Viruses
Viruses are responsible for many diseases and illnesses. Some are
deadly and others can cause permanent damage, yet while others still
again only cause temporary damage and your body can recover. Some
diseases caused by viruses: Polio, Influenza, AIDS, Warts, Rabies,
Colds, Hepatitis There are two main categories of viruses, DNA or
RNA.
Characteristics of a virus:
-Sub-cellular, (very small) 25nm to 250nm -They have no nucleus,
organelles, cytoplasm, ribosome’s, or membrane. -Show only one
characteristic of life: Reproduction -Viruses do not have: locomotion,
metabolism, any other life function. -Can only reproduce when in a
living cell.
Do viruses obtain and use energy?
A virus is not a cell, it does not need food. It does need material to
reproduce, but it does not require energy, technically.
Do they then grow, develop and die? In the host cell, the viruses are
produced as complete entities, technically there is no growth. Viruses
DO have the ability to adapt over time to a new environment, so one
could state that they do develop. Dying is also something a virus can
do. Over time it loses its ability to invade new host cells and
effectively “dies out”. Viruses like HIV only live a few hours when they
aren’t in reach of new host cells. Most viruses are quite weak, but
some have been known to stay dormant for a number of years.. Some
viruses can "sleep" inside the genetic instructions of the host cells for
years before reproducing. For example, a person infected with HIV
can live without showing symptoms of AIDS for years, but they can
still spread the virus to others..
1
Do viruses respond to their environment?
Well, yes and no. Viruses are inert. They can not move by themselves,
but always need some means of transport, be it a fluid or the wind. As
I said above, viruses adapt to their environment by constantly
changing their DNA or RNA. These mutations make it very hard to
effectively cure viral diseases – the virus is always trying to stay ahead
of the game..
So there it is, regarding a virus as a living organism could be argued.
“Are viruses alive?
This question is more difficult to answer because it depends on a
definition of life. Suppose our definition includes the idea that living
things are able to reproduce. A dog is obviously alive and is made up
of living cells, but a spayed dog cannot reproduce and its genetic
information dies with it; yet is alive. We may, on the other hand,
define life as the possession of specific genetic information capable of
functioning in living cells. Then the cells of the spayed dog are clearly
alive, and so are viruses, which can multiply in living cells. Viruses
reproduce and evolve if they have suitable host cells available. Are
viruses any different from animals or plants, which also require
specific external conditions to propagate their species? To the
biologist, a virus is alive because it participates in the adventure of
biological evolution.”
2
Components or Parts of a typical Virus



The Capsid - The Capsid of a virus is basically its "brains." It
contains an outer protein coat which is wrapped around a
central core of a highly complex chemical called nucleic acid.
Typically, the capsid is divided into distinct subunits called
capsomeres. X-rays have shown that viruses have an
icosahendron capsid (30 sides).
The Body - Viruses have a highly complex symmetry,
somewhat like the Surveyor space craft send to explore the
moon. Attached to the head (capsid) is a rod like structure that
consists of a retractible sheath surrounding a central hollow
core.
The Tails - At the very end of the core is a spiked plate carrying
6 slender tail fibers which help anchor the virus to its host.
3
Above is a schematic drawing of a type of virus which is called a
“bacteriophage”. Bacteriophages – or just phages - are typically large viruses,
which attack bacteria. The phages were first discovered around 1916. They
have been much used in the study of bacterial genetics and cellular control
mechanisms largely because the bacterial hosts are so easily grown and infected
with phage in the laboratory. Phages were also used in an attempt to destroy
bacteria that cause epidemic diseases, but this approach was largely abandoned
in the 1940s when antibacterial drugs became available. The possibility of
“phage therapy” has recently attracted new interest among medical researchers,
however, owing to the increasing threat posed by drug-resistant bacteria.
Summary of how viruses do their damage: the Lytic Cycle.
1. Attachment
2. Entry of nucleic acid into the host cell
3. Viral DNA replication
4. Synthesis of virus particles
5. Assembly of virus particles
6. Release of virus progeny (this may or may not destroy the host cell)
textbook.
4
Viral infection, what really happens?
Most viruses are harmless, and do not cause diseases. But there are viruses that
can cause paralysis (polio virus) and even cancer. There are viruses that make
you sick by releasing toxics that are the result of their cellular invasion, and
others make you sick, because they destroy cells that function to keep your
body from getting sick.
Let’s see what happens when you catch a cold.
1. An infected person sneezes near you.
2. You inhale the virus particle, and it attaches to cells lining the sinuses in
your nose.
3. The virus attacks the cells lining the sinuses and rapidly reproduces new
viruses.
4. The host cells break, and new viruses spread into your bloodstream and
also into your lungs. Because you have lost cells lining your sinuses, fluid
can flow into your nasal passages and give you a runny nose.
5. Viruses in the fluid that drips down your throat attack the cells lining your
throat and give you a sore throat.
6. Viruses in your bloodstream can attack muscle cells and cause you to have muscle
aches.
Why is HIV different?The HIV virus attacks T-cells in the immune system,
eventually causing AIDS (acquired immunodeficiency syndrome). There is no
such thing as an AIDS virus. HIV progressively destroys the body's ability to fight
infections and certain cancers. People diagnosed with AIDS may get lifethreatening diseases called opportunistic infections, which are caused by
microbes such as viruses or bacteria that usually do not make healthy people sick.
5
6
7