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Virus:
the Latin word meaning “poison”
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Was given this name by scientist named
Beijerinck
HISTORY
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In 1892, a Russian biologist named Ivanovski
studied the disease of tobacco plants. The disease is known as tobacco mosaic.
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Ivanovski noticed that when you rubbed
the juice from infected leaves on a healthy plant, the healthy plant contracted the disease.
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Next, Ivanovski filtered the juice from infected leaves
through a filter with pores small enough to filter out bacteria.
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He checked the fluid for bacteria and
found none.
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When this fluid was rubbed on the healthy
plant, they contracted the disease.
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During Ivanovski’s day, scientists thought that
bacteria were the smallest possible agents of disease.
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Six years later, Beijerinck repeated Ivanovski’s
work and concluded that an invisible agent must be causing the disease, the virus.
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In 1935, the virus was found! Dr. Wendell Stanley ground and removed fluid from over a ton of diseased tobacco leaves.
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From the fluid, he isolated a teaspoon
of needlelike crystals.
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When stored in a bottle, they did nothing. BUT, when placed in water and rubbed on leaves, they caused the disease.
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From Stanley’s work, it was determined that viruses were
very small and that they were non-living in crystal form. They needed to be placed
in living cells to be able to reproduce.
HOW BIG ARE VIRUSES?
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Viruses are larger than a molecule but smaller than
the smallest cells.
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All viruses have one thing in common: They enter living
cells, and once inside, use the machinery of the infected cells to produce more viruses.
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To see the tobacco mosaic virus, you need
an electron microscope. The virus then needs to be magnified 200,000 times just
to be seen.
WHAT MAKES UP A VIRUS?
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Viruses are composed of:
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The outer protein coat that is called the capsid.
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The protein coat carries the genetic material
that is either a strand of DNA or RNA.
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The capsid binds to the surface of a cell and “tricks”
the cell into allowing it inside.
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There are a wide variety of shapes for
viruses: oval shaped with tiny necks, some are shaped like spheres, cubes or needles.
VIRUSES ARE SUBCELLULAR!
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This means that they lack organization
of cells.
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They do not contain any of the structures of a cell
that is used for making protein or high energy molecules.
HOW
DO VIRUSES WORK? HOW DO VIRUSES REPRODUCE?
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All viruses have the same basic life cycles.
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When it is outside a cell, a virus does
not show any signs of life. (grow or carry on respiration).
TO BECOME ACTIVE, A VIRUS REQUIRES A HOST CELL!
§ A host cell is any cell that is attacked (has the viral DNA or RNA inserted into its
cytoplasm) by a virus.
§ VIRULENCE is a virus’s ability to cause disease.
§ Viruses only invade specific kinds of cells. This
is why they are classified according to their host cell.
§ There are bacteria, plant and animal viruses.
§ Tobacco mosaic attacks only certain types of plant tissue.
§ Polio only attacks one kind of nerve cell in the brain and spinal cord producing
paralysis.
§ Human papillomavirus infects cells in the outermost layer of the skin causing
warts.
VIRAL DISEASES CANNOT BE TREATED WITH ANTIBIOTICS!
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Prevention of viruses is by the
use of vaccines.
WHAT ARE OTHER VIRUSLIKE PARTICLES?
§ Viroids cause disease in plants.
§ These are single-stranded RNA molecules that have no surrounding capsids.
§ They disrupt the metabolism of the plant cell and stunt the growth of the entire
plant.
§ Prions cause disease in animals.
§ These are “protein infectious particles” which contain no DNA or
RNA.
§ They cause disease by forming protein clumps and eventually damage nerve tissue.
Lytic Cycle
1. Phage attaches tail down to special site called the receptor site on
a bacterias cell surface.
2. Tail is like a hypodermic. Sheath on
the tail penetrates the cell wall and membrane. The viral DNA is injected into
the bacterium.
3.
In minutes, the viral DNA serves as the genetic code and has complete control
of cell activity. New DNA sequence codes
for more viral DNA to be made.
4. The bacteria becomes a factory for new viruses.
During assembly, proteins coded for by phage DNA act as enzymes that put new viruses together.
5. An enzyme digests the bacterial wall from within. The cell breaks open releasing new
viruses. The new phages attack other bacteria.
Lysogenic Cycle
1. The virus attacks a bacterium. The DNA or RNA is injected.
2. Genetic material attaches to the bacterial chromosome or host DNA.
3. The virus does not cause the bacteria to produce more like the lytic virus. Instead, it acts like an extra gene. In this stage, the virus is called a prophage. The prophage replicates whenever the host bacterium reproduces.
4. As the bacteria divides and multiplies, the viral genetic material is being replicated too. There is a copy in every bacteria made. Each bacterial offspring
is infected with a prophage. It causes no harm
to the host cell.
5. When the environmental conditions are right, the prophage becomes virulent and
takes over the host cell. It enters the lytic cycle, proceeding with replication
and destroying the host cell.