1. Diversity of Living Things
2.2 Viruses

2. What is a Virus?
Microscopic particles capable of reproducing only within living cells
Lifeless outside of living cells, in living cells (host cell) they can reproduce.
Virus means poison in Latin

3. Viruses Classified as non-living matter
But has many characteristics of living matter
1934: early electron microscope allowed scientist to first see viruses
Less than 0.1 micrometers in diameter (1 micrometer=
10 -6 m)
5000 flu viruses fit on the head of a pin

4. Structure of Virus
Basic structure: Nucleic acid (DNA or RNA) in core, surrounded by capsid (protein coat)
Some viruses have lipid membrane around capsid (HIV)

5. Structure of Viruses
Bacteriophages or phages (category of viruses that invade and destroy bacteria cells) have unique shape and distinct head and tail regions
The capsid can display various shapes (See Figure 3 and 4 on p. 55)

6. Viral Diversity and Specificity
Most viruses are selective and host-specific
- Bacteriophages have a very restricted host range
- Most plant viruses can infect a wide range of host plants
- Some animal viruses have a broad range while others have a very narrow host range
swine flu - hogs & humans
rabies - many mammals and birds
common cold - only cells in the upper respiratory tract of humans
HIV - only certain types of human white blood cells

7. Classification and Phylogeny
Classified into orders, families, genera and species
Classified based on size, shape and type of genetic material

8. Origin of Virus Many theories about origins of viruses
Could have been parasitic organisms that depended less and less on their own cell components
Could have come from fragments of genetic material of other organisms
Another hypothesis is that virus-like particles existed before the first cells

9. Importance of viruses Cause many human diseases
Mild: common cold, chicken pox
Serious: AIDs, cholera and rabies
Ability to spread from person to person. Ex. Influenza can infect millions of people in a short amount of time.
Epidemic: large-scale outbreak of disease in a particular region
Pandemic: epidemic occurring on global scale

10. Importance of viruses cont’d
Small number of viruses have been linked to cancer. This can happen if the virus alters the host cell’s DNA leading to uncontrolled cell division. Hepatitis C has been shown to produce this effect in liver cells.
Viruses also cause disease in animals and plants
They can be useful in ecosystems by controlling populations of certain organisms

11. Viral Replication 4 basic steps in the “lytic cycle” 1. Attachment
2. Synthesis
3. Assembly
4. Release
See video:
Some viruses have a lysogenic cycle, where host cells are not destroyed
Cancer-causing viruses can act by adding specific genes to a host cell’s DNA, causing it to become a cancer cell
Transduction is when a virus transfers DNA from one bacteria cell to another. This only happens on rare occasions.
Attachment:Virus attaches to a specific receptor site on the host cell membrane. Whole virus or just DNA/RNA enters the host cell
Synthesis: Viral DNA/RNA directs host cell in replicating virus parts
Assembly: Viral parts are assembled into new virus particles
Release: New virus particles are released from host cell. Lysis (destruction/bursting) of host cell

12. Lytic Cycle

13. Lytic Cycle
Lytic cycle can take as few as minutes to produce as many as 300 new viruses

14. Lysogenic cycle & Transmission
Herpes virus remains dormant in body cells
During stress, virus can be activated and go through lytic cycle causing cold sores to form
Virus can then go in dormant stage, therefore person remains infected with the virus
Virus can be transmitted in many ways such as air, physical contact, insect bites. See Table 2 on p. 57

15. Vaccinations and Human Health
Viruses do not respond to treatment by antibiotics or other drugs, but some can be prevented by vaccines
Vaccines are mixtures that contain weakened or dead forms of a virus. B-cells retain memory of the disease so immune system can react quickly when exposed to real virus.

16. More on vaccinations
Vaccinations have dramatically improved human health
Some diseases have been completely eliminated such as small pox
It is not possible to create vaccines for all viruses. For example, there is no vaccine against HIV due to the virus structure and characteristics of the infection

17. Applications of viruses
Viruses are used in genetic engineering:
to treat diseases through gene therapy by inserting gene into individuals suffering from genetic disorder
to insert gene from one species to another species (to create GMOs and for genetic engineering of plants)
as capsules to deliver drugs to target cells in the body such as cancerous tumour cells

18. Viral Vectors
Viruses can be used as vectors (carriers) of genes into cells.
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