Multiplication cycle of bacteriophages: lytic vs lysogenic cycle
Lytic cycle or lytic phages called as
virulent phages multiplies inside the host bacterium and new viral particles
comes out by lysing or by rupturing the host bacterial cell wall.
Eg: T phages, T2, T4, T6 etc.
Lysogenic cycle or lysogenic phages
called as temperate phages does not undergo multiplication or induce lysis, here
the viral DNA gets integrated into the bacterial DNA without causing lysis. Eg
Lambda phages.
Lytic cycle in
detail:
Step I :
Adsorption: attachment
of adsorption of tail fibres of the phage on to a specific receptor site on the
bacterial cell wall.
Step II:
Injection: injection of
viral genome into the host through the hollow tubes of the tail
Step III:
Protein
synthesis: Inside the host, the viral genome directs the synthesis of viral
proteins using the machinery of the host. Viral genome generally encodes for
some enzymes and coat proteins.
Step IV:
Viral genome
synthesis: Viral genome replicates inside the host making several
copies.
Step V:
Packaging and
release: The viral genome gets packaged inside the protein
coat.
These intact mature
infectious particles are called virions. The crowding of virions inside the
host ultimately causes cell lysis and liberation of mature viral particles. ~
about 200 mature phages/cell are liberated.
Lysogenic cycle: only
differ from step III onwards.
Look at the figure
Step I:
Adsorption: attachment
of adsorption of tail fibres of the phage on to a specific receptor site on the
bacterial cell wall.
Step II:
Injection: injection of
viral genome into the host through the hollow tubes of the tail.
Step III:
Integration of viral genome
to the host genome: After the entry of viral genome, it gets
integrated into the bacterial genome of the host. The viral genome integrated
to the bacterial genome is termed prophage.
Step IV:
Viral genome
synthesis: Viral genome replicates along with the bacterial genome
replication and pass on to the daughter cells.
Step V:
Induction of lytic
cycle: Occasionally, integrated viral genome detaches and released into the
bacterial cytoplasm. This dissociation is called induction and
lytic cycle is followed releasing mature lysogenic phages. Induction can be
induced artificially using ultraviolet rays or heat treatment.
And that is the viral story of
destroying bacterium, the microbial super powers.
