1. Define VIRUS and give several examples.
2. Identify the structures and shapes of viruses.
3. Identify methods of infection on host.
4. Name and describe both replication cycles.
5. Describe retroviruses, viroids, and prions, with examples.
A VIRUS is a non-living particle that contains genetic information and needs a host cell in order to replicate. Many viruses are infectious agents, and cause diseases referred to as "viral" disease or infection. Common examples are the flu (influenza virus) and HIV (human immunodeficiency virus).
The basic structure of a virus is the GENETIC MATERIAL, or viral genome, surrounded by a protein coat called a CAPSID. Sometimes the virus also has an extra outer covering called an ENVELOPE, which is made of cell-membrane based material that it receives when the viruses leave the host cell. Some capsids have a cylindrical shape with teh genetic material inside the column, and these are named HELICAL. Some capsids are made of connected triangular panels to make a POLYHEDRON shape, again with the genetic material within. Some viruses are a combination of these shapes, and also have specialized structures to help them attach and inject their target host cell.
Some viruses, such as the BACTERIOPHAGE (which targets E. coli) land on the outside of the target host cell and inject the viral genome into it. Some viruses land on the outside of the target cell and are taken into the cell through a process called ENDOCYTOSIS. Either way, the virus infects the host cell with its own genetic material, which is vital to the cycle of the virus.
Replication occurs in two cycles: The short cycle is called the LYTIC cycle and involves these steps in order: Virus injects genetic material into host, inside the host the new viruses begin assembling, the newly assembled viruses are released either by breaking out (bursting) of the cell or by EXOCYTOSIS (the opposite process of endocytosis). The longer cycle is called the LYSOGENIC CYCLE and involves these steps in order: Virus injects genetic material into host, viral genome is incorporated within the host DNA, cell continues to function and replicate normally (sometimes for years). This period of time is called INCUBATION and during this time there are no symptoms of the virus in the organism... it is sort of just replicating and waiting. Then somethign will trigger an outbreak, and the virus switches to the LYTIC cycle- and by this time there are a great many number of cells rapidly producing and releasing viruses.
RETROVIRUSES are viruses that haveRNA and a molecule called a "REVERSE TRANSCRIPTASE"- which uses the RNA as a template to make DNA. HIV is an example of a retrovirus.
VIROIDS are viruses with a small circular RNA strand that interfere with RNA in the host cell. All known viroids are plant viruses.
PRIONS are very simple viruses that consist of just a protein, which somehow causes misfolding of the host cells proteins. Prions are viruses that affect the nervous systems of animals. Mad Cow Disease is a well-known example of a prion disease.