Viruses
I. Compared with either prokaryotic or eukaryotic cells, viruses are extremely simple
and extremely small. They have no organelles, no ribosomes and, though they may contain
one or two enzymes, they are not capable of carrying on the biochemical reactions needed
to replicate unless they are within either a prokaryotic (bacteria) or eukaryotic host
cell. A virus must infect a cell and use that cells biochemical machinery to make more
viruses. For this reason the are said to be obligate intracellular parasites.
The genetic material of a virus is limited, usually containing no more than 15-20 genes.
The bacteriophages are an exception in that they can have as many as 100 genes in their
DNA.
A. The genetic material of some viruses is DNA, while the genetic material of other
viruses is RNA. This genetic material is single stranded for some viruses while it is
double stranded for others. Viral particles that are capable of infecting a cell are
referred to as a virions. The structure of the virion will be discussed
shortly.
B. The proteins of a virion are unique to that type of virus. These proteins serve to
protect the nucleic acid in the virion and play a crucial role in the virus gaining entry
into a host cell.
II. Viruses are very specific about what they infect. Very few viruses are able to
infect the cell's of more than one or two closely related species. Those species that a
virus is capable of infecting defines the virus's host range.
A. Animal viruses, along with being able to only infect a limited range of species, are
also limited in the types of tissues that they can infect. In some cases one cell type
within a host will be infected while all other cell types are perfectly healthy.
B. Plant viruses are passed throughout the plant either by moving from cell to
cell through channels that connect plant cells to one another or by moving through the
fluid and nutrient transport systems (xylem and phloem).
C. Bacteriophages are viruses that infect bacterial cells. Like all
other viruses, they are capable of infecting a limited number of bacterial species. The
ability of phages to infect only particular strains of bacteria within a species can be
used to determine if an unknown bacteria is of a particular strain. This is referred to as
phage typing.
III. Viruses range in size from 20 - 100nm. (As a reference, a ribosome is around 15nm,
most bacteria are around 5000nm and a typical eukaryotic cell is around 50,000 to 100,000
nm in diameter.) Because of the small size of viruses they can not be seen under light
microscopes. Images are made using electron microscopes.
A. Long before the advent of electron microscopes, the presence of viruses had been
postulated. Experiments designed to determine the cause of certain diseases demonstrated
that the agent responsible for the disease was transmissible but would not grow in a Petri
dish nor could it be seen under a microscope.
B. It was also shown that these agents could pass through filters that were designed to
filter out bacteria (250nm pore size). Up until the 1940's viruses were usually referred
to as filterable agents.
C. Culturing of viruses in the laboratory relies upon growth of the virus in cells.
1. Initial culture techniques involved injection of viruses into chick embryos. Several
days later large quantities of virus could be harvested from the embryos. This is still
the method of cultivation used for cultivation of certain viruses (influenza virus is
almost sole cultivated in eggs).
2. In the mid-50's tissue culture methods allowed cells from freshly removed organs to be
grown in flasks. This type of culture is referred to as primary tissue culture.
Primary tissue culture could not be maintained for long so a constant supply of organs was
needed to provide cells. Recognition that cancerous cells divided continuously led to the
development of continuos cell lines which could be grown in flasks and
passed from one flask to another many times. This greatly reduced the work involved in
generation of tissue culture and increased the rate at which work on viral diseases could
be performed.
IV. The structure of most viruses is very simple and can be divided into three general
areas: the central genetic material (nucleic acid); a nucleocapsid
made of proteins that associates with the nucleic acid; an envelope of
phospholipids and proteins may or may not be present.
A. As mentioned earlier the genetic material of a virus may be RNA or DNA. The genes
for all the proteins that make up the virion, along with those proteins that allow the
virus to take over the cell and make copies of itself are found in the nucleic acid of the
virion.
B. In the virion the nucleic acid is found within a structure known as the nucleocapsid.
The nucleocapsid is made up of many repeating subunits of one or a few different proteins.
These protein subunits are referred to as capsomeres. The nucleocapsid
serves to protect the nucleic acid from digestion by extracellular enzymes. For those
virus referred to as "naked viruses" the virion consists only of
the nucleic acid contained within a nucleocapsid.
C. Enveloped viruses are surrounded by a phospholipid bilayer that is
similar to (and usually arises from) the cell's cytoplasmic membrane. Embedded in the
phospholipid bilayer are protein "spikes" that are specific for the virus.
The spike proteins are essential in allowing the virus to attach to cells so that they can gain
entry into that cell. A second protein commonly referred to as the
fusion protein is also found embedded in the envelope. This protein
allows the virus to fuse with the membrane of the cell it is going to
invade. A naked virus lacks the phospholipid
bilayer. The spikes and fusion protein of the naked virus extend from
the nucleocapsid.
1. Viruses that are enveloped are easily rendered harmless by any agent that will
disrupt fats (since the envelope is a phospholipid membrane and thus consists partially of
fat). Alcohol and strong detergents will usually reduce or eliminate infectivity of the
enveloped viruses.
2.Naked viruses on the other hand require agents that will destroy or
denature proteins to render them harmless. The most common of these agents is chlorine
bleach.
V. Multiplication of animal viruses involves the adsorption of the virus
to the cell, the internalization of the virus, the liberating of the nucleic acid into the
cytoplasm, the synthesis of virus specific proteins and nucleic acids, the assembly of
nucleocapsids that contain the viral nucleic acid and the release of the virus from the
cell.
A. Attachment involves interaction between
spike proteins on the outside of the virion with
proteins on the surface of the cell. These interactions are very specific. If the
particular protein is not present on the surface of the cell, the virus's
spike will not be able
to attach and the virus will not be able to infect that cell. This is one of the factors that limit
the host range of a virus.
