Viruses
a Brief History
Scientists
discovered viruses while studying a disease in plants called tobacco mosaic
disease. The disease could wipe out whole crops of tobacco, causing serious
economic losses for the growers. One of the scientists searching for the cause
of the disease was Russian botanist Dmitri Ivanovski. In 1892, Ivanovski
crushed infected tobacco leaves to extract juice from the plants. He then
passed the juice through a filter with very small pores. Because bacteria, the
disease-causing particles known at the time, could not pass through the small
pores, Ivanovski expected to find the germs trapped in the filter. To make sure
that they had been trapped and were no longer in the plant juice, he coated
healthy plants with the filtered juice. To his surprise, the healthy plants
became sick. Although Ivanovski realized that the infectious particles must
have been smaller than any known bacteria, he still believed that a type of
bacterium was causing the disease. Other scientist, a Dutch botanist Martinus
Beijerinck, showed that the disease-causing particles did not behave like
bacteria. Beijerinck named this small organism a virus.
In
the 1930's, scientists found ways to remove viruses from plant cells and study
them. With the invention of the electron microscope, it became possible to see
viruses for the first time. Using these new methods and tools, scientists found
many new types of viruses. Virologists, scientists who study viruses, also
found how viruses survive and reproduce in cells, changing themselves and the
cells that they infect.
Viruses
are very small and compact. Most can only be seen with an electron microscope.
The size of these tiny organisms is measured in nanometers. A nanometer is
about 1/25 millionth of an inch. The smallest viruses, such as the common cold
viruses, are 20 to 30 nanometers in diameter, while the largest viruses, such
as the smallpox viruses are 200 to 300 nanometers in diameter. To get an idea
of the size of viruses in relation to cells, think of a large ocean liner, a
tugboat, and a rowboat. The tugboat next to the large ocean liner would be
about the size of a large virus compared to a cell. The small rowboat next to
the ocean liner would be about the size of a small virus compared to a cell.
Viewing
viruses with the electron microscope, scientists were also able to examine
their shapes. Viruses come in several shapes: helical, polyhedral, and
complex. The tobacco mosaic virus, a helical virus, looks like a long rod
with a hollow core. Polyhedral viruses look like soccer balls. They look almost
spherical, but they are made up of many triangular sides, or faces.
Viruses
are the smallest creatures known. They cause diseases in humans, such as the
common cold, chicken pox, and AIDS. Viruses can infect not only humans, but
every living thing plants and animals. The name "virus," means
"poison." Viruses are fascinating creatures. Scientists are not even
sure whether they are alive or whether viruses are just organized packages of
chemicals that take over cells. Their existence prompts many questions: How can
such small and simple creatures survive and reproduce?
What
is a virus? Viruses can only live and reproduce within
the cells of other organisms. Because viruses usually give nothing useful to
the cell they infect and often damage it, we call them parasites. The skin
sores in chickenpox are an example of this cell damage. Viruses lack the basic
machinery of life. They cannot make their own essential molecules, such as
proteins and carbohydrates. They also cannot produce ATP, the basic energy
carrier of a cell. These tiny creatures act like cell pirates, taking over and
using the chemical factories of the host cell and most of its production
molecules, which are called enzymes, to survive.
Viruses
are made up of a piece of genetic code, such as DNA or RNA, and protected by a coating
of protein called a capsid. The capsid gives the virus its
characteristic shape. In some types of viruses, a protective membrane called an
envelope covers the protein coat. The virus gets this membrane, which is made
of fatty acids and proteins, from the cell it infects. Viruses that acquire
this membrane are called enveloped viruses. Viruses that do not acquire an
envelope are called naked or non-enveloped viruses.
Not
all envelopes are the same. Some viruses put their own identifying marks, in
the form of molecules, into the envelope. These molecules, called spikes, stick
out from the envelope like stiff hairs. Spikes are important to the virus
because they help it attach to other cells and enter them.
Once
a person is infected, viruses invade host cells within the body. They then use
the components of the host cell to replicate, producing more viruses. After the
replication cycle is complete, these new viruses are released from the host
cell. This usually damages or destroys the infected cells. Some viruses can
remain dormant for a time before multiplying again. When this happens, a person
appears to have recovered from the viral infection but gets sick again.
Antibiotics do not kill viruses and therefore are ineffective as a treatment
for viral infections. Antiviral medications can sometimes be used, depending on
the virus.
An
illness caused by a virus can occur only after an individual is directly
exposed to a virus. People, insects, food, and water can harbor viruses and
transmit them to others. But to become infected, individuals must not only be
exposed, they also must receive a sufficient dose of the virus. Some viruses
cause infection only when they are present in very high numbers; others that
are highly infectious cause illness when they are present in fairly low
numbers. Rarely does a single virus make you sick.
Multiplication
of viruses -Each virus needs a certain type of cell in
which to multiply: Plant viruses multiply only in plant cells; animal viruses
multiply only in animal cells.
After
infecting a host cell, a single virus can produce hundreds or even thousands of
similar viruses. The virus carries the essential information for its life cycle
in its genes, while the host cell supplies the energy, machinery,
building blocks, and production enzymes necessary for the virus to live and
multiply. The new viruses are made in an orderly, step-by step process. The
steps used by all viruses are a) attachment, b) entry, c) synthesis and
assembly of virus parts, and d) release.
a)
Attachment.
To
multiply, the virus must first attach itself to the surface of a cell. The
virus and the host cell must bind together tightly and fit each other exactly.
Bacteriophages, viruses that attack bacteria, use their tail fibers to bind to
molecules called receptors on the outside surface of bacteria.
b)
Entry.
Once
attached, viruses must free their genetic material inside the host cell so that
new virus parts can be made. Sometimes the whole virus will enter the cell
before gaining access to the host cell's protein-making processes. Other times
just the nucleic acid will enter the host cell.
The
method a virus uses to enter a cell depends on the type of virus. Some viruses
are engulfed and taken in by the host cell. Other viruses, such as
bacteriophages, inject their nucleic acid into the host. The tail of the
bacteriophage contracts like a syringe and shoots the nucleic acid into the
bacterium. The old protein coat of the virus is no longer needed and stays on
the outside of the bacterium. The virus, with its genetic material free and
open within the host cell, can now make new viruses.
c)
Synthesis and Assembly.
Once
freed inside the cell, the viral genes get to work and take over the cell
machinery. The host cell becomes a factory, with all its processes dedicated to
turning out new viruses. Copies of the viral nucleic acid are made instead of
the cellular nucleic acid. Then viral proteins that will make up the outside of
the virus-head, tail, and tail fibers are made. Soon each - copy of viral
nucleic acid is surrounded by many capsomeres. The capsomeres assemble to form
the capsid, and a complete, mature virus is produced.
d)Release.
The
final step of the virus life cycle is the release of the viruses from the host
cell. Some viruses destroy the cell when they are released. Bacteriophages make
molecules that weaken the walls of the large numbers of new bacterial viruses
cause the weakened cells to burst.
Viruses
can cause many diseases in humans, other animals, and plants. Some common
diseases in humans are the cold, chicken pox, measles, warts, and cold sores.
More serious diseases include hepatitis, rabies, polio, and AIDS. In other animals’
viruses cause, among other ailments, distemper, rabies, and foot-and-mouth
disease. Plants can have such virus diseases as peanut stunt, blueberry leaf
mottle, beet curly top, and cauliflower mosaic.
