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Chapter 10: Viruses
General Characteristics of Viruses
• Viruses are infectious agents that are too small to be seen with a light microscope and that are not cells
• When they invade susceptible host cells, viruses display some properties of living organisms and so
appear to be on borderline between living and nonliving
• Viruses can replicate, or multiply, only inside a living host cell (obligate intracellular parasites)
• Viruses contain DNA or RNA but almost never both
Components of Viruses
• Nucleic Acid Core (DNA or RNA)
• Capsid: Surrounding protein coat
• Envelope: Some viruses have this additional surrounding lipid bilayer membrane
• Virion: A complete virus particle
• Viruses use their nucleic acids (genome) to replicate themselves in host cells
• Capsids also play a key role in the attachment of some viruses. Each capsid is composed of protein
subunits called capsomeres.
• Enveloped viruses have a typical bilayer membrane outside their capsids and acquire their envelope
after they are assembled in a host cell and “bud” through host’s membrane
• Nucleocapsid comprises the viral genome together with the capsid
• Naked: viruses with a nucleocapsid and no envelope
• Spikes: projections that extend from the viral envelope that may aid in attachment to the host cell
• Glycoprotein: these surface projections serve to attach virions to specific receptor sites on susceptible
host cell surfaces
• Envelopes help the virus in evading detection by the host’s immune system
Viral Shapes
• Some viruses are variable in shape, but most have a specific shape that is determined by the
capsomeres or envelope
• Helical capsid: consists of a ribbonlike protein that forms a spiral around the nucleic acid
• Polyhedral capsid: many-sided, and one of the most common polyhedral capsid shapes is the
icosahedron
• Some viruses have a bullet-shaped capsid and some are spherical
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Classification of Viruses
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Historically, virologists classified viruses by the type of host/host structures they infected
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Bacteriophages: infect bacterial cells
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Plant viruses infect plant cells
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Animal viruses are subgrouped by the tissues they attack:
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Dermotrophic: if they infect the skin
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Neurotrophic: if they infect nerve tissue
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Discoveries into biochemical and molecular properties allowed for viruses to be classified based on:
• Type and structure of their nucleic acids
• Method of replication
• Host range
• Other physical and chemical characteristics
Nucleic Acid Classification
• Major groups of viruses are distinguished first by their nucleic acid content as either DNA or RNA
• RNA viruses can be single-stranded (ssRNA) or double-stranded (dsRNA)
• RNA viruses must either carry enzymes or have genes for those enzymes in order to copy RNA
genomes after infecting a host cell
• DNA viruses can also occur in single-stranded or double-stranded form
General Properties of RNA Viruses
• Many ssRNA viruses contain positive (+) sense RNA, and during an infection acts like mRNA and can
be translated by host’s ribosomes
• Other ssRNA viruses have negative (-) sense RNA and the RNA acts as a template during transcription
to make a complementary (+) sense mRNA
• Negative (-) sense RNA must carry an RNA polymerase within the virion
General Properties of DNA Viruses
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Like the RNA viruses, the animal DNA viruses are grouped into families according to their DNA
organization
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dsDNA viruses are further separated into families on the basis of:
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Shape of DNA (linear or circular)
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Their capsid shape
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Presence or absence of an envelope
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Only one family of viruses has ssDNA
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Viral Replication
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Adsorption: the attachment of viruses to host cells
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Penetration: entry of virions (or their genome) into host cells
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Synthesis: new nucleic acids, capsid proteins, and other viral components
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Maturation: assembly of newly synthesized viral components into complete virions
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Release: departure of new virions from host cells
Properties of Bacteriophages
• Can have their genetic information in form of either ds or ss DNA or RNA
• Relatively simple or complex in nature
• T-even phages (T2, T3 and T6): “T” stands for type
• Structurally, have a head with genome, tail sheath and plate and tail fibers
Phage Growth and Estimation of Phage Numbers
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Viral growth can be described by a replication curve
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Eclipse period: which spans from penetration through biosynthesis (mature virions undetected)
Latent period: spans from penetration up to the point of phage release
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Plaque assay: method to estimate phage number
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A sample of each dilution is inoculated onto a plate containing a susceptible bacterial lawn
General Properties of Lysogeny
• Temperate phages do not always undergo a lytic cycle
• The majority of the time they exhibit lysogeny, a stable, long-term relationship between phage and host
in which phage nucleic acid incorporates into host nucleic acid
• Prophage: this viral DNA within the bacterial chromosome
• Lysogen: the combination of a bacterium and a temperate phage
Penetration
• Follows quickly after adsorption of the virion to the host’s plasma membrane
• Unlike bacteriophage, animal viruses do not have a mechanism for injecting their nucleic acid into host
cells
• Most naked viruses enter cell by endocytosis in which virions are captured by pitlike regions on cell
surface
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• Uncoating: this process occurs after animal virus enters host cell’s cytoplasm and separates genome
from protein coat
Synthesis
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Generally, DNA animal viruses replicate their DNA in host cell nucleus with aid of viral enzymes
and synthesize their capsid and other proteins in the cytoplasm with aid of host cell enzymes
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Synthesis in RNA animal viruses takes place in a greater variety of ways than found in DNA viruses
(+) sense RNA acts as mRNA (e.g. picornaviruses)
dsRNA (+) sense are transcribed into ssDNA with help of reverse transcriptase (e.g. retrovirus –
HIV)
(-) sense RNA make (+) sense RNA which are mRNA (e.g. measles and influenza)
Maturation: Once an abundance of viral nucleic acid, enzymes, and other proteins have been
synthesized, assembly of components into complete virions begins
Release: The budding or new virions through a membrane may or may not destroy the host cell.
