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Kan 'n virus homself verander. [boodskap #37554] Ma, 04 Desember 2000 00:00
Ernst v Biljon  is tans af-lyn  Ernst v Biljon
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Geregistreer: Maart 1999
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Sue het my die vraag gevra na aanleiding oor Malaria. Sy het my gevra of
ek nie dink aan die HIV virus as ek praat van 'n organisme wat himself
aanpas nie. Daar is baie 'organismis' wat in die menslike liggaam leef.
Party is lewend en and 'dood'. Party kan net leef as hulle 'n 'host'
oorneem. 'n Virus is so 'n eenheid. Dit neem 'n sel oor en gebruik dan
die sel se 'kern' om homself dan 'n nuwe 'kern' te gee. Sien bygaande:

What The Heck is a Virus?

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First, let me tell you what a virus is NOT. A virus is not a bacterium,
nor an independently-living organism. A virus cannot survive in the
absence of a living cell within which to synthesize copies of itself
(replicate). Antibiotics do not harm a virus; it is for this reason that
treatment for the "flu" for example, is mainly to help ease the symptoms
of the illness rather than to kill the organism which causes the "flu"
(Influenza virus Please see: What the Heck is the "Flu"?).
Well then, what IS it?
Now, is there a simple explanation which can define what a virus IS?
Hmmmm... that's actually a tough question. A virus is not strictly
alive.. nor is it strictly dead... A virus has some fundamental
information (genes made of DNA or RNA) which allows it to make copies of
itself. However, the virus must be inside a living cell of some kind
before the information can be used. In fact, the information won't be
made available unless the virus enters a living cell. It is this
entrance of a virus into a cell which is called a viral infection. Too,
the virus is very, very small relative to the size of a living cell.
Therefore, the information the virus can carry is actually not enough to
allow it to make copies (replicate). The virus uses the cell's machinery
and some of the cell's enzymes to generate virus parts which are later
assembled into thousands of new, mature, infectious virus which can
leave the cell to infect other cells. Poliomyelitis virus for example,
may have over one million copies of its basic genetic information (RNA)
inside a single, infected human intestinal mucosal cell.

What does a virus look like?
Moving from the outside to the inside, here are some parts of a virus
which are common to many different kinds of viruses: capsid, core,
genetic material (DNA or RNA). The capsid is the outer shell of the
virus which encloses the genetic material within. The capsid is actually
made of many, many identical individual proteins which are assembled
very precisely to form the capsid structure. Sometimes there will be a
protein core underneath the capsid which also surrounds the genetic
material. Some viruses may have an additional covering on the outside
called an envelope. An envelope is kind of like skin around the outside
of the virus. The envelope is actually a lipid bilayer (membrane) with
proteins embedded within the membrane. If you examine a baseball, take
it apart, you will see how some viruses are assembled. The cover of the
baseball (envelope), the tightly-woven thread (capsid), and the rubber
core (genetic material) can be used to represent the parts of some

What do viruses actually do?
All viruses only exist and make more viruses. And with the possible
exception of bacterial viruses which kill harmful bacteria, all viruses
appear to be harmful because their replication leads to the death of the
cell which the virus entered. A virus enters a cell by first attaching
to a specific structure on the cell's surface via a specific structure
on the virus surface. Depending on the virus, either the entire virus
enters the cell, or perhaps only the genetic material of the virus is
injected into the cell. In either case however, the ultimate result of
viral infection is the exposure of virus genetic material inside the
entered cell. Then, the virus material essentially "takes over" the cell
and nothing but viral parts are made, which assemble into many complete
viruses. These viruses are mature and leave the cell either by a process
called "budding" (just one or a few viruses at a time leave the cell) or
by a process called lysis (the cellular membrane ruptures and releases
all of the virus particles at once).

