies and assembles new viruses
New viruses burst free from the cell
Makes a virus virulent

10. Lysogenic Cycle Virus incorporates itself into genome
Gets copied along with the host genome
Can alter the function of the host

12. Retroviruses
RNA viruses like HIV must first make DNA from RNA to enter lysogenic cycle
Reverse Transcriptase accomplishes this so DNA can be inserted
Drugs like AZT fight this action

14. Very Difficult to Fight Viruses
There’s nothing to attack!
Almost everything they use belongs to the host
Only mechanisms are vaccines to prevent viruses from entering cells and preventing reverse transcription

15. Viral Genetics
Viruses can reproduce extremely rapidly by using hosts

16. Viruses Evolve Quickly
Reproduce rapidly
Very little proofreading of errors (especially RNA viruses)
2 viruses in the same host can combine

17. Where Did they Come From?
We don’t know!
3 main hypotheses, each with some support
Cells that through evolution lost other genes and structures necessary for independent survival
Evolved from plasmids or transposons (jumping genes)
Co-evolved with the first forms of life

18. Transposons Jumping genes Can copy and paste or be cut and paste
Introduces repetition in patterns in addition to changing genes

19. What we Think Viruses are ancient and probably as old as life itself
Viruses probably do not share a common ancestor
There are millions of viruses

20. Why Should We Care About Viruses?
Effects on human health
Role in ecosystems
Important role in genetics and evolution

21. Role in Ecosystems
Viruses kill bacteria (estimated to kill about 20% of the biomass of the ocean every day)
Recycle nutrients and materials, prevent overgrowth of bacteria

22. Role in Genetics/Evolution
Viruses carry genes from one individual to another (can be different species!)
Transduction
Hugely important in increasing genetic diversity (especially before sex evolved)

23. The More Realistic Tree of Life
Genes have been transferred horizontally
Viruses account for much (not all) of this transfer

24. Impact on Humans
Much of our genome came from viruses (how much is debated, but current estimate is 8%)
May have inserted things like schizophrenia? dna-comes-virus-causes-schizophrenia
Cancer?
human-genome-cancer html

25. Crazy Virus Facts Every day 1016 copies of HIV are made
1 million viruses per ml of seawater
1030ish on the planet
End to end would be 200 million light years long
Smallest virus studied was 1,700 nucleotides long- made only 2 proteins
Some biologists believe the nucleus formed from a virus that invaded a bacterium (others think it was an archaean cell)