VIRUS REPLICATION
Animals were first used for experimental or diagnostic work, followed by chick embryos
and finally cell cultures. Numerous types of animal cell culture have found application in
virology. The choice of species, tissue of origin, and type of culture (primary, cell strain,
or cell line) depends on the virus and experimental objectives. Each animal virus can
replicate only in a certain range of cells. Among non-susceptible cells, some have a block
at an early step. (e.g. they lack suitable receptors or a factor required for expression of
viral genes), so that the expression of all viral functions is prevented (resistant cells)
Other cells lack a factor required for a later step, so that some, but not all viral activities
are expressed (non-permissive cells). In either case, a heterokaryon formed by fusing a
susceptible and a non-susceptible cell has the required functions and is usually
susceptible.
Role of nucleic acid - transfection
The cells of higher organisms can be infected by naked viral nucleic acid, yielding
normal virions. There are several important differences between infections by nucleic
acid (transfection) and by virions.
1. The efficiency of infection with nucleic acid is much lower, by a factor of 10-6 to 10-8
in ordinary media, showing the importance of the viral coat in infectivity. The efficiency
can be increased by the precipitation of viral DNA onto cells with calcium phosphate,
injection or packaging into liposomes.
2. The host range is much wider with nucleic acids, which can infect resistant cells e.g.
chicken cells, although resistant to poliovirus because they lack receptors for the virions,
are susceptible to its RNA, but only 1 cycle of viral multiplication takes place because the
progeny are again virions and cannot spread to other cells because of the lack of suitable
receptors or essential factors.
3. Infectious nucleic acid can be extracted even from heat- inactivated viruses in
which the protein of the capsid has been denatured; the nucleic acid can withstand much
higher temperatures than the protein. The ability of nucleic acid infectivity to survive
damage to the viral coat must be considered in the preparation of viral vaccines.
4. With some RNA viruses, a DNA copy of the viral RNA is infectious. e.g.
polioviruses, this permits the preparation of viral genomes, such as those of vaccine
strains in high quantities by avoiding the high mutation rate in replication of RNA and its
lability.
5. The infectivity of nucleic acid is unaffected by virus-specific ABs, which suggests
that this form of a virus could be an effective infectious agent even in the presence of
immunity. However, nucleases in the body probably greatly limit its role. Of the animal
, viruses, papovaviruses, adenoviruses, some herpesviruses, togaviruses and picornaviruses
yield infectious nucleic acid. With retroviruses, infectious DNA can be extracted from
infected cells or can be made by copying the viral RNA in vitro.
Steps in Viral Replication
The following steps take place during viral replication;-
1. Adsorption
2. Penetration
3. Uncoating
4. Viral genome replication
5. Maturation
6. Release
1. Adsorption
Animals were first used for experimental or diagnostic work, followed by chick embryos
and finally cell cultures. Numerous types of animal cell culture have found application in
virology. The choice of species, tissue of origin, and type of culture (primary, cell strain,
or cell line) depends on the virus and experimental objectives. Each animal virus can
replicate only in a certain range of cells. Among non-susceptible cells, some have a block
at an early step. (e.g. they lack suitable receptors or a factor required for expression of
viral genes), so that the expression of all viral functions is prevented (resistant cells)
Other cells lack a factor required for a later step, so that some, but not all viral activities
are expressed (non-permissive cells). In either case, a heterokaryon formed by fusing a
susceptible and a non-susceptible cell has the required functions and is usually
susceptible.
Role of nucleic acid - transfection
The cells of higher organisms can be infected by naked viral nucleic acid, yielding
normal virions. There are several important differences between infections by nucleic
acid (transfection) and by virions.
1. The efficiency of infection with nucleic acid is much lower, by a factor of 10-6 to 10-8
in ordinary media, showing the importance of the viral coat in infectivity. The efficiency
can be increased by the precipitation of viral DNA onto cells with calcium phosphate,
injection or packaging into liposomes.
2. The host range is much wider with nucleic acids, which can infect resistant cells e.g.
chicken cells, although resistant to poliovirus because they lack receptors for the virions,
are susceptible to its RNA, but only 1 cycle of viral multiplication takes place because the
progeny are again virions and cannot spread to other cells because of the lack of suitable
receptors or essential factors.
3. Infectious nucleic acid can be extracted even from heat- inactivated viruses in
which the protein of the capsid has been denatured; the nucleic acid can withstand much
higher temperatures than the protein. The ability of nucleic acid infectivity to survive
damage to the viral coat must be considered in the preparation of viral vaccines.
4. With some RNA viruses, a DNA copy of the viral RNA is infectious. e.g.
polioviruses, this permits the preparation of viral genomes, such as those of vaccine
strains in high quantities by avoiding the high mutation rate in replication of RNA and its
lability.
5. The infectivity of nucleic acid is unaffected by virus-specific ABs, which suggests
that this form of a virus could be an effective infectious agent even in the presence of
immunity. However, nucleases in the body probably greatly limit its role. Of the animal
, viruses, papovaviruses, adenoviruses, some herpesviruses, togaviruses and picornaviruses
yield infectious nucleic acid. With retroviruses, infectious DNA can be extracted from
infected cells or can be made by copying the viral RNA in vitro.
Steps in Viral Replication
The following steps take place during viral replication;-
1. Adsorption
2. Penetration
3. Uncoating
4. Viral genome replication
5. Maturation
6. Release
1. Adsorption
