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Topics: Virus, Virology, Microbiology Pages: 5 (1529 words) Published: June 24, 2013
UNIT 9: VIRUSES, VIROIDS, & PRIONS

Lesson 1 – Characteristics of Viruses
Depending on their characteristics, viruses may or may not kill the host cell. Viruses are too small to be seen with a light microscope and cannot be cultured outside their hosts. Viruses and Bacteria Compared.| Bacteria| Viruses|

| Typical Bacteria| Rickettsias/Chlamydias| |
Intracellular Parasite| No| Yes| Yes|
Plasma Membrane| Yes| Yes| No|
Binary Fission| Yes| Yes| No|
Pass through Bacteriological filters| No| No/ Yes| Yes| Possess both DNA & RNA| Yes| | No|
ATP-Generating Metabolism| Yes| Yes /No| No|
Ribosomes| Yes| Yes| No|
Sensitive to Antibiotics| Yes| Yes| Yes|

There are viruses that infect invertebrates, vertebrates, plants, protists, fungi, and bacteria. Viruses that infect bacteria are called bacteriophages, or phages. The nucleic acid of a virus is protected by a protein coat called the capsid. Each capsid is composed of protein subunits called capsomeres. In some viruses, the capsid is covered by an envelope, which usually consists of some combination of lipids, proteins, and carbohydrates. Depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrate-protein complexes that project from the surface of the envelope. Some viruses attach to host cells by means of spikes. Viruses may be classified into several different morphological types on the basis of their capsid architecture. * Helical Viruses

Resemble long rods that bay be rigid or flexible. The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure. The virus that causes rabies is a helical virus. * Polyhedral Viruses

The capsid is in the shape of a polyhedron with 20 triangular faces and 12 corners. An example is the poliovirus. * Enveloped Viruses
Capsid covered by an envelope. Usually a roughly spherical virus. When helical or polyhedral viruses are enclosed by envelops, they are called enveloped helical or enveloped polyhedral viruses. An example is the influenza virus. * Complex Viruses

An example is a bacteriophage. Complicated structures.

Lesson 2 – Viral Taxonomy
The International Committee on Taxonomy of Viruses groups viruses into families based on the following: i. Nucleic acid type
ii. Strategy for replication
iii. morphology
Lesson 3 – Isolation and Identification of Viruses
A number of procedures have been established for growing and isolating viruses. The hosts that the viruses infect will often determine how they are grown. Thus, bacteriophages will propagate in bacteria, whereas animal viruses may grow in animals, embryonated eggs, or cell cultures. Growing Bacteriophages in the Laboratory – The plague method A sample of bacteriophage is mixed with host bacteria and melted agar. The agar containing the bacteriophages and host bacteria is then poured into a Petri plate containing a hardened layer of agar growth medium. The virus-bacteria mixture solidifies into a thing top layer that contains a layer of bacteria approximately one cell thick. Each virus infects a bacterium, multiplies, and releases several hundred new viruses. These newly produced viruses infect other bacteria in the immediate vicinity, and more new viruses are produced. Following several multiplication cycles, all the bacteria in the area surrounding the original virus are destroyed. This produces as a number of clearings, or plaques, visible against a lawn of bacterial growth on the surface of the agar. While the plaques form, uninfected bacteria elsewhere in the Petri plat multiply rapidly and produce an opaque background. Each plaque theoretically corresponds to a single virus in the initial suspension. Growing Animal Viruses in the Laboratory

In living animals
* Some animal viruses can be cultured only in living animals, such as mice, rabbits, and guinea pigs. Animal inoculation may be used as a diagnostic...
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