Download Introduction to Agricultural Biotechnology AGR 0150 Viruses Part 3

The Viruses
Viruses
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Viruses may be defined as acellular organisms whose
genomes consist of nucleic acid,
• obligately replicate inside host cells using host
metabolic machinery and ribosomes to form a pool of
components
• which assemble into particles called VIRIONS, which
serve to protect the genome and to transfer it to other
cells.
• They are distinct from other so-called VIRUS-LIKE
AGENTS such as VIROIDS and PLASMIDS and
PRIONS
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VIRUS STRUCTURE
• range in size
• All viruses contain
– a nucleic acid genome (RNA or DNA) and
– a protective protein coat (called the capsid).
• may or may not have an envelope
Viral Shapes
• Three basic shapes
– Helical
– Icosahedral or Polyhedral
– Complex
Helical
Composed of capsomeres that bond together in
a spiral fashion to form a tube around the
nucleic acid; tobacco mosaic virus
Helical symmetry
Icosahedral or Polyhedral
Roughly spherical, with the shape similar
to a geodesic dome; common cold
VIRION
The icosahedral shape of a soccer ball.
penton subunits (black)
and
hexon subunits (white)
Complex
Have capsids of many different shapes that do not fit
into the two other categories; small pox virus with
several covering layers and bacteriophage T4, with
a icosahedral head and tail
Complex
Many bacteriophages contain the icosahedral - many
sided, three dimensional, hexagonal shape made
up of many small triangles – head, containing the
genome, attached to helical tails with tail fibers
Virus Facts
• viruses do not respire,
• nor do they display irritability;
• they do not move
• and nor do they grow,
Virus Facts 2
• Cause many infections of humans, animals,
plants, and bacteria
• Cannot carry out any metabolic pathway
• Do not respond to the environment
• Cannot reproduce independently
• Obligate intracellular parasites
Characteristics of Viruses
• Cause most diseases that plague industrialized
world; common cold, influenza, herpes, AIDS
• Virus
– Miniscule, acellular, infectious agent having one or
several pieces of either DNA or RNA (genome); never
both
• No cytoplasmic membrane, cytosol, organelles
• Have extracellular and intracellular state
Characteristics of Viruses 2
• Extracellular state – outside of the cell
– Called virion
• Protein coat (capsid) surrounding nucleic acid
– Nucleic acid and capsid also called nucleocapsid
– Some have phospholipid membrane called an
envelope
– Outermost layer provides protection and recognition
sites for host cells
• Intracellular state
– Capsid removed
– Virus exists as nucleic acid
How viruses are distinguished
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Type of genetic material they contain
Kinds of cells they attack
Size of virus
Nature of capsid coat
Shape of virus
Presence or absence of envelope
Viral Genetic Material
• Show more variety in nature of their
genomes than do cells
• May be DNA or RNA; never both
• Primary way scientists categorize and
classify viruses
• Can be dsDNA, ssDNA, dsRNA, ssRNA
• May be linear and composed of several
segments or single and circular
• Much smaller than genomes of cells
Viral Hosts
• Most only infect particular kinds of host’s cells
– Due to affinity of viral surface proteins or
glycoproteins for complementary proteins or
glycoproteins on host cell surface
• May only infect particular kind of cell in host
– HIV – T lymphocytes
• Generalists – infect many kinds of cells in many
different hosts
– Rabies – humans to bats
Viral Hosts 2
• All types of organisms are susceptible to viral
attack
• A bacteriophage is a virus that infects bacteria
• Most studies have focused on bacterial and
animal virus
• Some studies on viruses that infect crops
• Fungal virus studies are limited,
– It is known that they have no extracellular stage
Tobacco leaf infected with
tobacco mosaic virus;
bacteriophage attacking bacteria
Viral Size
Capsid Morphology
• Capsids – protein coats that provide
protection for viral nucleic acid and means
of attachment to host’s cells
• Capsid composed of proteinaceous
subunits called capsomeres
• Some capsids composed of single type of
capsomere (protein); others composed of
multiple types
The Viral Envelope
• Some viruses, particularly animal
viruses, have a membrane similar in
composition to a cytoplasmic membrane
surrounding their capsids
• Such a membrane is called an
envelope, and thus the virus called an
enveloped virus
• A virus without an envelope is called a
nonenveloped or naked virus
Enveloped Viruses
• Enveloped viruses acquire their envelope from
the host cell during viral replication or release
• Envelope is a portion of the membrane of the
host cell
• Composed of phospholipid bilayer and proteins
coded for by host DNA
• Some of the proteins are virally coded
glycoproteins, which appear as spikes
protruding outward from the envelope’s surface
Envelope
• The envelope’s proteins and glycoproteins
often play a role in the recognition of host
cells
• The envelope does not perform other
physiologic functions of cell membranes
Enveloped Virus
Coronavirus with helical - spiral in form- capsid
Enveloped Virus
Togavirus with icosahedral capsid
Viral Replication
• Dependent on host’s organelles and
enzymes to produce new virions
• Replication cycle usually results in
death and lysis of host cell → lytic
replication
• Stages of lytic replication cycle
– Attachment
– Entry
– Synthesis
– Assembly
