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Viruses
CHAPTER 19
YOU MUST KNOW
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The components of a virus.
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The differences between lytic and lysogenic cycle.
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How viruses can introduce genetic variation into host
organisms.
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Mechanisms that introduce genetic variation into viral
populations.
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Viruses, viroids, and prions are formidable pathogens in
animals and plants.
A virus consists of a nucleic acid surrounded by a protein coat.
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Smaller than ribosomes, the tiniest viruses are about 20 nm across.
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The two essential components of a virus are a protein shell or capsid that
surrounds the genetic material (either double or single stranded DNA or
double or single stranded RNA, depending on the type of virus).
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Many viruses found in animals have membranous viral envelopes that
surround the capsid and aid the viruses in infecting their hosts.
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The capsid may be rod-shaped, polyhedral, or more complex in shape, built from a large
number of protein subunits called capsomeres.
Viral envelopes are derived from the membranes of the host cell, contain host cell
phospholipids and membrane proteins.
Bacteriophages, or phages, are viruses that infect bacterial cells.
Viruses replicate only in host cells.
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Viruses have a limited host range. This means
they can infect only a very limited variety of
hosts.
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EX. Human cold virus infects only cells of the upper
respiratory tract.
Viral reproduction occurs only in host cells.
Two variations have been studied in
bacteriophages.
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Lytic cycle
Lysogenic cycle
Lytic Cycle
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Ends in the death of the host cell by rupturing it (lysis). In this cycle, a
bacteriophage injects its DNA into a host cell and takes over the host
cell’s machinery to synthesize new copies of the viral DNA as well as
protein coats. These self-assemble, and the bacterial cell is lysed,
releasing multiple copies of the virus (to infect heathy cells).
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Virulent phage: a phage the replicates only by a lytic cycle.
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However bacteria are not defenseless.
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Natural Selection favors bacterial mutants with
receptors that are no longer recognized by a
particular type of phage.
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Restriction enzymes: their activity restricts the
ability of the phage to infect the bacterium.
Lysogenic Cycle
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The bacteriophage’s DNA becomes incorporated into the hosts
cell’s DNA is replicated along with the host cell’s genome. The
viral DNA is known as a prophage. Under certain conditions,
the prophage will enter the lytic cycle.
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Unlike the lytic cycle, the host cell is not destroyed.
Temperate Phages (lambda)
Viral Envelopes
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Animal viruses are equipped with an
envelope (an outer membrane), which is
used to enter the host cell.
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Viral Glycoproteins protrude from the
outer surface that bind to specific receptor
molecules on the surface of a host cell.
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Here is an example of a ssRNA genome
virus.
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The virus RNA genome serves as a
template for mRNA synthesis.
Herpes virus
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Some viruses like Herpes virus, do not have envelopes derived
from plasma membranes. They are temporarily cloaked in
membrane derived from the nuclear envelope of the host,
which is shed once inside and a new envelope made from
membrane of the Golgi apparatus.
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dsDNA genome that replicates inside the host cell nucleus,
using a combination of viral and cellular enzymes to replicate
and transcribe their DNA.
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They can remain dormant until stress or other triggers begin a
new round of active virus production.
http://biology-forums.com/index.php?topic=10481.0
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Retroviruses: are RNA viruses that use the enzyme
reverse transcriptase to transcribe DNA from an RNA
template. The new DNA then permanently integrates
into a chromosome in the nucleus of an animal cell.
The host transcribes the viral DNA into RNA that may
be used to synthesize viral proteins or may be released
from the host cell to infect more cells.
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Ex. HIV is a retrovirus.
http://bcove.me/64avjj9s
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Viruses have the ability to introduce genetic change into
organism as well as to undergo rapid genetic change
themselves. Moving from one host to another, viruses pick up
pieces of the first host’s DNA and carry it to the next cell to be
infected. This is very common in bacteria infected by viruses,
where the process is called transduction.
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RNA viruses lack replication error-checking mechanisms and
this have higher rates of mutation. Mutations may accumulate
rapidly and give rise to diverse clones of the virus within one
organism, as occurs in humans with AIDS, or result in new
genetic strains that may cause disease. This rapid mutation of
viruses explains why there is no vaccine against the common
cold.
Prions: are misfolded, infectious proteins that cause the
misfolding of normal proteins in the brains of various
animal species. Their damage to the brain accumulates
over time and eventually leads to death.
• Examples of diseases caused by prions include mad cow
disease and, in humans, Creutzfeldt-Jakob disease.
• Viroids: are smaller than a virus, consisting of very short,
circular, single stranded naked RNA.
• They only infect plants, causing errors in the regulatory
system. Signs would be stunted or abnormal growth.
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