LESSON 1: INTRODUCTION Flashcards
1
Q
General description of a virus:
A
- Obligatory intracellular infectious
agents,
-size from 20 to 400 nanometer (nm) - Filterable agents
-Nocellular organization and do not have organelles
2
Q
are thesmallest viruses (20nm)
A
picornaviruses
2
Q
The picornaviruses (e.g.)
A
Foot and Mouth-Disease virus
3
Q
are the largest viruses
(300nm)
A
poxviruses
3
Q
Viruses cannot be seen by light microscope because of their small size except
A
poxviruses
4
Q
Contain only one type of nucleic acid
A
DNA or RNA
5
Q
Viruses multiply by a complex process involving
A
protein synthesis and nucleic
acid production
6
Q
Viruses are unaffected by
A
antibiotics
7
Q
Threecategories:
A
-DNA viruses,
-RNA viruses and
-Viruses that utilize both DNA
and RNA for replication
8
Q
the viruses that infect bacteria
A
Bacteriophages or phages
9
Q
an infectious extracellular virus particle consists of nucleic acid (DNA or RNA) that is covered by a protein coat called capsid
A
Virion
10
Q
a shell of subunits of proteins called capsomere that encloses the
genome of vertebrate viruses
A
Capsid
11
Q
capsid functions
A
protection
attachment
antigens
12
Q
are the two types of capsid symmetry
described in viruses (Fig. 3). But large viruses with large genome have
complicated symmetry which is neither icosahedral nor helical such as poxviruse
A
Icosahedral and helical symmetries
13
Q
the term used to refer to the combined nucleic acid and capsid
which can either be naked or covered with a membrane termed an envelope
A
Nucleocapsid
13
Q
:the proteins that make up the subunit of capsid.
A
Structural proteins
13
Q
The viral
genome also codes for important enzymes
- required for viral replication but are not incorporated in the virion
A
non-structural proteins
13
Q
are generally assembled in the host cell prior to incorporation of the viral nucleic acid.
A
Icosahedral capsids
14
Q
are formed by the insertion of protein units between each turn of
the nucleic acid helix, incorporating the RNA in the tubular package. The length
of the helix is determined by the length of the RNA molecule
A
Helical capsids
14
Q
a lipid bilayer and associated glycoproteins that cover a nucleocapsid
A
Envelope
14
Q
is acquired when the nucleocapsid buds through a cellular membrane,
endoplasmic reticulum, the Golgi apparatus or the nuclear membrane.
A
Envelope
14
Q
are usually susceptible to detergent and are rendered non
infectious following damage to the envelope
A
Enveloped viruses
14
Q
the proteins encoded by viral nucleic acid for binding to
receptors on host cells, membrane fusion, uncoating of the virion and destruction
of receptors on host cells
A
Glycoproteins
14
Q
are knob-like projections from the envelope formed from
the oligomers of glycoproteins.
A
Peplomersor spike
14
Present in certain viruses including
coronaviruses, retroviruses, orthomyxoviruses, rhabdoviruses and
paramyxoviruses, and used to bind to cell receptors or may have enzymatic
activity
15
a layer of protein present between the nucleocapsid and the
envelope in some enveloped viruses that provides additional rigidity to the virion.
Matrix protein
15
papovavirus
papilloma
15
vacuolating
polyoma
15
pico/small–rna–virus
picornavirus
15
Coronaviruses
(halo or corona/crown of spikes),
15
Togavirus
(Toga/cloak),
16
Fourorders containing viruses of animals are so far recognized:
Mononegavirale
Herpesvirales
Picornavirales:
Nidovirales
16
Rhabdovirus
(Rhabdo/Rod-shaped)
17
Calicivirus
(Calix/cup-shaped depression
18
have common attributes
including a single stranded, non-segmented, negative sense
RNA genome, similar replication strategies.
order Mononegavirale
18
e is made up the families Paramyxoviridae,
Rhabdoviridae, Bornaviridae and Filoviridae.
19
comprising the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae
Herpesvirales
20
comprising the families Picornaviridae, Iflaviridae,
Dicistroviridae, Marnaviridae and Secoviridae;
Picornavirales
21
are infectious particles, which can transmit a disease, composed mainly
of a protein without any detectable nucleic acid.
