1. Viruses
Are they alive?

2. The word ‘virus’ comes from Latin meaning poison
A virus is so tiny it cannot be seen with a light microscope, which makes it much smaller than a bacterium

3. Are they Alive?
Viruses are not considered living by many biologists. Why not? Well….
Living things cannot crystallize, but viruses do
Living things can reproduce on their own (asexual reproduction) or with members of their own species (sexual reproduction) Viruses cannot do either
Living things grow and develop, viruses do not
Living things obtain and use energy, viruses don’t
Living things respond to their environment, viruses don’t

4. Structure Very simple Have a core of genetic material (DNA or RNA)
Core is surrounded by a protein coat called a capsid
Many different shapes – See page 479 – sketch a few on the back of your notes!

5. Tricking their way into a host cell
Proteins bind to surface of host cell
Host cell is ‘tricked’ into letting virus in
Once virus is in, it takes over, kind of like a Trojan horse
Viruses are very specific to host
Plant viruses infect plant cells (i.e. tomato viruses only infect tomatoes)
Animal viruses infect animal cells (i.e. pig viruses only infect pig species)
Bacterial viruses infect bacterial cells (these viruses are called bacteriophages,or ‘phages’)

6. Changing Hosts
Sometimes a virus suddenly starts to infect a species that is not it’s normal host
This is bad for the new species as it has no defense against the virus
The mechanism for this is being researched
Examples:
HIV (crossed from chimps to humans)
Swine flu (crossed from pigs to humans)
Avian flu (crossed from poultry to humans)

7. Infection Cyles Two processes: Lytic infection cycle Lysogenic cycle
Fast
Lysogenic cycle
Slower (can be years)

8. Lytic Infection Cycle Steps
Virus binds to host cell
DNA is injected into host cell, remaining separate from host DNA
Virus takes over host cell, causing new viral parts to be made
New viruses assemble
Viruses leave host cell
Phages lyse (burst) bacterium cell wall and host dies
Other viruses do not necessarily cause lysis

9. Lysogenic Infection Cycle Steps
Virus binds to host cell
DNA is injected into host cell
DNA inserts itself into host DNA – called a prophage
Every time host divides, more viral DNA is made
Can stay in the lysogenic (prophage) stage for a long time
Virus is inactive
Certain factors can activate DNA of prophage (ultraviolet light, chemicals,…)
Viral DNA separates from host
Enters Lytic cycle, taking over host cell

10. Retroviruses Nasty little particles Contain RNA
When retroviruses infect a cell, they make a DNA copy of their RNA
called retroviruses, because they copy their genetic info backwards
usually it’s DNA to RNA
Can remain dormant for years
Eventually become active, direct production of new viruses and cause death of host cell

11. HIV HIV = Human Immunodeficiency Virus
This is a retrovirus
It attacks the cells of the immune system
The immune system can no longer work properly, allowing many other diseases to flourish
HIV causes the disease called AIDS
AIDS = Acquired Immunodeficiency Syndrome
This is the disease, not the virus
To sum it up: HIV is the name of the virus, AIDS is the name of the disease

12. Treating Viral Infections
Very few drugs available (but there are some)
Antibiotics don’t work (they’re for bacterial infections)
Can usually just treat the symptoms
Prevention is best ex:
Vaccinations, hand washing