Adenoviruses bud from host cell in a controlled manner (e.g. shedding) which does not lyse host cells
• Latent viral infections: this ability is held be all herpesviruses. These dsDNA viruses exhibit a lytic
cycle and are activated by a cold, fever, stress or immunosuppression
Culturing of Animal Viruses
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Two discoveries greatly enhanced the usefulness of cell cultures for virologists and scientists
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The discovery and use of antibiotics made it possible to prevent bacterial contamination
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The discovery of proteolytic enzymes (e.g. trypsin) can free animal cells from surrounding tissues
without injuring freed cells
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Subculturing: the process by which cells from an existing culture are transferred to new containers
with fresh nutrient media
Types of Cell Cultures
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Three basic types of cell cultures are widely used in clinical and research virology:
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Primary cell cultures: come directly from the animal and if repeatedly subcultured, one cell
type will become dominant (cell strain)
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Diploid fibroblast strains: Most widely used strain and support growth of a wide range of
viruses
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Continuous cell lines: consists of cells that will reproduce for an extended number of
generations and the most famous is the HeLa cell line
The Cytopathic Effect (CPE)
• The visible effect viruses have on cells
• Cells in culture show several common effects, including changes in cell shape and detachment from
adjacent cells or culture container
• CPE can be so distinctive that an experienced virologist can use it to make a preliminary ID of the
infecting virus
• Syncytia: giant, multinucleate cells caused by fusion of adjacent cells (e.g. paramyxoviruses)
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Viruses and Teratogenesis
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Teratogenesis: the induction of defects during embryonic development
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Teratogen: a drug or other agent that induces such defects
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Three human viruses account for a large number of teratogenic effects:
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Cytomegalovirus
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Herpes simplex virus type I and II
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Rubella
Viruslike Agents: Satellites, Viroids, and Prions
Satellites
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Small, single-stranded RNA molecules, which lack genes required for their replication
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In the presence of a helper virus, they can replicate
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There are two types:
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Satellite viruses: most are associated with plant viruses
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Satellite nucleic acids (virusoids)
Viroids
• 1971: plant pathologist T.O. Diener described a new type of infectious agent when studying potato
tuber spindle disease
• No virions could be detected
• Proposed the concept of a viroid, an infectious RNA particle smaller than a virus
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Viroids have been found to differ from viruses in six ways:
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Consists of a single circular RNA molecule of low molecular weight
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Exist inside cells, usually inside of nucleoli as particles of RNA without capsids or envelopes
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Do not require a helper virus
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Viroid RNA does not produce proteins
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Viroid RNA is always copied in the host cell nucleus
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Not apparent in infected tissue without use of special techniques to ID nucleotide sequences in
the RNA
Proteineacous Infectious Particles (Prions)
Protein structure model of the two forms of prion protein (PrP).
The protein helices are represented as spiral ribbons. A: harmless form and B: harmful form
Viruses and Cancer
• 1911: viruses could cause some cancers was discovered by F. Peyton Rous
• Rous sarcoma virus caused sarcomas (neoplasms of connective tissue) in chickens
• DNA tumor viruses can exist as proviruses and the major CPE is the uncontrollable division of the
infected cells
• Process is called neoplastic transformation
Oncogenes
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Proteins produced by tumor viruses that cause uncontrolled host cell division
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Proto-oncogene: A normal gene that, when under the control of a virus, can cause uncontrolled cell
division and can act as an oncogene
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Work in two ways:
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Product of oncogene can disrupt normal cell function, leading to uncontrolled cell divisions
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Controlled by viral regulators near the site of their integration into the host cell’s chromosome
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