Transcript Viruses and Bacteria

Viruses
Viruses
• no cellular structure, no
cytoplasm, organelles or
cell membranes
• no metabolism on their
own, cannot grow or
respire
• Therefore are not
classified as living
things
Viruses continued
• Viruses can reproduce, a
basic characteristic of
life
• They can take over
control of a host cell
• Can direct cell to make
new virus particles
• Mobile genes that
attack cells
Parts of a virus
Most viruses consist of 4 parts
1) Core - located in the center of the virus and contains
the virus’ DNA or RNA wrapped together with
some proteins
2) Capsid - made of protein and forms a shell around the core,
protects DNA from enzymes of host cell
3) Matrix - forms a layer between the capsid and the envelope
4) Envelope - consists of lipids stolen from the cell membrane
of the host cell that the virus is growing in
Note: some viruses do not have an envelope and others do not have
a capsid
Classifying Viruses
Often viruses are named for:
•
Type of disease
caused (poliovirus)
•
Those who discovered
them (Epstein-barr
virus)
•
Site of replication
(rhinovirus or tobacco
mosaic virus)
•
Where they were
isolated (sendai-virus)
Classification is now based on
type of nucleic acid, shape,
size etc.
Shape and Size
--> determined by the type and
arrangement of proteins in the capsid
• Cylindrical
• Polyhedral
• Spherical
Site of Replication
• Specific organism and cell
• Wide variety of organisms
e.g. Tobacco mosaic virus
Type of disease they cause
• 21 different groups of viruses infecting humans
differ in set of genes (genome) and method of
replication
e.g. influenza virus
Type of nucleic acid
DNA viruses
– Less virulent
– follow the lytic cyle
– Eg. Warts, shingles,
chicken pox
RNA viruses
– Virulent
– Follow lysogenic cycle
– AIDS, rubella, rabies,
measles
How big are viruses?
• Video
Viral Replication
• It depends on the metabolism of a prokaryotic or eukaryotic cell
to replicate its DNA or RNA and to make protein coats foe each
newly formed virus particle
• Attaches to specific receptors on the plasma membrane of the
host cell and can only enter particular cells with specific
receptor sites
• Enter host in one of two ways
a) the virus injects its nucleic acid into host cell (lytic cycle)
b) membrane of host engulfs virus creating a vacuole inside host
• Host metabolism replicates the viral DNA or RNA and protein
coat and new virus particles are assembled
•
New particles are released from the host cell in one of two
ways:
a)
The host cell breaks open (Lysis) and releases the new viruses
which then infect neighbouring cells; Host cell is destroyed in
process
a)
The host cell releases new viruses without being destroyed
Bacteriophage
• Virus that infects a
bacteria
• Video
• Same video
different site
T4 Bacteriophage
• Specific to E.coli
• Head  capsid containing DNA
• Sheath  support and pump to
move DNA
• Base plate  cut or bite a hole in
cell
• Tail fibres  find site for
attachment
• Core  pipeline to conduct DNA
from head to cytoplasm of
cell
• Collar attach tail to head
Viral Replication:
Lytic and Lysogenic Cycles
Video
Viroids
• Very small infectious pieces of RNA
responsible for some serious plant
diseases
• Differ from viruses as RNA does not
code for proteins
• May interfere with the normal formation and functioning of RNA
in host cell
Eg. Viroid outbreak killed more than
10 million coconut plants in
Philippines
Prions
• Abnormally shaped infectious protein responsible for some brain
diseases in mammals, including humans
• When tissues are eaten by another animal, prions enter blood
stream and go to its brain
• Prions interact with normal proteins causing them to become
abnormal and infectious
• E.g. Bovine spongiform encephalopathy (BSE)
(Mad Cow Disease)
Humans - Creutzfeldt-Jakob Disease
(CJD)