Download Replication of Viruses

Replication of Viruses
Viruses are specific to a particular host cell because part of the capsid bind to the receptor
on the host cell’s plasma membrane in a lock and key fashion. The virus then will inject
its viral nucleic acid into the host cell and it will take over the metabolic reaction of the
cell so new viruses will be made.
Replication of Bacteriophages
Bacteriophages are viruses that are parasitic to bacteria. These phages must use bacteria
to replicate. Phages have two cycles. The lytic cycle is when the host bacterial cell lysis
and breaks open to release the newly formed viruses. The lysogenic cycle is when there
is viral replication, but will be complete some time in then future.
Lytic Cycle
There are five key stages to the lytic cycle.
1) Attachment Stage – Capsid combines with the bacteria’s receptor in a lock
and key fashion.
2) Penetration Stage - Enzymes from the virus start to eat away the bacterial cell
wall and will inject its’ viral DNA in the bacterial cell.
3) Biosynthesis Stage – at this stage the virus inactivates the bacterial genes and
takes over the metabolic functions to produce copies of DNA and produce the
capsid.
4) Maturation Stage – During this stage, the viral DNA and capsids are put
together.
5) Release Stage – Lysosomes are made and produce hydrolytic enzymes to eat a
hole in the cell wall of the bacteria. At this time, the cell will lyse and the
newly formed viruses will leave the host cell.
Lysogenic Cycle
After the attachment the penetration stage, the viral DNA attaches to the bacterial DNA,
but does not replicate and destroy the bacterial DNA. During the lysogenic cycle, the
phage becomes a prophage that is integrated into the host genome (DNA). At a later
time, the phage may start the lytic cycle. When a virus is latent, not replicating, it is
called a prophage. At any time, factors such as x-rays or ultraviolet light may initiate the
prophage to start the lytic cycle.
Replication of Animals Cells
Animal cells are quite different when replicating. In fact, the entire animal virus enters
the bacterial cell. Once inside, the animal virus loses its capsid and either the viral DNA
or RNA and biosynthesis occurs. Another difference that occurs in animal’s cells is that
when animal viruses leave the cell, it leaves by a process called budding. This means that
a portion of the animal cells membrane is wrapped around the virus.
Retroviruses
Retroviruses are viruses that have the nucleic acid RNA. This virus contains the enzyme
called reverse transcriptase. This enzyme allows the RNA to do DNA transcription. The
resulting DNA strand is a copy of the viral genome. New viruses are produced when the
DNA is transcribed and is released from the cell by budding. An example of a retrovirus
is HIV and some cancers.