Prions
21
comprising the families Coronaviridae, Arteriviridae and
Ronivirida
Nidovirales
22
apparently have no virion
structure or genomes and evoke no immune response in the infected host.
Prions
23
These are extremely resistant to inactivation by
heat, disinfectants, and radiation.
24
The prions are causative agents of slow viral infections, such as
Subacute spongiform encephalopathy
24
more resistant than bacteria to chemical disinfectants such as
phenol
25
After long incubation period of years, they produce
a progressive disease that causes damage to the central nervous system,
leading to
subacute spongiform encephalopathy
26
Active virucidal agents include
formaldehyde and betapropiolactone
27
most active antiviral disinfectants.
hydrogen peroxide
potassium permanganate,
hypochlorite, and
organic iodine compounds
27
Theviruses usually remain viable in a pH range of --- but are sensitive to
extremes of acidity and alkalinity
5–9
28
Most of the viruses with few exceptions are highly heat labile.
They are inactivated within seconds at
-within seconds at 56°C, within
-minutes at 37°C, and
-within days at 4°C
29
Radiations: the viruses are readily inactivated by
sunlight, ultraviolet (UV)
radiations, and ionizing radiations.
29
Lipidsolvents:
are active against enveloped
viruses but are not active against non-enveloped, naked viruses.
chloroform, and detergents
30
The replicative cycle of a virus may range from
6 to 40 hours
31
The replicative cycle of a virus may range from 6 to 40 hours. Within hours of
infection, an occurs
eclipse phase
32
After this eclipse phase, it is followed by the
---- as new viral particles are formed and released from the cell
wherein the number of viral particles increases exponentially
productive stage
32
Steps in virus replication
1. attachment
2.entry
3. uncoating
4.biosyntesis
5. maturation or assembly of virus
6. release daughter virion
33
the initial stage of virus replication
whereby the infecting virus loses its physical identity and most or all of its infectivity
eclipse
34
: Initial virus–cell interaction is a random event, which relates
to the number of virus particles present and the availability of appropriate receptor
molecules.
1. Attachment:
35
Virus–cell
interaction determines both the host range and the tissue tropism of viral species.
36
Virus receptors on cells could be
glycoproteins or glycolipids; proteoglycans,
glycoconjugates with terminal sialic acid residues, integrins and the IgG superfamily
of transmembrane proteins
37
is the process of separation of viral nucleic acid from its protein
core for transcription to take place.
Uncoating
37
Mechanism of entry:
A. Endocytosis:
B. Fusion
C. Direct introduction of viral genome into the cytoplasm (injection) t
37
ollowing attachment, the virus gains access to the host cell internal
environment where replication takes place
2. Entry
37
However, in certain viruses, transcription may
proceed without complete release of the viral genome (ex:
reoviruses
38
In enveloped
viruses, in which the nucleocapsid is discharged directly into the cytoplasm,
transcription can usually proceed without complete uncoating
---
39
. In non-enveloped
viruses, uncoating involves conformational changes,, proteolytic enzyme activity ,
progressive loss of structural proteins and weakening of intermolecular interactions.
40
Uncoating may occur on the
cell membrane
cytoplasm, or nucleus
41
while some viruses uses their
own enzymes.
Most RNA viruses generate its own enzyme (------) to
trancribe and replicate mRNA
polymerase
42
The synthesis of viral
proteins by host cells, which is the central event in replication of viruses, requires the
production of
Biosynthesis or Replication of nucleic acid.
viral mRNA
43
Some viruses (most DNA viruses) makes use of the host
cells enzymes (transcriptases) to synthesize mRNA,
44
DNA viruses replicate their DNA in the
nucleus of the host cell by using viral
enzymes
45
However, they synthesize their
capsid and other proteins in the cytoplasm by using host cell enzymes
cytoplasm
46
Most
of the DNA viruses
herpesvirus, papovavirus, adenovirus, and hepadnavirus
47
is an exception,
because all of its components are synthesized in the cytoplasm
Poxvirus
48
The RNA viruses replicate in the cytoplasm except
orthomyxoviruses and Borna disease virus w
49
of each segment
is transcribed to produce individual mRNA molecules
negative-sense strand
50
Transcription occurs in the cytoplasm under the
direction of a
viral transcriptase
51
single stranded RNA viruses can act directly as
mRNAfollowing infection
positive-sense
52
The assembly of the protein capsid is the
first step in viral maturation. The mechanisms for the assembly and release of
enveloped and non-enveloped viruses are distinct.
