Transcript Viruses

Viruses
• Discovered in 1898 as
the cause of hoof-andmouth disease in cattle.
The disease-causing
organism was much
smaller than bacteria—it
could pass through filters
that stopped all bacteria.
• Viruses that attack
bacteria are called
“bacteriophage”, or just
“phage”. Phage means
“eater”.
Virus Structure
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Viruses consist of nucleic acids (DNA or
RNA) wrapped in a protein coat. Sometimes
there is also a surrounding membrane. The
membrane is covered with protein spikes.
The spikes are used to attach the virus to
the cells it is invading.
Viruses have no internal metabolism: no
ATP, no enzymes transforming chemical
compounds to new forms. Viruses
reproduce by invading living cells and taking
over their metabolic processes.
Are viruses living or non-living? The answer
to this question is closely related to the
observation that there are 2 kinds of people
in this world: those who divide the world into
2 categories and those who don’t. Viruses
are both and neither: they have some
properties of life but not others. For
example, viruses can be killed, even
crystallized like table salt, but they can’t
maintain a constant internal state
(homeostasis).
Most scientists consider viruses to be
parasites that evolved from cells: essentially
parasitic cells that degenerated.
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However, there is some thought that perhaps
very simple viruses or just RNA molecules
evolved before the appearance of
metabolism.
Virus Replication
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Viruses are very specific as to which
species they attack: humans rarely
share viral diseases with other naimls,
for instance.
The general virus life cycle involves 5
steps:
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1. attachment to the cell
2. penetration by either the whole virus
or just its DNA or RNA
3. replication of new viral proteins and
nucleic acids
4. assembly of the new viruses
5. release of the new viruses into the
environment
Bacteriophage inject their nucleic acid,
and they lyse (break open) the
bacterial cell when replication is
finished.
Virus Replication In Eukaryotes
• Eukaryotic viruses
usually have
membranes. The
membranes are
derived from the cell
membrane. They
coat the virus as it
buds out of the cell.
The cell remains alive
and permanently
infected.
Viral Latency
• Some viruses have the ability
to become dormant inside the
cell. They are inactive for long
periods of time, and then
activate to produce new
viruses in response to some
external signal.
• In bacteriophage, the phage
DNA incorporates itself into the
bacterial chromosome. When
an activation signal occurs, the
phage DNA comes out of the
chromosome and starts
replicating.
Latency in Eukaryotes
• Some eukaryotic viruses move
into nervous system tissues
and remain dormant for many
years. Chickenpox (caused by
the virus Varicella zoster) is a
childhood infection that can
reappear later in life as
shingles, a painful itching rash
limited to small areas of the
body.
• Herpes viruses also become
latent in the nervous system. A
herpes infection lasts for a
person’s lifetime.
Retroviruses
• Retroviruses contain RNA as
their genetic material, but
convert it into DNA inside the
cell. This process is carried
out by the enzyme “reverse
transcriptase”. The retroviral
DNA then inserts itself into one
of the cell’s chromosomes,
where it makes more copies of
the retroviral RNA.
• HIV, the AIDS virus, is a
retrovirus. So is Feline
Leukemia Virus
Viroids
• Some plants are
infected by small
circular RNA
molecules that have
no protein coat.
These are called
viroids.
• Viroids resemble the
introns that are
spliced out of most
eukaryotic genes.
Prions
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Prions are “infectious proteins”.
They are normal body proteins
that get converted into an
alternate configuration by contact
with other prion proteins, They
have no DNA or RNA.
The main protein involved in
human and mammalian prion
diseases is called “PrP”. Similar
proteins have been found in other
organisms, including some with
useful, positive effects.
(For the literary types: prions work
approximately like “ice-9” did in
the novel Cat’s Cradle, by Kurt
Vonnegut.)
Prion Diseases
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When normal proteins are converted to the
prion form, they form insoluble deposits in
the brain. This gives rise to rapid neural
degeneration, similar to a fast acting form of
Alzheimer’s disease.
Mad cow disease (bovine spongiform
encephalitis: BSE). Apparently started as
scrapie, a disease of sheep. In England,
cattle were fed ground up sheep carcasses
and developed BSE. Later, humans eating
the infected cows developed a form of
Creutzfeld-Jacob Syndrome, a human prion
disease. After this, virtually all cattle in
England were slaughtered to prevent spread
of the disease.
Deer and elk in the US (including northern
Illinois and Wisconsin) get “chronic wasting
disease”, a prion disease. So far, no cases
of transfer to humans.
People in New Guinea used to suffer from
kuru, which they got from eating the brains
of their enemies. Once this practice ended,
kuru disappeared.