Transcript 3. Viruses 2010

Viruses
Relative sizes
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
 Viruses are one of
the smallest
biological structures
known
 Between 20 and 50
nanometers in size.
 The average animal
cell would be about
10,000 times bigger
T4 bacteriophage infecting an E. coli cell
Viruses are cellular terrorists
The Latin root for virus means
“poison”
They take over host cells, but
they are VERY specific in
the types of cells that they
attack
0.5 m
Bacteriophage= a type of
virus that attacks bacteria
Are they alive?
In pairs, look at the set
of pictures and
diagrams related to
viruses and make a
hypothesis to answer
the question:
Are viruses alive? Why
or why not?
H1N1 video
Tobacco Mosaic Virus 
Viral Structure
The head also called a
capsid, is made
mostly of protein and
protects the genetic
material inside
The remaining portion is
called the tail and it
acts in recognition of
the host cell and
attachment
Some viruses are
covered in a fatty
envelope
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
Viral structure
DNA
Tail
fiber
Glycoprotein
18  250 mm
20 nm
(a) Tobacco mosaic virus,
a rod shaped, helical
virus
Glycoprotein
70–90 nm (diameter)
50 nm
(b) Adenoviruses, an
icosahedral virus
with 20 triangular
faces
80–200 nm (diameter)
50 nm
(c) Influenza
viruses, have a viral
envelope
surrounding them
80  225 nm
50 nm
(d) Bacteriophage T4,
attack bacteria
Diversity (of genetic material)
 Viruses can have DNA
(double stranded or
single stranded) or RNA
as their genetic material
 Not all viruses cause
disease.
 What is a disease causing
agent called?
 A pathogen
Influenza
Classes of Animal
Viruses
Specificity
 Viruses have a restricted host range.
 Some animal viruses will have a broad host
range. For instance the rabies virus can
infect many mammalian species
 Some have a narrow host range. For
instance the human cold virus tends to
infect the cells of the upper respiratory
tract. The AIDS virus affects the T cells of the
human immune system
Viral Replication
Purpose of replication = to make more viruses
Two requirements for replication, since viruses
have 2 components. More protein is
needed for the outside and more genetic
material is needed for the inside.
Two possible lifecycles:
 The lytic cycle
 The lysogenic cycle
The Lytic Cycle: Viral Lifecycle
1. Attachment: Virus recognizes host cell and attaches
2. Entry: Genetic material is inserted inside
3. Synthesis: Viral DNA takes over the hosts cellular
machinery for:
o Making genetic material by replication
o Making more capsids by protein synthesis
4. Assembly: Hundreds of new viruses come together
(cell becomes virus factory!)
5. Release: Virus ruptures the host (lysis)
Called a virulent virus E.g. influenza
The lytic cycle of phage T4, a virulent phage
1 Attachment.
The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outside of
the E. coli cell.
5
2 Entry of DNA
DNA injected into
the cell and leaving an empty
capsid outside. The cell’s
DNA may be broken up.
Release.
The phage damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
Phage assembly
4 Assembly.
Proteins form themselves into phage
heads, tails, and tail fibers. The phage
genome is packaged inside the capsid.
Head
Tails
Tail fibers
3 Making components.
The phage DNA
tells the cell how to make
proteins and copies of the
genetic material.
The Lysogenic Cycle: Viral Lifecycle
1. Attachment: Virus recognizes host cell and attaches
2. Entry: Genetic material is inserted inside
Enters lysogeny: Virus integrates into the hosts DNA and
becomes dormant. Now each time the host
divides, each of its daughter cells will have a copy
of the virus. At some point, it re-enters the lytic
cycle…
3. Synthesis: Viral DNA takes over the hosts cellular
machinery
4. Assembly: Hundreds of new viruses come together
5. Release: Virus ruptures the host (lysis)
Called a temperate virus E.g. Herpes
The lytic and lysogenic cycles of phage
, a temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Reproductive strategies
Difference between temperate and virulent
phages?
Which strategy is more successful for
reproduction? In the short term? In the long
term?
Back to the start… Living?
Yes, its alive…
No, its not alive…
• Interacts with its
environment
• No cells
• Reproduces
• No homeostasis
• Adapts or evolves
• No growth or
development
• No metabolism
• No response to stimuli
 Is it a cellular parasite?
Human Health
What makes viruses so challenging?
1. Most are difficult to treat and are not destroyed
by sulfa drugs or antibiotics that have been so
effective at treating bacterial illnesses.
2. Some viruses remain dormant for years before
symptoms appear (due to the lysogenic cycle)
Vaccines
 Are used to prevent some viral diseases.
 Contain weaken or killed virus, or parts of the
virus
eg. Polio, smallpox, hepatitis B
Mode of action: The body reacts to the vaccine
as if it were a real virus and produces
antibodies, and as a result make us immune to
the disease.
Homework
Worksheet on Viruses and Human Health