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Viruses
Warts are a skin virus!
Herpes mouth virus:
Other Viral Diseases
•
•
•
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Measles
Polio
Smallpox
Influenza
Viruses & Cancer
• Hepatitis B Virus
• Human Papilloma
Virus
• HPV
Tree Man - HPV
Is a Virus a Living Organism?
Properties of life
– Cellular Respiration
– Reproduction
– Metabolism
– Homeostasis
– Heredity
– Responsiveness
– Growth and development
Are viruses living or non-living?
Nonliving
• Not cells
• No respiration
• No growth
• No development
• No metabolism
• Can be
crystalline,
dormant
Living
• Replicate using a
host
• Contain DNA,RNA
• Contain protein
coat
Virus Characteristics
• Non-living:
– Do not grow
– Do not develop
– Do not carry out
respiration
• All they can do is
replicate but they
can’t to that without a
host cell
Viruses are very small, even
compared to bacteria, and they
can only be viewed with the
electron microscope. The
smallest viruses are about 20nm
and the largest about 300nm.
Virus Structure
• Virus means Poison
in Latin
• Composed of nucleic
acids (DNA or RNA)
wrapped in a protein
coat called a capsid
• Some viruses have
an extra layer of
protection called an
envelope
Viral Structure
Nucleic Acid – RNA or DNA
Capsid – protein coat that surrounds the DNA
or RNA in a virus
Lipid Membrane – a membrane around the
capsid in many kinds of viruses; helps the
virus enter cells (“enveloped” viruses;
without the membrane, the virus is “naked”)
– Made of proteins, lipids, and glycoproteins
Origin of Viruses
• Scientists suggest viruses might have
originated from their host cells
• May be nucleic acids that broke away
from their host but maintain the ability
to replicate parasitically within the host
cell
Viruses are host cell specific.
Most viruses are restricted to certain kinds
of cells (those that infect plants cannot
infect animal cells).
Why?
Scientists think that viruses originated from
escaped genetic material from host cells.
Plant peach virus
Viruses can
infect
• Plants
• Animals
• Bacteria
Differences in Viruses
Species Specific Viruses:
• Ex. Smallpox
(eradicated)
• Non-species specific:
Flu & West Nile
Cell-type Specific:
• Ex. Polio – only
infects intestinal &
nerve cells
Naming Viruses
• Viruses are named
after the diseases
they cause or the
tissues or organs they
infect
• Ex. Bacteriophage –
Infects a bacterium
Virus Categories
• DNA viruses – stable, do not mutate rapidly
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Single-stranded or double-stranded
Smallpox, Hepatitis B
Warts
Chickenpox
mononucleosis
• RNA viruses – mutate rapidly, unstable
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Single-stranded or double-stranded
Rhinovirus
Human immunodeficiency virus (HIV)
Influenza viruses
Rabies
Virus Shapes
Helical
Rodlike with capsid proteins winding around the
core in a spiral
Tobacco Mosaic Virus
Virus Shapes
Polyhedral
Has many sides
Most polyhedral capsids have 20 sides
and 12 corners
Virus Shapes
Polyhedral capsid attached to a helical tail.
Chicken Pox
& Polio Virus
Rabies virus
Hepatitis B
(Liver)
How do viruses replicate?
2 methods of replication:
1. Lytic Cycle – the virus enters the cell,
replicates itself hundreds of times, and
then bursts out of the cell, destroying it.
2. Lysogenic Cycle – the virus DNA
integrates with the host DNA and the
host’s cell helps create more virus
DNA when it divides. An environmental
change may cause the virus to enter the
Lytic Cycle.
Attachment
• Attachment proteins
on virus recognize &
attach to the receptor
site on the plasma
membrane of the host
cell
• Puzzle pieces
• http://www.slic2.wsu.e
du:82/hurlbert/micro1
01/images/101VirusF
usionAn.gif
Viruses Enter Living Cells
Viruses enter bacterial cells by punching a
hole in the cells wall and injecting its DNA
Viruses Enter Living Cells
Viruses enter plant cells through tiny rips in
the cell wall.
