Transcript BTY328: Viruses

BTY328: Viruses
Dr William Stafford
[email protected]
Viral hosts and disease
Viruses that infect eukaryotes: plants
and animals
Diversity of viruses that infect
vertebrates
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Microbiologists first began to
classify animal viruses in terms
of host preferences. However,
many viruses will infect a
variety of animals, and a
particular animal can be
invaded by several dissimilar
viruses....
Modern classifications are
primarily based on virus
morphology, the physical
and chemical nature of virion
constituents, and genetic
relatedness.
Reproduction of human phages
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The reproduction of animal viruses is very similar in
many ways to that of phages: stages: adsorption,
penetration and uncoating, replcation of virus nucleic
acids, synthesis and assembly of virus capsids, and
release of mature viruses.
The capacity of a virus to infect a cell depends on its
ability to bind target receptors.
Host and tissue specificity e.g poliovirus receptors are
found only in the human nasopharynx, gut, and spinal
cord; whereas, measles virus receptors are present in
most tissues.
Viral entry
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Some naked viruses such as the poliovirus undergo
major change in capsid structure on adsorption to the
plasma membrane, and only their nucleic acids are
injected into the cytoplasm.
Many enveloped viruses enter cells through
engulfment by receptor-mediated endocytosis to
form coated vesicles. These vesicles fuse with
lysosomes where viral uncoating is completed.
The envelope of paramyxoviruses, membrane fusion
occurs- membrane lipids rearrange and adjacent
membranes merge, and a proteinaceous fusion pore
forms. Then nucleocapsid enters the host cell
cytoplasmic matrix, where uncoating is completed.
Strategies of viral entry
Viral genome replication
strategies
Replication and Transcription in DNA Viruses
The early phase of infection aims to take over the host
cell and to the synthesise viral DNA and RNA. Some
virulent animal viruses inhibit host cell DNA, RNA, and
protein synthesis, although cellular DNA is not usually
degraded.
Parvoviruses (canine and feline leukopenia)- small,
ssDNA molecule about 4.8kbp with overlapping genes;
directs the synthesis of only three capsid polypeptides.
Since the genome does not code for any enzymes, the
virus must use host cell enzymes for all biosynthetic
processes (viral DNA can only be replicated in the
nucleus during the S-phase of the cell cycle, when the
cell replicates its own DNA).
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Replication and Transcription of DNA Viruses
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Herpesviruses- icosahedral, enveloped, dsDNA
viruse with a genome of 160 kbp coding for 50-100
genes. Immediately upon infection, the DNA is
transcribed by host RNA polymerase to form mRNAs
directing the synthesis of viral proteins.
Poxviruses such as the vaccinia virus are dsDNA
and the largest viruses known, the genome is 190kbp
coding for 250 genes. The viral core contains both
DNA and RNA polymerases that synthesize early
mRNAs, which then produce viral proteins. The
complete reproductive cycle in poxviruses about 24
hours.
Replication and Transcription of RNA Viruses
ssRNA viruses, except retroviruses, use a viral replicase
that converts the ssRNA into adouble-stranded RNA called
the replicative form that then directs the synthesis of
new viral RNA genomes
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Picornaviruses (e.g poliovirus) (+) strand ssRNA. Use
their RNA genome as a large mRNA, that results in the
synthesis of an enormous polypeptide that is processed.
Orthomyxoviruses (e.g Influenza) (-) strand ssRNA.
Use a viral RNA-dependent RNA polymerase to synthesize
mRNA and subsequently proteins.
Retroviruses (e.g.HIV) (+) strand ssRNA but differ from
other RNA viruses that replicate their genome by means of
DNA intermediates. The virus has an RNA-dependent DNA
polymerase or reverse transcriptase.
Replication and Transcription of RNA Viruses
dsRNA viruses use a RNA-dependent RNA
polymerase that copies the negative strand of their
genome to generate mRNA. Normally use a single
polymerase for replication and transcription.
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Reoviruses use a different strategy. The virion
contains 10 to 13 different dsRNAs, each coding for an
mRNA. Late in the reproductive cycle, these mRNA
associates and are copied by the viral replicase to
form a double-stranded genome that is incorporated
into a new virion.
Viral release
Mechanisms of virion release is different for naked and
enveloped viruses.
Naked virions appear to be released most often by host
cell lysis, causing cell death.
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In contrast, the release of enveloped viruses is usually
by membrane budding and the host cell may continue to
release virions for some time.
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Host actin filaments can also aid in virion release (e.g.
vaccinia virus) by moving virus in the cytoplasm and
propelling it through the plasma membrane without
destroying the cell.
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Summary: Influenza virus
Life cycle of
Influenza virus
Influenza virus release
Viruses that infect Plants
Tobacco Mosaic Virus (TMV)
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TMV is a (+) ssRNA virus
that uses complex
replication strategy involving
intermediates.
After the coat protein and
RNA genome have been
synthesized, they
spontaneously assemble
into complete TMV virions in
a highly organized process
Central RNA core with protein protomers
arranged in a helical spiral.
TMV infection and reproduction
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TMV-infected cells produce microscopically visible
intracellular inclusions, usually composed of virion
aggregates. The host cell chloroplasts become
abnormal and often degenerate and new chloroplast
synthesis is inhibited. Leaves may appear chlorotic.
Reproduction within the host depends on the virus's
ability to spread throughout the plant. Viruses can
move long distances through the plant vasculature
(usually phloem). The spread in nonvascular tissue is
hindered by the presence of tough cell walls. TMV
does spread slowly from cell to cell through the
plasmodesmata
Transmission of Plant Viruses
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Since plant cells are protected by cell walls, plant
viruses have a considerable obstacle to overcome
when trying to establish themselves in a host. TMV
and a few other viruses may be carried by the wind or
animals and then enter when leaves are
mechanically damaged.
The most important agents of transmission (vectors)
are insects that feed on plants, particularly sucking
insects such as aphids and leafhoppers. Other vectors
such as soil nematodes can transmit viruses (e.g., the
tobacco ringspot virus) while feeding on roots.
Acute and persistent infections
Many virus infections (e.g. influenza) are acute
infection- fairly rapid onset and last for a relatively
short time. However, some viruses can establish
persistent infections lasting many years. Persistent
infections are:
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chronic - virus is almost always detectable and
clinical symptoms may be either mild or absent for
long periods (e.g hepatitis B and HIV).
latent- virus stops reproducing and remains dormant
for a period before becoming active again. During
latency, no symptoms, antibodies, or viruses are
detectable. (e.g. herpes simplex virus, varicella-zoster
virus, cytomegalovirus, and Epstein-Barr virus.)
Effects of viral infection