Transcript Viruses and Bacteria

In ancient Rome, ‘virus’ meant “poison” or “venomous”.
This name was passed down when
these extremely small particles were
discovered in the late 1800’s.
This is the image of a
bacterium being
attacked by viral
particles. You can see
the contrast in size
easily.
Find a lower-case ‘i’ on your
notes page….
• Viruses are extremely small non-cellular, infectious
agents composed of genetic material (DNA or RNA)
enclosed in a protein coat.
• You will see Bacteriophages (bottom left) in many
examples....
Bacteriophage Viruses
Flu Viruses
Viral Structure
Capsid
• A virus has an inner core of
nucleic acid, either RNA or DNA
and an outer protein coat called a
capsid.
Nucleic acid
Capsid
Nucleic
acid
Envelope
• Some larger viruses,
(human flu viruses), have
an additional layer
surrounding their capsids,
called an envelope
(usually made of membrane
remnants of previously
infected cells).
Vertebrate immune systems can detect
certain viral proteins, HOWEVER
the genes for viral proteins tend to mutate
at very high frequencies, so viruses can
often elude immune fighters
For instance Influenza mutates often, that is why a
new vaccine is created each year.
All Living Things, as a species…:
• Are made of cells, and possess a system of organization
• Require, can metabolize, and use energy.
• Grow and develop.
• Reproduce.
• Can adapt to changing environments.
* viruses do not metabolize, grow, nor are they composed of cells.
Also, they don’t possess a way to reproduce without a host.
However, THAT is true of many parasites.
• Viruses are PATHOGENS- infectious, diseasecausing agents that invade target organisms and
multiply inside them or on them.
• Disease follows when the pathogens take over
metabolic activities of other cells- damaging body
tissues and interfering with their normal functioning.
Can a Virus Infect Any Cell?
Nope, not if it can’t get in!
So…how does it get into the host cell?
It works like a lock and key. If the key doesn’t match a lock, you can’t
open the door
The “lock” of the cell is called a receptor site.
If the shape of the virus does not match the host’s receptor
site, the virus can not inject it’s nucleic acid or enter the cell.
Virus
INJECTION
ENDOCYTOSIS
Lytic Cycle:
Faster cycle that quickly destroys the host.
Steps of the Lytic Cycle:
1.
Virus attaches to host
2.
Virus injects nucleic acid
3.
Viral nucleic acid takes over and re-programs host’s DNA
4.
New viruses form
5.
Lysis – cell bursts and releases new viruses
Lysogenic Cycle:
Slow cycle in which the viral DNA
hides in the host cell
• A lysongenic cycle begins in the same way as a lytic cycle.
Virus attaches to a host cell
Viral nucleic acid integrates itself
into the host’s DNA
Viral nucleic acid enters the host
Lysogenic Cycle:
Slow cycle in which the viral DNA
hides in the host cell
At any time, this provirus can enter
the Lytic cycle….
This part can go on for years….
The host cell will divide, and each time it
replicates it’s DNA, it also makes a copy of
the viral nucleic acid.
RNA Viruses