Viruses

Class Notes

Do viruses qualify as living things?

They are much smaller and less complex than cells.
They consist of either DNA or RNA enclosed in a protein coat called a capsid.
Virons do not grow.
They have no nucleus, cytoplasm, or membranes.
They do not carry out cellular functions.
Virons do not generate metabolic energy.
They are obligate intracellular parasites, meaning that they require a host cell to reproduce.

Viruses can be harmful.

Virulent - disease causing.
Temperate - not immediately disease causing.
Viruses play a role in causing some cancers.

bacteriophage, a virus that infects bacteria cells

Viruses may be helpful.

A temperate virus introduces nucleic acid from its former host into a new host, thus changing the genetic code of the host. By this process of transduction, viruses cause genetic variation within a host population.
Certain varieties of flowers have been developed using viruses to alter the genetic code.

Characteristics used to classify viruses.

Type of nucleic acid: viruses have either single-strand RNA or double-strand DNA, but never both.
Physical structure: viruses are found in several general shapes.

There are two basic shapes:

icosahedron (the capsid has 20 triangular faces)
helix (the capsid resembles a coiled spring).

The diagram at the top of the page is a virus that commonly infects bacteria cells. This virus is usually referred to as a bacteriophage.

Envelope: some viruses use lipids from the cell to form a layer around the capsid during replication.
Type of host cell: most viruses are very specific about the type of cell they invade. There are specific sites called receptor sites that the viruses attach to on the host cell's surface.

Compare Lytic and Lysogenic Cycles:
All viruses invade a host cell and reproduce through one of these cycles.

Phases of the Lytic Cycle of a Virulent Virus:

Adsorption: Virus attaches itself to the cell.
Entry: Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty caspid outside the cell. Many viruses actually enter the host cell intact.
Replication: Viral DNA takes control of cell activity.
Assembly: All metabolic activity of the cell is directed to assemble new viruses.
Release: Enzymes disintegrate the cell in a process called lysis, releasing the new viruses.

The Lysogenic Cycle of a Temperate Virus:

The virus attaches itself and injects its DNA into the cell.
The viral DNA attaches itself to the host DNA, becoming a new set of cell genes called a prophage.
When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits.
Now there are two possibilities:

The prophage survives as a perminant part of the DNA of the host organism.
Some external stimuli can cause the prophage to become active, using the cell to produce new viruses.

Some Diseases Caused by Viruses
Animals	Plants	Humans
Rabies	Tobacco mosaic diesase	Common cold and flu
Foot and mouth disease in cattle	Tomato bushy stunt	German measles amd mumps
Newcastle disease in chickens	Maise dwarf	Chickenpox
Distemper in dogs	Alfalfa mosaic disease	Mononucleosis
Cowpox	Sugar beet curly top	Cold sores, hepatitis, warts
Influenza in cows, horses, and sheep	Dwarfism in rice	Herpes and AIDS

viruses particles and bacteria cells Size Of Viruses:

Viruses are very small, ranging in size from 20 nanometers to 250 nanometers. A nanometer is equal to 0.00000004 inch (4X10^-8 in). The smallest of all bacteria is about the size of the largest virus. This picture shows round virus particles along with rod-shaped bacteria cells.

Controlling Viruses:

Virus particles are usually easy to destroy while outside living organisms. For instance, the "AIDS" virus, outside the body, can be destroyed with a solution of bleach that is almost weak enough for you to drink. But once inside a host, most substances that destroy the virus are also harmful to the host organism. For this reason, viral infections in animal cells are extremely hard to cure. Viral infections in plant cells are almost impossible to cure.

Compared to the number of vaccines developed to treat bacterial diseases, there are very few vaccines for viral infections. Virus vaccines are made with either inactivated or attenuated viruses. Inactivated viruses do not replicated in a host cell. Attenuated viruses have been genetically altered so they are not able to cause disease. The first viral vaccinations were for measles, mumps, and rubella. There are now vaccines for hepatitis A and B, chickenpox, smallpox, and rabies.

Virologist Antiviral drugs interfere with the synthesis of viral nucleic acid or with the formation of viral capsids during replication.
Antibiotics specifically attack the metabolism of a bacterial cell. Since viruses use only the reproductive machinery of a cell, antibiotics are of no use in destroying viruses.

Your body does have some natural ability to inhibit viral infections. There are two limited ways the body fights viruses:

White blood cells - engulf viruses in the blood and "digest" them.
Interferon - a protein produced by cells when exposed to a virus. This protein binds to the cell membranes of neighboring cells and "interferes" with the ability of a virus to enter the cell.

Complete the virus worksheet .

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