What things can become infected by a virus
So far, there is not a living thing identified that doesn't have some
sort of susceptibility to a particular virus. Plants, animals,
bacteria - every living thing, whether multicellular or single-celled,
can be infected with a virus specific for the organism. And, within a
species, there may be 100 or more different viruses which can infect
that species alone. So, whenever viruses are discussed, they are
discussed as being either plant, animal or bacterial viruses - which
means that an animal virus only infects a certain animal, and a plant
virus only infects a particular plant. We say that a virus is specific
for a particular thing if the virus infects only that thing. So, there
are viruses which infect only humans (smallpox), some which infect
humans and one or two additional kinds of animals (influenza), some
which infect only a particular kind of plant (tobacco mosaic virus), and
some which infect only a particular species of bacteria (lambda
bacteriophage which infects E. coli).

How many kinds of viruses are there?
Viruses come in all shapes and sizes and have an enormous array of
different kinds of organization of basic genetic material within them.
And, it is the arrangement and type of genetic material which is the
method used for sub-classification of a given group of viruses. For
example, the animal virus group can be sub-divided into the following
sub-groups: double-stranded DNA; single-stranded DNA; double-stranded
RNA; single-stranded RNA, and, retroviruses (a very unique kind of
single-stranded RNA virus). An example of a human double-stranded DNA
virus is Epstein-Barr virus (EBV). An example of a human single-stranded
RNA virus is Influenza virus, Type A). An example of a human retrovirus
is Human Immunodeficiency virus (HIV). As you can see, unlike humans,
the arrangement and kind of genetic material within viruses can be very
diverse. However, for all viruses, regardless of the kind or arrangement
of genetic material, the virus is capable of replicating within a living
cell and can produce progeny (offspring) which are usually absolutely
identical to the original virus. You may wish to take a look at David
Sander's information about viruses. Please see: David Sander's Complete
Virology Information, Tulane

Do viruses change
Sometimes during the process of viral replication, mutations occur. If
the mutation is harmful, the new virus particle might no longer be
functional (infectious). However, because a given virus can generate
many,. many copies of itself, even if 200,000 particles are no good, 100
might still be just fine. Further, some mutations don't lead to harm to
the virus, but instead lead to a functional but now brand-new strain of
virus (Influenza virus can do this; consequently, there are several
different strains of this virus which have to be identified each year in
order to make a vaccine against the particular strain which might cause
the "flu").

What protects things against viral infection?
Humans are protected in a couple of ways. First, if a particular virus
infects one or more cells of a given tissue in our body, the infection
leads to the synthesis and secretion of substances called interferons.
Interferons are proteins and may be designated as alpha, beta, or gamma
interferon). These proteins interact with adjacent cells which help
adjacent cells become more resistant to infection by the virus.
Sometimes, this resistance isn't quite good enough to prevent the spread
of the virus to more and more cells, and we begin to feel sick (we are
now experiencing a disease caused by the viral infection). Now however,
the body's immune system takes over and begins to fight the infection by
killing the virus on the outside of the cells, and kills the infected
cells, too. The killing of the infected cells prevents the spread of the
virus, since as was stated above, a virus requires a living cell in
order for the virus to be able to replicate. Eventually, the virus will
be completely removed, and we'll get over the illness. HIV is an
exception to this situation because HIV infects cells of the immune
system which are necessary to kill the infected cells. So, although HIV
does not itself directly cause the condition known as AIDS, the eventual
death of immune cells due to infection with HIV allows other infections
to harm a person.

Recently, there have been agents designed in the laboratory and isolated
from natural sources which are being used to fight certain viral
infections. These agents are not called antibiotics however, since they
are effective only for viruses and have not been isolated from other
organisms capable of killing a virus. So far, no agents have been
identified which are secreted by a cell which actually kills a virus.
You may be familiar with the drug called Acyclovir which is used to
inhibit the replication of Herpesvirus; and, AZT and HIV protease
inhibitors which are used to inhibit the replication of HIV.

Plants are protected from certain viruses by substances which coat
leaves and stems and "closing-off" systems which generate a walled-off
area within the plant at the source of the infection. Bacteria can be
protected from bacterial-specific viruses through the action of enzymes
inside the bacterium's cell. However, if a bacterial virus (called a
bacteriophage) infects one cell, usually within a very short time, all
of the bacterial cells will be killed. If there are no other bacterial
cells of that particular species around for that particular virus,
however, the virus will die, too.
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