Transcript Viruses

VIRUSES: The Ultimate Parasite
Virus is
Latin for
“poison”
Are Viruses Alive?
• Are NOT cells!
• They are “lifeless” particles
with no metabolic functions
of their own
• They cannot reproduce on
their own and need to
“infect” other cells
• They do not display the
essential characteristics of
life
• They are considered to be
between nonliving and
living matter
They form parasitic relationships with living organisms’ cells:
the virus benefits at the expense of the living organism
Viruses are tiny…even much smaller then bacteria
We don’t measure them in µm (microns),
but nm (nanometres), which is a billionth of a metre.
Virus Morphology (Form or structure)
Viruses are far less complex then the simplest living organism.
There basically a piece of DNA or RNA surrounded
by a protein coat (or capsid).
The protein coat is sometimes surrounded by an
envelope containing “spikes”
The virus’s spikes enable the virus to attach to the surface of a host cell.
Naked
Vs.
Enveloped Viruses
Viruses get their envelopes from the host’s cells when they infect them.
Bacteriophages (Phages)
Tail
A type of virus that specifically infects bacteria cells.
Phages have a unique “tail” structure they use to
help them attach to host cells.
Viral Replication
The Lytic Cycle:
1st step:
Virus spike attaches
to cell receptor
2nd step:
Virus inserts it’s own
genetic material
(DNA or RNA) into
the host cell
Virus Spike
Host Cell Receptor
Viral Replication
3rd step:
The host cell is tricked into replicating the viral DNA/RNA
and producing the other parts of its structure.
4th step:
The viruses components are assembled and packaged into full virus particles.
Viral Replication
5th Step: Lysis – the destruction or bursting open of a cell
Hundreds or even thousands of the new virus particles
are released from the infected host cell, killing it in the
process
The lytic cycle can happen as quickly as 25 to 45 minutes!
Lysogenic Cycle
• Bacteriophages can also
insert their own genetic
material into DNA of
the bacteria they are
infecting
• The bacteria cells
replicate without
initially being harmed,
as the virus is in a state
of dormancy called
lysogeny
Lysogeny
• Infected bacteria can
remain like this until an
environmental factor
triggers the lytic cycle,
once again producing new
virus particles
• Example: Herpes Virus
• Once you get herpes, it
remains dormant in your
body for life
• Environmental factors
cause the virus to become
active and cold sores to
appear
Host Range
All viruses need to infect cells to replicate,
but they are limited by which cells they can successfully invade.
The number of different cells, tissues or species that a virus can infect is called
it’s “host range”.
Viruses can have broad host range (are able to infect many)
or narrow host ranges (infect very few)
Example:
Rabies Virus can infect many mammals such as rodents, dogs, and humans.
Swine Flu can infect pigs and humans.
The Human cold virus (rhinovirus) only infects the cells of the upper respiratory tract.
HIV (Human immunodeficiency virus) infects the human immune system.