Download Lytic Virus-Cell Interaction

VIRAL
REPLICATION
Lecture 2
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Introduction
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Viral replication is the formation of
biological viruses during the infection process in the
target host cells.
Viruses are intracellular obligate parasites which
means that they cannot replicate or express their
genes without the help of a living cell.
A single virus particle (Virion) is in and of itself
essentially inert.
It lacks needed components that cells have to
reproduce.
When a virus infects a cell, it organizes the cell's
ribosomes, enzymes and much of the cellular
machinery to replicate.
Virus-Host Cell Interaction
For a virus to multiply it must obviously infect
a cell.
 Viruses usually have a restricted host range
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◦ i.e., animal and cell type in which this is possible.
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All viruses must make proteins with 3 sets of
functions
◦ Ensure replication of the genome,
◦ Package the genome into the virus particles, and
◦ Alter the metabolism of the infected cell so that
viruses are produced.
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Virus-Host Cell Interaction
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The two most commonly observed virushost cell interactions are:
◦ The lytic interaction:
 which results in virus multiplication and lysis (or death)
of the host cell
◦ The transforming interaction:
 which results in the integration (or existence as
episome) of viral genome into the host genome and
permanent transformation or alteration of the host cell
(e.g. morphology, interaction with other cells, growth
habits, etc.)
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Lytic Virus-Cell Interaction
During this cycle the virus enters host cell,
multiplies and is released.
 This cycle is repeated many times when a
virus particle infects an organism, until, for
one reason or another, further multiplication
is arrested or the host dies.
 The following steps is involved in a complete
lytic cycle:
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Lytic Virus-Cell Interaction
1- Recognition of a target host cell
(Adsorption)
◦ The receptor-binding site on virus particle reacts
specifically with a corresponding receptor on a cell
surface.
◦ The receptors on cells are glycoproteins or
glycolipids.
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Lytic Virus-Cell Interaction
2- Internalization of the virus
(Penetration and Uncoating)
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After virus attachment to host cell, virus
penetrates the plasma membrane and releases its
genome (uncoating) inside cytoplasm.
Virus entry is accomplished in one of three ways:
1. Fusion of the viral membrane and the plasma
membrane with the release of viral nucleic acid into the
cytoplasm (Figure 1A).
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Viruses have fusion proteins, e.g., measles and mumps
2. Internalization of the whole virion by viropexis (or
pinocytosis) and release of its nucleic acid (Figure 1B).
3. Naked viruses appear to pass or slide through the
external plasma membrane directly.
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Lytic Virus-Cell Interaction
3- Transcription and Replication
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The replication strategies employed by the
different viruses are different
◦ RNA viruses
 (+) stranded RNA acts as mRNA
 (-) stranded RNA, a virus associated polymerase
makes a complementary (+) copies that act as
mRNA
◦ DNA viruses
 Transcription “early” mRNA from parental DNA.
 Late mRNAs are transcribed from newly replicated
progeny DNA molecules
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Lytic Virus-Cell Interaction
4. Processing of mRNA
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Viral primary RNA transcripts may need to be
processed before they are translated into
proteins:
a) Primary transcripts splicing is common among
DNA viruses, e.g., adenoviruses.
 Some retrovirus and influenza virus mRNAs are also
spliced.
b) Addition of a sequence of 50-200 adenosine
residues to the 3΄ terminus of the mRNA molecule.
c) Addition of a 7-methyl guanosine “cap” to the 5΄
terminus of the transcript.
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Lytic Virus-Cell Interaction
5. Synthesis of viral proteins
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Viral mRNAs are translated on normal host cell
ribosomes to produce viral structural and
non-structural proteins.
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Host cell protein
synthesis machinery is
responsible for reading
the genetic message of
the viral mRNAs, in a
triplet code with start
and stop codons, as in
reading normal cellular
mRNAs.
Lytic Virus-Cell Interaction
5. Synthesis of viral proteins
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A single viral protein is synthesized from a
single mRNA,
Some viruses have a second strategy, whereby a very
large viral poly-protein is first formed, which is then
cleaved at specific sites by viral or cellular proteolytic
enzymes to give a series of smaller viral proteins.
