Biology 11
Ms. Jamieson
Virus Replication
Viral replication is the formation of new viruses during the infection
process in a host cell.
• A virion cannot reproduce by itself. In order to reproduce, a
virion must invade, or infect, a living cell. It uses the host
cell’s materials and enzymes to produce new virions.
• Viruses are parasites: organisms that depend entirely upon
other living organisms for their existence in such a way that
they harm that organism.
1)
There are 2 main types of virus replication. Most viruses use one or
the other, a few viruses may use both.
We will observe the T4 bacteriophage as an example:
A) Lytic Replication
• The virion enters the cell, makes many copies of itself, and
then releases the offspring by lysis (bursting the cell) or by
budding
• Usually results in the destruction of the infected cell
• Viruses that reproduce by this method are referred to as
virulent
Stages in Lytic replication:
1. Attachment - the virion attaches to the bacterium’s cell
wall
2. Injection – The bacteriophage injects its genetic material
into the bacterium
3. Replication/Synthesis – the genetic material of the
bacteriophage takes over the bacterium’s metabolism, to
synthesize new bacteriophage proteins and nucleic acids
4. Assembly – newly synthesized bacteriophage proteins and
nucleic acids are assembled into complete bacteriophages
5. Lysis – A bacteriophage enzyme lyses the bacterium’s cell
wall, releasing new bacteriophages
2)
3)
4)
At 37°C, a Lytic infection takes 16 – 20 minutes from
Attachment to Lysis
5)
B) Lysogenic Replication
• The virion enters the cell and inserts itself into the host
cell’s DNA.
• It does not directly make new copies of itself, but is
passively replicated with the host cells genome, and
passed on to all the offspring.
• Viruses that reproduce by this method are referred to
as temperate
Stages in Lysogenic Replication:
1) Attachment - similar to Lytic Infection
2) Injection - similar to Lytic Infection
3) Insertion - the bacteriophage genetic material is
inserted into the DNA of the host cell
• the virus is now called a prophage (or a
provirus if its an animal virus)
4) Replication - the prophage stays inactive in the host
cell’s DNA for many generations. When the host cell
DNA is replicated, so is the virus.
1) a virus in this state is referred to as latent.
5) Reactivation - occasionally, environmental conditions
will trigger the viral DNA to become an active virulent
lytic virus
6) Synthesis and Assembly – similar to Lytic infection
7) Lysis – similar to Lytic Infection
Benefits of lysogenic infection for the host cell:
• The prophage blocks entry of other viruses into
the cell
• The prophage adds useful genes to the host
cells’ DNA that helps the cell survive
Drawbacks of lysogenic infection:
• Occasional reactivation, turning the prophage
into a lytic virus, and leading to some of the
cell’s descendants being lysed
1)
2)
3)
4)
5)
6)
7)
Variations in Virus Replication
Enveloped Animal Viruses
• These viruses are surrounded by an envelope that came from
the host cell membrane when the virus was formed.
• Attachment and Entry: The viral envelope fuses with the cell
membrane to bring the virion into the cell
• Once inside the cell, the capsid is then removed, releasing
the genetic material
• Release: These viruses do not lyse the host cell when they
are release. Newly formed viruses “bud” from the cell,
wrapping themselves in the host cell membrane in the
process
Episomes
(from “epi” = “additional” and “some” = “body”)
• Small circular DNA molecules that are not part of a host cell’s DNA
• Some viruses persist in the cell in an inactive (latent) state as episomes
Nuclear Episomes
• are found within the nucleus
• eg: herpesviruses, adenoviruses, polyomaviruses
• During cell division (mitoses) the viral DNA attaches itself to
host cell chromosomes
• the viral DNA is replicated along with the host cell DNA and
kept within the nuclear membrane
Cytoplasmic Episomes
• some virus episomes are found in the cytoplasm
• eg: poxviruses
Reactivation
• At some point, latent episomes can be reactivated by
environmental triggers (eg: UV radiation, stress hormones)
and become virulent
Retroviruses
(retro = “backward”)
• contain RNA as their genetic information
• when they infect a cell, they produce DNA from RNA by the process of reverse transcription
• the new viral DNA is then inserted into the host cell DNA.
• The virus is now called a provirus
• eg: HIV