Download Viruses

Viruses
Are they alive?
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
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
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!
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’)
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)
Infection Cyles
Two processes:
Lytic infection cycle
Fast
Lysogenic cycle
Slower (can be years)
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
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
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
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
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