Maturation/Assembly of virus:
53
The envelope
develops around the capsid by a process
The envelope
develops around the capsid by a process
54
are present intracellularly as fully
developed viruses, but in case of enveloped viruses, only the nucleocapsid is
complete
nonenveloped viruses
55
nucleocapsid is surrounded by an envelope, which
is derived from the host cell membrane during the process of budding
56
Release from the cell can occur either b
exocytosis or by cytolysis.
57
are released
into the surrounding environment and may affect new host cells
Progeny virions
57
Abnormal replicative cycles may occur in four ways:
1. Incomplete viruses
2. Pseudovirions:
3. Abortive infections:
4. Defective viruses:
57
defect during assembly of viral components, some of
the daughter virions that are produced may not be infective.
Incomplete viruses
58
Example: an influenza
virus that shows a
high hemagglutination titer but with a low infectivity
59
are the viruses that occasionally enclose host cell
nucleic acid instead of viral nucleic acid, therefore, are non infective and lack the
capability to replicate
Pseudovirions
60
In this type of infection, the virus components may be
synthesized but the maturation is defective maybe due to infection of the wrong host
cells by the virus
. Abortive infections
61
These are viruses that produce fully mature virions only in the
presence of helper viruses which supplement the genetic deficiency in the defective
viruses
Defective viruses
61
Ex: Hepatitis D virus (defective virus) replicate only in the presence of
hepatitis B virus (helper viruses)
62
Spontaneous and random errors in the copying of viral nucleic acid,
termed mutations, can occur during the replication of viruses.
Mutation
63
genomic structure by two principal methods—
—mutations and
recombination.
64
It is the most important
mechanism by which a virus can be genetically modified which results in production
of new viral strains showing properties different from parental or wild-type virus such
as
inactivation of viruses,
altered antigenicity and pathogenicity of the virus, and
nduce drug resistance in viruses.
65
Mutation may be induced by mutagens like
X-rays,
UV irradiation or
chemical agents, or may even occur spontaneously
66
resulting from single nucleotide substitutions, are the most common type
of mutation.
Point mutations,
66
. Less common types of mutation result from the
deletion or insertion of
one or more nucleotides
66
a new area of antiviral research wherein those RNA viruses with
inherently high mutation rates are administered with mutagenic agents to drive viral
extinction through violation of the error threshold and error catastrophe
Lethal mutagenesis
67
is the extinction of an organism as a result of excessive
mutations.
Error catastrophe
67
a virus mutant which can replicate only under defined
permissive conditions.
Conditional-lethal mutants
:
68
can multiply most
efficiently at temperature ranges different from parental virus.
temperature-sensitive mutants
69
s are used extensively for the study of viral genetics and are also
evaluated for possible use in live viral vaccines.
temperature
sensitive mutants
70
viruses that replicate in the presence of antibody.
Because of altered antigenic surface determinants, the mutants are unaffected by
neutralizing antibodies induced by the ---
wild-type virus
70
variant strains showing differences in the tissue type and
species of target cells affected by viruses.
Host-range mutants:
70
viruses that replicate in the presence of antibody.
Because of altered antigenic surface determinants, the mutants are unaffected by
neutralizing antibodies induced by the wild-type virus.
Antibody escape mutants
:
71
rendering a virus towards low viral load and low viral fitness by
subjecting it to a combination of mutagenic agents and antiviral compounds.
Viral suppression
72
a virus with decreased infectious titer despite a high
number of viral particles. This mutant promote the establishment and maintain
persistent infections.
Defective-interfering mutants
:
73
—variant strains that cause less serious infections in humans
and animals.
Attenuated mutants
74
the exchange or transfer of genetic material between different
but closely related viruses infecting the same cell simultaneously, or between virus
and host cell.
Recombination
75
occurs between two closely related DNA or RNA
viruses.
