The Replication of Viruses
(Answer Sheet)
Viruses must infect cells in order to reproduce themselves. Much of
what we know about viral replication comes from studies of bacteriophage
viruses that infect bacteria. The common intestinal bacteria Escherichia
coli can be infected by T-phage who go through a lytic cycle (transduction),
or by Lambda phage who stay dormant once they enter a cell. This is called
LYSOGENY.
How T-Bacteriophage Infect Bacteria
The first stage of T-phage infection is attachment to the cell wall of
a bacterium. The phage does this using its tail
fibres
. The tail
of the phage then contracts downward. The tail releases an enzyme
penetrates the wall and nucleic
acid
which
is injected into the bacterial
cytoplasm. This stage of infection is called penetration
of the virus is left outside the cell.
Figure 1 – The Lytic Cycle
. The capsid
Figure 2: The Lytic
Cycle
Now the synthesis stage begins. Using building blocks found in the bacterial
cytoplasm, numerous copies of the nucleic
acid
and many protein
molecules are produced. In the next stage of the cycle (the assembly
stage), these molecules will be used to form new bacteriophages. The cycle
ends with the release of the new virus particles by cell
It is this rupture that gives the lytic
curve is plotted, an initial latent
lysis
.
cycle, its name. If a growth
period is seen. This corresponds to
the stage of infection which takes place inside
the bacterium. A rapid
increase follows due to the release of many new T-phage viruses. The plateau
of the growth curve results when all of the bacteria in a culture are
infected.
Summary
1:Bacteriophage finds a suitable host;
2:The viral DNA is ejected through the tube into the bacteria;
3:The viral DNA takes over the bacterial machinery to produce new protein
and DNA for the virus;
4:The bacterial particles assemble in the bacterial cell; and
5:The bacterial cell lyses to perpetuate the cycle of the bacteriophage.