– Release
Classification of Viruses
• Viruses are categorized by their type of
nucleic acid, presence of an envelope,
shape and size
• Taxa and classification system still
under development as viruses are not
yet fully understood
Viral Replication
• Dependent on host’s organelles and
enzymes to produce new virions
• Replication cycle usually results in
death and lysis of host cell → lytic
replication
• Stages of lytic replication cycle
– Attachment
– Entry
– Synthesis
– Assembly
– Release
Lytic Replication of Bacteriophages
Attachment
• T4 bacteriophage of
Escherichia coli
• Attachment dependent
on random collisions
• Complementary fit of viral
tail proteins and receptor
proteins on host’s cell
wall
– Ensures virus will only
attach to correct host cell
Entry
• Releases lysozyme, protein
carried in capsid, to weaken
peptidoglycan of cell wall
• Tail contracts forcing
internal hollow tube through
cell wall and membrane
• Genome moves into
bacterium
Synthesis
• Viral enzymes degrade
the host’s DNA into
nucleotides
• Viral genome starts
transcription and
translation of its own
proteins, using host’s
ribosomes
• Production of protein
components for new
virions made
Assembly
• Capsomeres accumulate
in cell and spontaneously
attach to form new
capsids within the cell
• Tail assembles and
attaches to head
• Genome inserted after
assembly
Release
• Newly assembled virions
are released as lysozyme
completes its work on the
cell wall
• Bacterial cell disintegrates
Lysogeny
• Some bacteriophages have a modified
replication cycle in which the infected
host cells grow and reproduce for many
generations before they lyse
• Additional portion of replication cycle is
called lysogeny (lysogenic replication
cycle)
• Phages are called lysogenic phages or
temperate phages
Lysogeny
1. Attachment
2. Entry
3. Lysogeny
a. Prophage
incorporation
b. Replication
4.
5.
6.
7.
Induction
Synthesis
Assembly
Release
Lysogeny
Attachment, Entry and Prophage
• Similar to the lytic replication cycle except the
bacterial genome is not destroyed
• Viral genome remains dormant and is called
a prophage
• Prophage incorporates into bacterial DNA
Replication (Lysogony)
• Cell reproduces
reproducing prophage
in every daughter cell
• All daughter cells are
thus infected with the
quiescent virus
• Can continue for
many generations
Induction, Assembly and
Release
• Some induction agent causes the prophage to
proceed forward with assembly and lysis
• Induction agents are generally the same ones
that damage DNA; UV light, carcinogenic
chemicals
Nonviral infectious agents
• Prions
– PIECE OF PROTEIN
– CAUSE OF MAD-COW
DISEASE
– CAN INFECT ANIMALS –
INCLUDING HUMANS
• VIROIDS
– Single strand of RNA
– Causes plant diseases
Viroids and Virusoids
• Infectious agents,
• Differ from viruses in several ways.
– they have a single-stranded circular, RNA
genome.
– Their genomes are very small and do not
code for proteins.
– Viroids replicate autonomously inside a
cell, but virusoids cannot.
Viroids, the Smallest Infectious
Units
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An infectious particle,
similar to but smaller than a virus,
Lack a capsid
consists solely of a strand of RNA
is capable of causing disease in plants.
Viroids
• Very small, covalently closed, circular RNA molecules
capable of autonomous replication and induction of
disease
• Sizes range from 250-450 nucleotides
• No coding capacity - do not program their own
polymerase
• Use host-encoded polymerase for replication
• Mechanically transmitted; often seed transmitted
• More than 40 viroid species and many variants have
been characterized
• “Classical” viroids have been found only in plants
Viroids
Left is T7 genome with small potato spindle tuber
viroids in between; right is potatoes stunted by
PSTV
Characteristics of Prions
• Prions = Proteinaceous infectious particles
• Composed of single protein called PrP
• All mammals contain gene that codes for
primary sequence of amino acids in PrP
• Two stable tertiary structures of PrP
– Normal functional structure with α-helices called
cellular PrP
– Disease-causing form with β-sheets called prion
PrP
• Prion PrP converts cellular PrP into prion PrP
by inducing conformational change
Characteristics of Prions
• Normally, nearby proteins and
polysaccharides force PrP into cellular
shape
• Excess PrP production or mutations in
PrP gene result in initial formation of
prion PrP
• When prions present, they cause newly
synthesized cellular PrP to refold into
prion PrP
Prion Diseases
• All involve fatal neurological degeneration,
deposition of fibrils in brain, and loss of
brain matter
• Large vacuoles form in brain;
characteristic spongy appearance
• Spongiform encephalopathies – bovine
spongiform encephalitis (“mad cow”
disease) in cows, scrapie in sheep
Creutzfeldt-Jakob disease in humans
Prion Diseases
• Transmission through ingestion of
infected tissue, transplantation of
infected tissue, or mucous membrane
contact
• Only destroyed by incineration; not
cooking or sterilization
• There is no known treatment
Prion Diseases
Scrapie in sheep; “mad cow” disease; CJ disease
Other Autonomous or Semi-Autonomously
Replicating Genomes
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Retrons
Bacterial and fungal plasmids
Satellite nucleic acids
Satellite viruses
Viroids
Prions
Prions (Diseases)
• "small proteinaceous infectious
particles which resist inactivation by
procedures that modify nucleic acids".
• spongiform encephalopathies
Retroid Elements and
Retroviruses