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Viral Structure and Function - Coggle Diagram
Viral Structure and Function
Defining Characteristics/History
Viruses are classified as
obligatory intracellular parasite
and are small enough to pass through
0.2u filters.
A virus contains a nucleic acid, which can be either
RNA or DNA
, and a
protein coat
. Some viruses also possess an
envelope
.
Unlike bacteria, viruses do not have a plasma membrane, do not reproduce by
binary fission
, and cannot generate their own
ATP
. Viruses are also sensitive to
antibiotics
.
In 1886, Adolf Mayer demonstrated that
Tobacco Mosaic Disease
could be transmitted from an affected plant to a healthy plant.
In 1892,
Dimitri Iwanoski
filtered the sap of infected plants and found the fluid was still able to transmit the disease, leading to the term
filterable agent.
The first human disease associated with a filterable agent was
yellow fever,
which was also the first disease found to be caused by a virus.
In 1901,
Walter Reed showed that Yellow Fever
was the first disease demonstrated to be transmitted by a
mosquito
(a Vector).
Viral Structure & Genetics
A
virion
is defined as a complete, fully developed, infectious viral particle composed of
nucleic acid
and a
protein coat.
The
protein coat
surrounding the nucleic acid core is the capsid, which is composed of subunits called capsomeres.
The
envelope
may be present around the outside of the capsid. It consists of part of the host cell membrane along with
proteins coded for by the virus.
Nonenveloped viruses
consist only of the nucleic acid and capsid.
Spikes
are carbohydrate and protein complexes that project from the envelope surface. In the Influenza virus, spikes are used to clump RBCs together, a process called
hemagglutination
.
Four main types of viral genetic material:
ssDNA (few):
Parvoviridae
(e.g., Parvovirus)
dsDNA (many):
Herpesviridae
(e.g., Simplexvirus)
+ssRNA (many):
Picornaviridae
(e.g., Polio, Rhinovirus)
dsRNA (few):
Reoviridae
(e.g., Rotavirus)
Viral Morphology & Host Range
Most viruses are
species specific
, but a few viruses have a broad host range.
Bacteriophages (phages) infect only
bacteria
. Their attachment sites include part of the
cell wall, fimbriae, or flagella.
Receptor sites for animal viruses are proteins and glycoproteins of the
plasma membrane
of the host cells.
Viral Morphology types include:
Polyhedral Virus
: Often an
Icosahedron
(20 equilateral triangles), such as
poliovirus or Adenovirus
.
Helical Virus
: Ex. include
filovirus
(Ebola) and lyssavirus (Rabies).
Icosahedral
: Examples include
Herpesviridae
(causing cold sores and chicken pox/shingles).
Complex Viruses
: Characterized by being dsDNA, enveloped, and very large, such as the
T-bacteriophage.
Viral Replication
In the multiplication of
Animal Viruses
, entry occurs by
pinocytosis
or fusion (for enveloped viruses). Uncoating involves the digestion of the protein coat, often by
lysosomal
enzymes.
For most animal DNA viruses, DNA replication occurs in the
nucleus
of the host cell. The exception is
POX viruses,
where components are synthesized in the cytoplasm.
The
lytic cycle
(e.g., T4 Phage, a virulent phage) ends in the
death
of the host cell.
The
lysogenic cycle
(e.g., Lambda phage, a temperate phage) involves the insertion of phage DNA into the host genome, forming a
prophage
. In this cycle, the phage remains
latent
, and the host cell remains
lysogenic cells.
A significant result of lysogeny is
phage conversion
, where the host cell may exhibit new properties, such as Corynebacterium diphtheria producing the disease only when a
prophage
is present that codes for the cytotoxin.
Diseases & Special Viral Mechanisms
Yellow Jack Fever
is caused by a Flavivirus (ss RNA, envelope). It is a
Zoonotic Disease.
Sylvatic cycle
involves mosquitoes transmitting the disease from monkeys to monkeys and from monkeys to humans.
Urban Cycle
involves
Aedes aegypti
mosquitoes transmitting the disease from human to human.
Prevention includes a vaccine (Attenuated yellow fever
17D strain
) and
Dakar Stain
.
Influenza
(Family: Orthomyxoviridae) has a genome composed of 8 separate RNA segments.
The
H spikes
allow the virus to attach to specific host cell receptors.
The
N spikes
help the virus separate from infected cells after replication.
Influenza is spread by
aerosol transmission.
Treatment (TMT) includes:
Prevent uncoating- reduce symptoms of A-type influenza
Neuraminidase inhibitors - decrease the length of the disease by 3 days
Antigenic Shift
is caused by major genetic recombination. Ex. if swine are infected with both avian and human influenza, leading to a mixing of the RNA segments. This results in a major change in the H and/or N portions and can cause:
Antibodies formed against one strain will usually not be protective following an antigenic shift
Pandemics occurs when the virus acquire a new hemagglutinin and/or neuraminidase spike
Antigenic Drift
reflects minor annual variations in the antigenic makeup of the influenza virus. This often involves an alteration in a
single a.a
of the H or N spike. Vaccines are slightly different each year to accommodate antigenic drift/shift.
Latent Viral Infections
are often activated by
stress
and may not recur for months to years. Examples include cold sores and shingles, caused by Human Herpes viruses.
Retrovirus
: RNA Virus (like HIV) carry the enzyme
reverse transcriptase
which uses viral RNA as a template to produce complementary double-stranded
DNA
. This viral DNA can be inserted into a host cell chromosome as a provirus. The
provirus
can remain in a latent stage and replicate with the host DNA.