Intramolecular recombination
:
75
a recombination between positive-sense single
stranded RNA viruses and occurs through a template switching mechanism; RNA
polymerase switches between template strands during synthesis of the
complementary negative-sense strand
Copy-choice (template switching)
:
75
The alteration of genetic information may result from
intramolecular
recombination,
copy-choice recombination, reassortment or
genetic reactivation.
75
has led to the formation of western equine encephalitis virus, another togavirus.
recombination of Sindbis and eastern equine encephalitis virus
75
:
is another process of genetic recombination.
Reassortment
76
segmented
influenza virus A and B
(8 segments),
77
Reoviridae
(10–12 segments)
78
Bunyaviridae
(3 segments), a
79
Arenaviridae and
Birnaviridae
(2 segments)
80
An exchange of segments occurs between these viruses,
resulting in production of new
hybrid strains.
80
infectious progeny are produced from parental viruses, of which one or
both are non-infectious, following mixed infection of a ce
Reactivation recombination
81
when infectious progeny are produced from related viruses inactivated by lethal
mutations at different loci in their genomes
Multiplicity reactivation:
82
occurs when an inactivated virus becomes capable of replicating after acquiring
genetic material from an infective virus
Cross-reactivation or genome rescue
83
is widely used for virus propagation; inoculation of chick
embryos and experimental animals is employed for the isolation and production of
particular viruses.
Tissue culture
84
Tissue culture: growth and maintenance of living tissue
in vitro
84
a tissue fragment is used to isolate viruses from animals
with persistent infection.
A. Explant cultures
85
the tissues are digested into individual cells by mechanical
cutting followed by digestion with enzymes such as
trypsin
85
is required for the isolation and identification of
viruses involved in disease, for the titration of viruses for vaccine production and for
the provision of stocks for research purposes.
Propagation
86
Three types of cell
culture:
a) Primary cell culture
b) Semi-continuous
c) Continuous cell cell culture
86
derived directly from tissues and contain many cell
types such as epithelial cells, fibroblasts, keratinocytes, melanocytes,
endothelial cells, muscle cells, hematopoietic cells, mesenchymal
stem cells, etc
Primary cell culture:
87
diploid cell lines retain
their characteristic diploid chromosomal constitution and can support
the growth of a wide range of viruses.
Semi-continuous:
87
or immortal cell lines are derived from
either normal or neoplastic tissue and can be passaged indefinitely.
c) Continuous cell cell culture:
87
Such cell lines can be obtained commercially from organizations like
American Type Culture Collection (ATCC).
88
American Type Culture Collection (ATCC). Examples
HeLa cells, or
Madin Darby bovine kidney (MDBK)
89
How to detect viral growth in cell cultures
1. Using light microscopy, microscopic changes or cytopathic effect (CPE)
2. Serological tests using flourescein-labelled antibody
90
to be
observed in virus-infected cells are change in shape, cell detachment, fusion leading
to syncytium formation, the presence of inclusion bodies and cell death.
cytopathic effect (CPE)
90
induce cell lysis and cellular transformation in cell culture
Burster (lytic) virus
91
Two types of virus according to CPE production:
a) Burster (lytic) virus:
b) Creeper virus
92
these induce formation of multinucleated giant cells.
b) Creeper virus:
92
though no longer extensively used, this remains
the preferred method for isolation of influenza A viruses and for many avian
viruses.
Inoculation on embryos
92
Modes of inoculation:
via the allantoic cavity,
the amniotic cavity or the
yolk sac,
chorioallantoic membrane (CAM), or intravascularly.
93
Routes for inoculation of viruses into embryonated eggs.
1, Into allantoic
cavity;
2, into amniotic cavity;
3, into yolk sac;
4, on to chorioallantoic membrane.
93
However, for several virus families, animal
inoculation either on laboratory animals or natural hosts remains the preferred
procedure for the following studies:
(a) detection of arthropod-borne viruses and
for rabies virus,
(b) inoculation of the natural host species as requirement for the
isolation of some viruses,
(c) challenge experiments in the natural host species
to evaluate vaccines,
(d) the production of antisera,
(e) investigation of the
pathogenetic mechanisms relating to viral infections and the subsequent immune
response of the host
