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Virus Structure & Function - Coggle Diagram
Virus Structure & Function
Characteristics & Structure
viruses are classified as
intracellular obligatory parasites
and are small enough to pass through 0.2μ filters
some viruses have lipid envelope
viruses contain a
nucleic acid
of DNA or RNA & a protein coat
viruses do not have a plasma membrane, don't reproduce by binary fission, & can't generate their own ATP
viruses are sensitive to
interferons
(tells other cells of the virus); bacteria are sensitive to antibiotics
virion
- a complete, fully developed, infectious viral particle composed of nucleic acid & protein coat
the protein coat surrounding the nucleic acid core is the
capsid
which consists 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
non-lipid envelope
viruses consist of only the nucleic acid & capsid
spikes
- carbohydrate & protein complexes that project from the envelope surface
most viruses only infect one specific type of cell
most viruses are species specific, but a few viruses have a broad host range
Bacteriophages (phages)
onfect only bacteria; they attach on part of the cell wall, fimbriae, or flagella
receptor sites for animal viruses are proteins and glycoproteins of the plasma membranes of the host cell
History
1886-
Adolf Mayer
demonstrated that
Tobacco Mosaic Disease (TMD)
could be transmitted from an infected plant to a healthy one
1892-
Dimitri Iwanoski
filtered the sap of infected plants & found the fluid was still able to transmit the disease, leading to the term
filterable agent
1903-
Walter Reed
showed that Yellow Fever was first disease transmitted by a mosquito (vector)
First human disease associated with a filterable agent was
Yellow Fever
with was also first disease to be caused by a virus
1930s-
Wendell Stanley
isolated the
Tobacco Mosaic Virus (TMV)
& electron microscope was invented; TMV causes TMD
Main Viral Genetic Material Types
ssDNA (few):
Parvoviridae
(Parvovirus)
dsDNA (many):
Herpesviridae
(Simplexvirus- HHV1 and 2)
+ssRNA (many):
Picornaviridae
(Polio, Rhinovirus, Enterovirus)
dsRNA (few):
Reoviridae
(Rotavirus- gasteroenteritis)
ssRNA (many):
Filoviridae
(Filovirus- Ebola)
Viral Morphology
Polyhedral virus
: often an
icosahedron
(20 equilateral triangles), such as Poliovirus (+ssRNA) or Adenovirus (dsDNA, spikes)
Helical Virus
: includes Filovirus- Ebola (+ssRNA) and Lyssavirus- Rabies (ssRNA)
Enveloped Icosahedral Virus
: includes Herpesviridiae (dsDNA with envelope) causing cold sores & chicken pox/shingles
Complex Virus
: characterized by dsDNA, enveloped, & very large, such as the T-bacteriophage
Animal Viral Reproduction
entry occurs by
pinocytosis
or fusion (for enveloped viruses)
uncoating
involves the digestion of the protein coat, often by
lysosomal enzymes
attachment to plasma membrane proteins & glycoproteins
DNA replication occurs in the nucleus of the host cell (except for POX viruses where components are synthesized in cytoplasm)
animal virus replication can result in:
lytic infection
- death of cell & release of virus;
persistent infection
- slow release of virus w/o cell death;
latent infection
- always starts as lytic that becomes latent;
creation of tumor cells
- like cancer
Bacteriophage Viral Reproduction
Lytic Cycle
- (T4 Phage or virulent phage) ends in death of host cell
Lysogenic Cycle
- (Lambda Phage or template phage) involves insertion of phage DNA into host genome, forming
prophage
; the phage remains latent & host cell remains alive; cycle is part of transduction in horizontal gene transfer
result of
lysogeny
is
phage conversion
where host cell may exhibit new properties, such as
Corynebacterium diptheria
producing the disease only when a prophage is present that codes for the cytotoxin
Special Viral Mechanisms
Antigenetic Shift
- caused by major genetic recombination (ex:
swine
get infected with both
avian
&
human influenza
, leading to mixing of RNA segments resulting in major change in
H
and/or
N
portions; can cause pandemics)
Antigenetic Drift
- reflects minor annual variations in the antigenic makeup of the influenza virus; involves an alteration in a
single amino acid
of the
H
or
N spike
(these are missense mutations); vaccines differ slightly each year to accommodate the drift/shift
Latent
viral infections are often activated by stress & may not recur for months--years (ex: cold sores-
Human Herpes
viruses, and chicken pox/shingles-
Varicella/Zoster
virus
Retrovirus
- RNA viruses (like HIV) carry the enzyme
reverse transcriptase
which uses viral RNA as a template to produce complementary double-stranded DNA; this DNA can be inserted into a host cell chromosome as a
provirus
& can remain latent and replicate with the host DNA or be expressed
Human DNA Viruses
Adenoviridae
- respiratory disease
Poxviridae
- small pox, cow pox (large, viral replication in cytoplasm)
Herpesviridae
- cold sores, genital herpes, mono, chicken pox/shingles
Papovaviridae
- warts
Influenza
Family:
Orthomyxoviridae
; Genus:
Influenza virus
; Capsid: protein coat; Envelope: lipid bilayer; Genome:
8 segments RNA
; Surface Spikes:
neuraminidase (N spike) & hemagglutinin (H spike)
H spikes
allow the virus to attach to specific host cell receptors; host antibodies are targeted against these spikes
Signs/symptoms- chills, fever, headache, muscle aches; recovery in a few days
50,000-70,000 americans die each year
in elderly can cause death/disability due to secondary lung infections
N spikes
help the virus separate from infected cells after replication
spread by aerosol transmission
treatment includes
neuraminidase inhibitors
or prevent uncoating (for flu A)
flu types are based on
protein coat
Influenza A
- responsible for regular outbreaks; infects pigs, horses, chickens, ducks, some wild birds
Influenza B
- causes sporadic outbreaks of illnesses in limited areas (like nursing homes)
Influenza C
- common but seldom causes disease symptoms
prevention: injected vaccine grown in embryonic eggs & attenuated nasal spray (
FluMist
) which stimulates
IgA
(
Antibody A
) production
Yellow Fever
caused by
Flavivirus
(ssRNA, envelope)
zoonotic
disease
endemic in Tropics (Mexico, South Am., Africa)
impacted building of Panama Canal
mosquitos are the
vector
Sylvatic Cycle
- involves monkeys to mosquitos to monkeys, and monkeys to humans
Urban Cycle
- involves
Aedes aegypti
mosquitos from human to human
Vaccine (Attenuated yellow fever
17D strain/Dakar strain
)
Spanish Flu 1918
avian
combined with
10 changes in amino acids
strain became endemic in swine population
originated in Kansas
killed 675,000 ppl in U.S. & 20-50 million worldwide (number is estmate due to WWI)
produced hemorrhagic pneumonia