B. The binding of the virus to the cell's surface triggers the internalization of the
bound virus. This is usually accomplished by endocytosis.
C. Once internalized the endocytotic vesicle is acted upon by enzymes that result in a
drop in the pH of the solution within the vesicle (acidification). This drop
in pH triggers the fusion of the virion's envelope (or nucleocapsid in the case of naked
viruses) with the membrane of the vesicle. This fusion exposes the nucleocapsid to the
cytoplasm.
D. Fusion of the envelope with the vesicle membrane appears to trigger the rapid
falling apart of the nucleocapsid. This results in the release of the viral nucleic acid
into the cytoplasm.
E. The genetic material of the virus can now direct the synthesis of proteins
needed both for the control of the cell and for making more virus particles. Before more
virus particles can be assembled there must be production of multiple copies of its
nucleic acid.
1. If the virus contains DNA, the DNA must travel to the nucleus. In the nucleus it
will be used by the cell's RNA polymerase as a template to synthesize those mRNA molecules
that will code for the proteins of the virus. These proteins will be used to take over the
cell and make new virions.
2. The nucleic acid of some RNA viruses acts like a mRNA molecule. Ribosomes use it as the
would any mRNA molecule to synthesize proteins. The proteins that are synthesized are
those proteins needed to take over the cell and make more virions.
3. One particular class of RNA viruses are the retroviruses. Within the
nucleocapsid of these viruses, along with the RNA are several enzymes known as reverse
transcriptase. Upon introduction into the cytoplasm, the RNA is used by the
reverse transcriptase as a template for the synthesis of a complementary strand of DNA
molecule. This DNA then moves into the nucleus where it is "spliced" into the
genetic material of the host cell through the action of an enzyme known as integrase. The virus responsible for AIDS is a retrovirus.
F. Maturation involves the assembly of nucleocapsids with a copy of the virus's
original nucleic acid inside. This process usually occurs in the cytoplasm.
G. The
final step of the viral life cycle is the release of virus particles from the
cell. This sometimes results in
the rupture of the host cell. More
commonly the viral particle leaves the cell by the process of exocytosis
or budding.
Budding does not immediately kill the cell though continued budding
stresses the energy supply of the cell and the cell’s ability to maintain
the integrity of it’s cell membrane. Usually
high levels of viral replication will eventually kill the cell.
This is referred to as cytopathic
effect.
VI. When viruses infect cells several different reactions may occur. These reactions can
range from death of the cell (cytopathic effect) to no apparent change in
the cells.
A. For viral infections in which a clinically acute phase occurs some level
of cytopathology (cell or tissue damage) occurs. This cellular destruction can
be due to the direct effect that the
virus exerts on the cell. Some viruses produce proteins that shut down the cells ability
to make proteins. This will result in the death of the cell in several days. Other viruses
exhaust the cell's metabolic machinery and disrupt the membrane as they multiply and
mature. In many viral infections the widespread cytopathology that is observed and
contributes to the clinical signs and symptoms caused by the virus is the result of
cellular destruction mediated by the body's immune response to the virus. Cytotoxic T
cells and natural killer cells (NK cells) destroy cells which are infected with viruses.
Furthermore, antibodies bound to viral proteins on the surface of infected cells activate
the complement cascade against these cells and direct our own defensive cells
to kill the infected cells.
1. With many DNA virus infections the nucleus of infected cells will display an
abnormal appearance referred to as an inclusion body. This often is a
diagnostic clue for identifying the causative agent of a disease.
2. A second pathological clue to the presence of a viral infection is the formation of a
mass of fused cells referred to as a syncytia.
B. In many viral infections the viruses do not kill the cell or have any immediate
impact on its function. The DNA of these viruses often lies quietly in the nucleus of
cells for years without producing more virus particles. This type of infection is referred
to as a latent viral infection. Several viruses appear to establish an
infection in which the virus continually produces viral particles but the immune system is
unable to fight the disease. This type of infection is referred to as a persistent
viral infection.
C. It was once felt that viral latency or persistance presented little or no clinical
significance since no disease was immediately associated with the infection. This is no
longer felt to be true due to the recognition that the presence of viral DNA in the
nucleus may increase the chances that a cell will become cancerous. This process is
referred to as cellular transformation. In many animals this type
relationship between viral latency and cancer has been firmly established. The current
knowledge about the relationship between viral latency and cancer in humans is not yet as
clear cut due to ethical constrains on human experimentation.
VII. Though the immune system will ultimately be needed to eliminate a virus from the
body, the initial line of defense against viral spread within the body lies in the ability
of an infected cell's ability to produce several proteins collectively referred to as interferon.
Released from an infected cells, they bind to the surface of neighboring cells resulting
in production of cytoplasmic enzymes within these cells which will destroy the nucleic
acid of a virus as it enters that cell. Several classes of interferon are currently
recognized (alpha, beta and gamma). Alpha and beta interferon are produced in many cells
types in response to viral infection. Gamma interferon is produced by lymphocytes in
response to exposure to viral antigens.
Want to know more? Here are some good links!!!!
Medical Microbiology Textbook
Introduction
to Virology http://gsbs.utmb.edu/microbook/intoviro.htm
Structure
and Classification of Viruses http://gsbs.utmb.edu/microbook/ch041.htm
Multiplication
http://gsbs.utmb.edu/microbook/ch042.htm
Effects
on Cells http://gsbs.utmb.edu/microbook/ch044.htm
Washington
State University Viruses
http://www.wsu.edu:8080/~hurlbert/pages/Chap11.html
University
of Connecticut http://www.sp.uconn.edu/~terry/229sp98/viruses.html