Viruses enter animal cells by endocytosis.
Virus Cycles
• Once a virus enters a cell
it either enters into the
Lytic or Lyosogenic Cycle
• Lytic Cycle – takes over
host cell immediately &
kills cells by (bursting or
lysis)
• http://whyfiles.org/132aid
s2/images/virus_movie.gif
Viral Replication: Lytic Cycle
Lysogenic Cycle
• Viral DNA becomes
incorporated into host
cells DNA & becomes
a Provirus
• Provirus is replicated
with host cells
chromosome
• http://goldiesroom.org
/video_archive.htm
• At any time a provirus
can be activated &
enter into a lytic cycle
– Physical Stress
– Sunburn
– Emotional Stressanxiety
– Immuno-depressed
Provirus Examples
• Cold sores (Herpes
simplex I)
• Genital Herpes
(Herpes simplex II)
• Hepatitis B
• Chicken Pox
(Varicella zoster)
• Shingles – painful
infection of some
nerve cells
Also important…
Any agent (not just viruses) that causes
disease is a pathogen.
When a virus inserts its genetic material into
a host’s DNA, it is called a provirus.
Some viruses replicate very slowly and only
cause damage when the conditions are
“right”. (cold sores)
Mutating viruses
Viruses can mutate when they copy the
genetic material
– Copy something wrong
– Mistake proves useful
– More powerful virus (more infectious)
Viruses don’t mutate often, except…
– Influenza
– HIV
Retroviruses
• Have RNA as their
nucleic acid – most
complex replication
cycle
• Ex. HIV
• Make DNA from RNA
using the enzyme
reverse transcriptase
HIV
• Double-stranded
DNA enters into
host cells
chromosome &
becomes a Provirus
• Can test for the
enzyme reverse
transcriptase to see
if you have the virus
The spread of West Nile virus (1999 –
2002) – bird, horse, mosquito or human
Viruses can be beneficial…
Bacteriophages – attack & destroy bacteria
Baculovirus – ebola-like virus that attacks
insects
– Could use for pest control in crops
• Cabbage loopers eat
cabbage crops
• Virus can kill pests in days
– (it’s really gross)
… and then there are those that are not so good….
Viroids
• Infectious particles of naked RNA.
• Similar to viruses, but lack a capsid.
• Ex:
– Coconut Blight
– Chrysanthemum Wilt
Viroids
• Composed of a single
circular strand of RNA
with no protein coat
• Diseases in Plants
• Enter through wounds
or insect bites
• Don’t use surface
recognition
• Doesn’t undergo lytic
or lysogenic phases
• May cause stunted
growth & yield losses
• Mosaic viruses are
not harmful cause
beautiful patterns
What is a prion?
• Prions have recently been implicated in diseases called
Encephalopathies which affect the brain. The most
common ones that are know today are Bovine
Spongiform Encephalopathy or BSE in cattle, Scrapie in
sheep and new variant Creutzfeldt-Jakob disease in
humans.
• Prions are infectious agents but they are not like bacteria
or viruses. Prions are pieces of protein that can transfer
the disease from one organism to another.
• IT is simply a protein where the chains have folded
incorrectly. (secondary structure) The chains have the
same amino acid sequence as the normal protein. It is
not known how the malfunction in protein folding occurs
Prion Action
Prions
• Proteins but no
nucleic acids
• Cause proteins to fold
incorrectly
• Animal diseases:
– Mad Cow Disease
– Creutzfeldt-Jacob
disease
Treatments for Viruses
• Vaccines (preventative).
• Some Drugs
– Ex: Ara-A
Acyclovin
• Comment - some treatments are working
on the reverse transcriptase.