In a third strategy, two virus proteins may be encoded
by a single mRNA (see overlapping genes) since the
mRNA may be read in different reading frames.
Lytic Virus-Cell Interaction
6. Assembly
Virus structural proteins, such as envelope
and matrix proteins, migrate to the plasma
membrane (budding viruses) or, alternatively,
may assemble in the cell cytoplasm (lytic
viruses) or the nucleus.
 Carbohydrates and other groups are added
to newly synthesized proteins by cellular
enzymes.
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Lytic Virus-Cell Interaction
7. Release
In case of budding, viral proteins are inserted in
the external plasma membrane of the host cell.
 The proteins and nucleic acid assemble in the
host cell and bud by protrusion through the
cellular plasma membrane
 The cell may continue to produce successive
waves of new viruses, as many as 10,000 virions
may be produced per cell in as a few as 6 hours.
 Some viruses, e.g., poliovirus, may assemble
completely in the cytoplasm and are released
only after lysis and death of the cell.
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GENETIC VARIATION
OF VIRUSES
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Since viruses are made up of nucleic acid
molecules surrounded by a protective coat,
◦ they undergo mutations,
◦ interact with the host nucleic acid,
◦ and interact with other viruses in mixed infected cells.
This chapter covers the mechanisms by which
genetic changes occur in viruses.
 Two principal mechanisms are involved:
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◦ Mutation
◦ and recombination
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Mutations
In general, the base sequence of a genome
must be preserved from one generation to
the next, otherwise progeny might not be
able to synthesize all the proteins required
for their own survival and reproduction.
 Yet changes do occur in the genomes, and
they may affect one or more characters of
the organism.
 Said another way, they give rise to variation
in phenotype.
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Mutations
One mechanism by which this change occurs
is called gene mutation.
 This is a stable change in the nucleotide
sequence of the organism genome.
 This might be a deletion, addition, or
substitution of one to several bases in the
sequence of a gene.
 Even a change in a single nucleotide might
lead to significant consequences!
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Mutations
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Induced Mutations
◦ Some gene mutations are induced by mutagens,
environmental agents that can attack a nucleic acid
molecule and modify its structure.
 Ultraviolet, radiation, ionizing radiation and certain chemicals
are examples of mutagens.
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Spontaneous Mutations
◦ Other gene mutations are spontaneous; they are not
induced by agents outside the cell or organism.
◦ These are the mutations which occur naturally during
viral replication.
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Mutations
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Virus Mutants
◦ “Strain”, “type”, “mutant” and even “isolate” are all
terms used interchangeably to differentiate them
from original “parental”, “wild type” or “street”
viruses.
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Mutations
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Mutation Rates and Outcomes
◦ DNA viruses have spontaneous mutation rates similar
to those of eukaryotic cells because, like eukaryotic
DNA polymerases, their replication enzymes have
proofreading functions.
◦ The error rate for DNA viruses is about 10-8 to 10-11
error per incorporated nucleotide.
◦ With this low mutation rate, replication of even the
most complex DNA viruses, which have 2x105 to
3x105 nucleotide pairs per genome, will generate
mutants rather rarely.
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Mutations
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The RNA viruses, however, lack a proofreading function
in their replication enzymes, and some have mutation
rates of 10-3 to 10-4 per incorporated nucleotide.
Even the simplest RNA viruses, which have about 7400
nucleotides per genome, will generate mutants
frequently
Not all mutations that occur persist in the virus
population.
Mutations that interfere with the essential functions of
multiplication cycle are rapidly lost from the population.
Only mutations that do not cripple essential viral
functions can persist or become fixed in a virus
population
Phenotypic Variation by Mutations
Mutations that alter the viral phenotype but
are not deleterious may be important.
 For example mutations can create novel
antigenic determinants which may then
enable the virus to infect a previously
immune host
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◦ e.g H5N1, H1N1, H3N1 strains of influenza virus.
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Additionally, mutation has been a principal
tool in developing attenuated live virus
vaccines