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Viruses
16.5
Why are viruses considered
non-living?
• Do they have organelles?
• Do they carry out life processes?
– Grow, take in food, make waste?
– How do they reproduce?
• Are they cells?
Viruses and bacteria can cause
infection.
• Any disease-causing agent is
called a pathogen.
• Let’s talk size-
1 nanometer (nm) = one
billionth of a meter
– viruses are 100x smaller than bacterial cells.
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm
Virus classification
• Type of nucleic acid
– DNA or RNA but not both
• Their shape
– Based on the structure of the capsid- outer protein coat.
– Two basic shapes• Rod
• Spherical
• How they reproduce
– Lytic ( lysis) cycle
– Lysogenic ( latent) cycle
• What they infect– Organisms/species
– Type of cells
Viruses differ in shape and
how they enter host cells.
• Viruses have a simple structure.
– genetic material- DNA or RNA
– Capsid- outer protein shell
– maybe a lipid envelope, a protective outer coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
Basic Viral Structure- T-phage
Bacteriophage- virus that
infects bacteria
capsid
DNA
tail sheath
tail fiber
Viruses enter cells in various ways:
bacteriophages pierce host cellsinjecting their DNA
colored SEM; magnifications:
large photo 25,000; inset 38,000x
Viruses of eukaryotes -can fuse with
cell membranes- endocytosis
Viruses cause two types of
infections.
• A lytic infection causes the host cell
host bacterium
to burst.
The bacterophage attaches
and injects it DNA into a host
bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The viral DNA
forms a circle.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
A lysogenic infection does no
immediate harm- latent infectionexamples: herpes, HIV
At some point-the prophage
leaves the host’s DNA and
enter the lytic cycle.
The viral DNA ( a prophage)
combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
Viruses such as a bacteriophage are
capable of reproducing in two general
ways, the lytic and lysogenic cycles.
.
Viruses cause many
infectious diseases
• There are many examples of viral
infections.
– common cold
Viruses cause many
infectious diseases
• There are many examples of viral
infections.
– common cold
– influenza
Viruses cause many
infectious diseases
• There are many examples of viral
infections.
– common cold
––Sars
– influenza
Viruses cause many
infectious diseases
• There are many examples of viral
infections.
– HIV
• The body
has
natural
defenses
against
viruses.
HIV-infected
white
blood cell
HIV-AIDS
• HIV =virus
– Human Immunodeficiency Virus
• AIDS= disease– Acquired Immune Deficiency Syndrome
• An immune disease- immune cells attacked- T4 white blood
cells.
• Symptoms (damage of host immune cells) occurs when a
switch from the lysogenic cycle to the lytic cycle occurs.
– 5-10 years later.
• HIV- retrovirus-RNA virus
– Flow: RNA-DNA-RNA
– How: they have reverse transcriptase
• An enzymes that synthesizes DNA from RNA
HIV
HIV, a retrovirus, uses immune system cells to
reproduce itself. These host cells are eventually
destroyed, weakening the patient's immune system.
•
PreventionAntibiotics have no effect
• Vaccines can be effectiveDeactivated varieties or small pieces of
pathogens that stimulate the immune
system to respond by producing a
memory response when the actual
pathogen is met.
• Edward Jenner developed the first vaccine
(cowpox) against smallpox.
• Not all viruses can be prevented with a
vaccine because of the rapid mutation
rates of these viruses.
Edward Jenner-
Development of smallpox virus scheme.
It worked because smallpox and cowpox are
very
• similar –same antigens( cause an
antibody response from the body.
1979- smallpox was eradicated
Vaccines are made from weakened
(attenuated) pathogens.
• A vaccine stimulates the body’s own immune
response.
• Vaccines prepare the immune system for a future
attack.- memory response
• Vaccines are the only way to control the
spread of viral disease.
• What is herd mentality?
Antivirals?
• What are they?