BIOLOGY
LESSON #45
VIRUS REPRODUCTION
1. Read pages 288-290
in the textbook.
2. Record the following terms in your Biology Journal: bacteriophage,
receptor sites, prophage, transduction.
3. Read over
the Lecture Notes below.
4. Finish the Section Questions on page 290 1-5.
5. Diagram the Lytic Cucle.
LECTURE
NOTES:
VIRUSES-A GROUP
OF INTRACELLULAR PARASITES
Structure of Virus
Viruses
can be considered as the smallest organized, infective structures capable of
replicating themselves in a living cell system. They are not cells, but
packages of genetic information that insert themselves into a host cell and
direct its metabolic machinery to make more viruses. They are composed of
either DNA or RNA, containing necessary genetic information for the replication
and assembly of identical progeny within the host cell. In order to protect the
genetic material, the virus possesses a coat of protein or lipoprotein
molecules assembled according to precise geometrical or morphological designs.
Viral nucleic acid can infect cells on its own and produce complete viruses
possessing protein coats.
Life Cycle
Entering
the Host Cell
Because
viruses are acellular and do not use ATP, they must
utilize the machinery and metabolism of a host cell to reproduce. They are
obligate intracellular parasites. Before a virus has entered a host cell, it is
called a virion, a package of viral genetic material.
Virions can be passed from host to host through
direct contact or through a vector (an animal carrier who is not affected by
the virus--often insects). Inside the organism, the virus can enter the cell in
various ways. Bacteriophages have capsoids
with tails that attach to the cell wall at specific protein markers on the wall
surface. Once attached, enzymes on the tail core act
like lysozome enzymes and make a small hole in the
cell wall. The virus injects its DNA into the cell like a syringe and leaves
its empty ghost behind, attached to the outside of the cell wall. Other viruses (like HIV)
have capsoids enclosed in a lipid bilayer
envelope and enter the host via endocytosis. Glycoproteins stud the outer leaflet of the membrane
envelope and bind to receptor sites along the host cell's plasma membrane.
After entering the cell, the capsoid is broken down
and the virus's genetic material begins the destructive process of taking over
the cell and forcing it to produce new viruses
Replication
of the Viral Genome
There
are three different ways genetic information can be reproduced, and they are
governed by the kind of nucleic acid molecules and enzymes the virus carries in
its capsoid. Some viruses have DNA, which once inside
the host cell is replicated by the host along with its own DNA. Bacteriophages are DNA-containing viruses. There are two
kinds of reproduction by viruses containing RNA. One kind copies the RNA
directly with an enzyme (RNA replicase) that uses the
copy as a template to make hundreds of duplicates of the original RNA from the
host cell's nucleotide material. The second group of RNA-containing viruses is called
retroviruses. Instead of copying the RNA directly, they use an enzyme (reverse
transcriptase) which assembles a complimentary strand, so that the virus's
genetic information is contained in a molecule of DNA rather than RNA. This is
very different from normal transcription, because DNA is made from RNA rather
than vice versa, hence the name "reverse" transcription. HIV is an
example of a retrovirus.
Proviruses, Prophages, and the Lytic Cycle
Once the virus's genetic material has entered the
cell, two things can happen. Some viruses destroy the host's DNA, breaking it
down into smaller nucleic aid chains that can be used for replicating the
virus's own genome. Other viruses leave the host DNA intact and incorporate
their own into the host's chromosomes. Once spliced into the host genome, the
virus's genetic material is copied along with the host's during regular mitosis
or meiosis. Several generations can go by where the offspring of the infected
host, each containing viral genetic material, show no signs of viral infection.
When viral DNA is incorporated into the host's chromosome as a latent form,
this is called a provirus in the case of animal and plant viruses and a prophage in the case of bacterial viruses. The herpesvirus
can remain latent as a provirus in a host indefinitely only to be excised
from the host's genome in times of stress. This is why someone can have
recurrent infections long after he has seemingly been cured. Bacteriophages can remain latent as prophages
until a disturbance such as ultraviolet radiation or chemicals remove them from
the host genome. An outwardly healthy colony of bacteria which contain latent prophages that can be awakened by such a disturbance is
called lysogenic bacteria.
Protein
Production
When
the virus has taken over the cell, it immediately directs the host to begin
manufacturing the proteins necessary for virus reproduction. The host produces
three kinds of proteins: early proteins--enzymes used in nucleic acid
replication, late proteins--proteins used to construct the virus coat, and lytic proteins--used to break open the cell for viral exit.
The capsoid material is made from the host's
resources. The final viral product is assembled spontaneously, that is, the
viruses do not grow their parts as most organisms do,
their protein parts are made separately by the host and come together by
chance. This self-assembly is often aided by "molecular chaperones",
proteins made by the slaving host that help the capsoid
parts come together but are not a part of the virus itself.
THE LYTIC
CYCLE
The lytic
cycle, although slightly different for various viruses generally follow these
steps:
1. The phage attaches itself to the cell
2. It injects the DNA into the host
3. DNA is replicated by the host cell.
4. Instructed by the viral DNA, the host manufactures viral proteins.
5. Self-assembly occurs and the new viruses burst out of the
cell-destroying the cell.
6. After lysis, the cycle begins again.