Cow pox vaccination 1749
• Acquired
Immunization
• Artificial
injection of a
small amount
of virus
• Body’s immune
response
makes
antibodies
Human Immunodeficiency Virus
(HIV)
Acquired Immuno Deficiency
Syndrome (AIDS)
Basic Structure
• Viral envelope – lipid bilayer; glycoproteins
protrude from surface
– Glycoproteins enable virus to recognize
surface proteins of special immune cells and
to enter the cell (like a key to the cell’s door)
• 2 strands RNA – only 9 genes; 3 are found
in many viruses (structural proteins)
• Reverse Transcriptase – turns RNA into
DNA (this makes HIV a retrovirus); DNA
instructs cell to make more viruses
HIV Virus
HIV virus infects T-cells
• HIV virus
Weakens the
immune
system
• AIDS patients
die of
“common”
diseases
when T cell
(WBC) count
falls
AIDS = break down of the immune
system & death due to common
diseases versus death by AIDS virus
B Memory cells……..
Activation of the Immune Response
AIDS Virus on a T Cell
Activation of the Immune Response
AIDS virus destroys
Helper T cells !
Loss of T Cells makes the patient
susceptible to common infections
Prevent
attachment
Prevent Replication
Prevent
Coat
formation
How Is HIV Spread?
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Sexual contact
Sharing contaminated needles
Blood transfusions
Breast feeding (mother to baby)
Mother to baby during pregnancy or birth
Think about it…
• In the US, there is better than a 1/1000 chance of
contracting HIV during unprotected sex
• A person can be contagious for more than 10 years before
any sign of the disease is apparent
• HIV becomes AIDS when the number of immune cells drops
below a predetermined number
• No one dies from HIV or AIDS; people die from secondary
infections (ranging from the common cold to cancer)
• More than 3 million people (size of Chicago) die each year
• There are approx. 14,000 new cases of HIV
worldwide every day
VIRUSES
Review
Viruses are not usually included in
classification systems as they are non-cellular
and they are dependent on a host cell for their
replication and metabolic processes.
What is a
virus?
• Viruses have very
simple structures -
• Most of them are simply
a protein coat
surrounding a core of
nucleic acids.
• Could be DNA or RNA
LIVING or NOT LIVING?
• Viruses are not capable of living as free
entities; in fact they are generally inert and
inactive outside of their host cells, which
are needed only for the virus to replicate.
• Viruses can infect all types of cells
animals, plants and bacteria, but each
different type of virus is specific to its
particular type of host cell.
Bacterial Viruses
• Best understood of all viruses.
• May show two types of virus life cycles:
– Lytic Cycle
– Lysogenic Cycle
VIRUS
REPLICATION
Once inside the host cell the
virus hijacks the cells genetic
machinery and uses it to make
more viruses.
The lytic cycle – short,
replicates quickly, destroys
host cell when releasing
thousands of new viruses.
Cold, flu, Ebola.
The lysogenic cycle
– virus enters cell,
replicates DNA,
and WAITS.
Herpes, wart, HIV
Your immune system responds
to the infection, and in the
process of fighting, it produces
chemicals called pyrogens that
cause your body temperature to
increase.
Lytic Mode Triggers
• Switches the host from Lysogenic to Lytic
Cycles
• ex: radiation, chemicals, stress
Herpes Viruses
• Use nuclear membrane.
• Viral DNA integrated into Host DNA as
a provirus.
• Shows both lytic and lysogenic life
cycles.
RNA Viruses
• Classes III - VI
• Class VI - Retrovirus use Reverse
Transcriptase to make DNA from an RNA
template.
Human Immunodeficiency Virus
• HIV - causes AIDS.
• Retrovirus from chimps.
• Destroys the body’s immune system,
allowing other diseases to kill.
Plant Viruses
• Can be an important agricultural
problem.
• Ex: Tobacco Mosaic Virus
Tulip Flower Breakage