Lesson 1 (page 13 -18)

Question 1

Viruses with defective assembly of viral components resulting in some daughter virions that are produced may not be infective.

Example: an influenza virus that shows a high hemagglutination titer but with a low infectivity.

Answer 1

Incomplete viruses

Question 2

Viruses that occasionally enclose host cell nucleic acid instead of viral nucleic acid, making them non-infective and lacking the capability to replicate.

No example provided.

Answer 2

Pseudovirions

Question 3

Infections where virus components may be synthesized but the maturation is defective, possibly due to infection of the wrong host cells by the virus.

No example provided.

Answer 3

abortive Infections

Question 4

These are viruses that produce fully mature virions only in the
presence of helper viruses which supplement the genetic deficiency in the defective
viruses. Ex: Hepatitis D virus (defective virus) replicate only in the presence of
hepatitis B virus (helper viruses).

Answer 4

Defective viruses:

Question 5

Viruses, like other living beings, obey the law of genetics. The viruses show variation in their genomic structure by two principal methods—_______ and ___________.

Answer 5

Mutation and Recombination

Question 6

Spontaneous and random errors in the copying of viral nucleic acid,
termed mutations, can occur during the replication of viruses

Answer 6

Mutation

Question 7

MUTATION
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

Answer 7

1. Inactivation of viruses,
2. Altered antigenicity and pathogenicity of the virus
3. Induce drug resistance in viruses.

Question 8

, resulting from single nucleotide substitutions, are the most common type
of mutation. Less common types of mutation result from the deletion or insertion of
one or more nucleotides.

Answer 8

Point mutations

Question 9

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

Answer 9

Lethal mutagenesis

Question 10

: is the extinction of an organism as a result of excessive
mutations.

Answer 10

Error catastrophe

Question 11

a virus mutant which can replicate only under defined
permissive conditions. Example: temperature-sensitive mutants can multiply most
efficiently at temperature ranges different from parental virus. These temperature- sensitive mutants are used extensively for the study of viral genetics and are also
evaluated for possible use in live viral vaccines

Answer 11

Conditional-lethal mutants

Question 12

rendering a virus towards low viral load and low viral fitness by
subjecting it to a combination of mutagenic agents and antiviral compounds.

Answer 12

Viral suppression

Question 13

variant strains showing differences in the tissue type and species of target cells affected by viruses

Answer 13

Host-range mutants

Question 14

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. Such a selection process may
facilitate persistent or recurring infections.

Answer 14

Antibody escape mutants

Question 15

a virus with decreased infectious titer despite a high
number of viral particles. This mutant promote the establishment and maintain
persistent infections

Answer 15

Defective-interfering mutants:

Question 16

variant strains that cause less serious infections in humans and animals.

Answer 16

Attenuated mutants

Question 17

the exchange or transfer of genetic material between different
but closely related viruses infecting the same cell simultaneously, or between virus
and host cell. The alteration of genetic information may result from intramolecular
recombination, copy-choice recombination, reassortment or genetic reactivation.

Answer 17

Recombination

Question 18

Recombination: the exchange or transfer of genetic material between different
but closely related viruses infecting the same cell simultaneously, or between virus
and host cell. The alteration of genetic information may result from

Answer 18

Intramolecular recombination
Copy-choice recombination
Reassortment or genetic reactivation.

Question 19

occurs between two closely related DNA or RNA
viruses. For example, recombination of Sindbis and eastern equine encephalitis virus
has led to the formation of western equine encephalitis virus, another togavirus

Answer 19

Intramolecular recombination

Question 20

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. This process can occur in picornaviruses,togaviruses and coronaviruses

Answer 20

Copy-choice (template switching)

Question 21

: is another process of genetic recombination. It occurs between
viruses with segmented genomes, such as influenza virus A and B (8 segments), Reoviridae (10–12 segments), Bunyaviridae (3 segments), and Arenaviridae and
Birnaviridae (2 segments). An exchange of segments occurs between these viruses, resulting in production of new hybrid strains

Answer 21

Reassortment

Question 22

infectious progeny are produced from parental viruses, of which one or
both are non-infectious, following mixed infection of a cell

Answer 22

Reactivation

Question 23

when infectious progeny are produced from related viruses inactivated by lethal
mutations at different loci in their genomes.

Answer 23

Multiplicity reactivation

Question 24

occurs when an inactivated virus becomes capable of replicating after acquiring
genetic material from an infective virus.

Answer 24

Cross-reactivation or genome rescue

Question 25

is widely used for virus propagation; inoculation of chick embryos and experimental animals is employed for the isolation and production of
particular viruses.

Answer 25

Tissue culture

Question 26

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.

Answer 26

Propagation

Question 27

growth and maintenance of living tissue in vitro

Answer 27

Tissue culture

Question 28

: a tissue fragment is used to isolate viruses from animals with persistent infection.

Answer 28

Explant cultures

Question 29

the tissues are digested into individual cells by mechanical
cutting followed by digestion with enzymes such as trypsin. Three types of cell
culture:

Answer 29

Cell cultures

Question 30

: 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,

Answer 30

Primary cell culture

Question 31

cells of semi-continuous or diploid cell lines retain their characteristic diploid chromosomal constitution and can support the growth of a wide range of viruses. However, such cell lines, tend
to die out between the 30th and 50th passage.

Answer 31

Semi-continuous

Question 32

or immortal cell lines are derived fromeither normal or neoplastic tissue and can be passaged indefinitely. Such cell lines can be obtained commercially from organizations like American Type Culture Collection (ATCC).

Examples: HeLa cells, or
Madin Darby bovine kidney (MDBK

Answer 32

Continuous cell cell culture

Question 33

How to detect viral growth in cell cultures?

Answer 33

1. Using light microscopy, microscopic changes or cytopathic effect (CPE) 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. For non- cytopathic viruses, hemadsorption or binding of erythrocytes to the surface of
   infected cells can be observed.

Question 34

How to detect viral growth in cell cultures?

Answer 34

1. Using light microscopy, microscopic changes or cytopathic effect (CPE) 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. For non- cytopathic viruses, hemadsorption or binding of erythrocytes to the surface of
   infected cells can be observed.
2. Serological tests using flourescein-labelled antibody

Question 35

: though no longer extensively used, this remains
the preferred method for isolation of influenza A viruses and for many avian
viruses.

Answer 35

Inoculation on embryos

Question 36

: though no longer extensively used, this remains
the preferred method for isolation of influenza A viruses and for many avian
viruses.

Answer 36

Inoculation on embryos

Question 37

Inoculation on embryos
Modes of inoculation:

Answer 37

via the allantoic cavity, the amniotic cavity or the
yolk sac, chorioallantoic membrane (CAM), or intravascularly. Evidence of the
effect of viral growth includes death or dwarfing of the embryo and the formation
of pocks on the CAM.

Question 38

Routes for inoculation of viruses into embryonated eggs.

Answer 38

1. Into allantoic cavity
2. into amniotic cavity
3. into yolk sac
4. on to chorioallantoic membrane.

Question 39

Use of experimental animals: For ethical reasons, experimental infection
of animals is now infrequently used. However, for several virus families, animal
inoculation either on laboratory animals or natural hosts remains the preferred
procedure for the following studies:

Answer 39

(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