Chapter Capsule

A. Viruses are infectious particles and not cells; lack protoplasm, organelles, locomotion of any kind; are large, complex molecules; can be crystalline in form. A virus particle is composed of a nucleic acid core (DNA or RNA, not both) surrounded by a geometric protein shell, or capsid; combination called a nucleocapsid; capsid is helical or icosahedral in configuration; many are covered by a membranous envelope containing viral protein spikes; complex viruses have additional external and internal structures.

B. Shapes/Sizes: Cuboidal, spherical, cylindrical, brick- and bullet-shaped. Smallest infectious forms range from the largest poxvirus (0.45 mm or 450 nm) to the smallest viruses (0.02 mm or 20 nm).

C. Nutritional and Other Requirements: Lack enzymes for processing food or generating energy; are tied entirely to the host cell for all needs (obligate intracellular parasites).

A. The two major types of viruses are DNA and RNA viruses. These are further subdivided into families, depending on shape and size of capsid, presence or absence of an envelope, whether double- or single-stranded nucleic acid, and antigenic similarities.

B. Animal viruses: Common names of major DNA animal viruses include poxviruses (smallpox), herpesviruses, adenoviruses, papillomavirus (wart), hepatitis B virus, parvoviruses. RNA animal viruses include poliovirus, hepatitis A virus, rhinovirus (common cold), encephalitis viruses, yellow fever virus, rubella virus, influenza virus, mumps virus, measles virus, rabies viruses, and HIV (AIDS virus).

C. Other noncellular infectious agents cause spongiform encephalopathies such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle. The agents are probably protein particles called prions.

A. A unique method of multiplication occurs—no division is involved. Viruses make multiple copies of themselves through:

1. attachment, or adsorption, to a host cell; they must have special binding molecules for receptors on the host cell;

2. penetration of a cell by means of whole virus or only nucleic acid;

3. assuming control of the cell’s genetic and metabolic components, thus

4. programming synthesis of new viral parts;

5. directing assembly of these parts into new virus particles, and

6. releasing complete, mature viruses.

B. Bacteriophages are viruses that attack bacteria; they penetrate by injecting their nucleic acid; they are released as virulent phage upon lysis of the cell.

C. Enveloped viruses penetrate by being engulfed into a vesicle. They are released by budding off with an envelope.

D. Some viruses go into a latent or lysogenic phase in which they integrate into the DNA of the host cell and later may become active and produce a lytic infection.

A. Medical: Viruses attach to specific target hosts or cells. They cause a variety of infectious diseases, ranging from mild respiratory illness (common cold) to destructive and potentially fatal conditions (rabies, AIDS). Some viruses can cause birth defects and cancer in humans and other animals.

B. Agricultural: Hundreds of cultivated plants and domestic animals are susceptible to viral infections, often with adverse economic and ecologic repercussions.

C. Research: Because of their simplicity, viruses have become an invaluable tool for studying basic genetic principles.

• Viruses exist at a level between living things and nonliving molecules.

• They consist of a capsid and nucleic acid, either DNA or RNA, not both.

• Lack metabolism and respiratory enzymes.

• Multiply inside host cells using the assembly line of the host’s synthetic machinery.

• Are genetic parasites.

• Can pass through fine filters.

• Are ultramicroscopic and crystallizable.

• Some viruses are enveloped, others are naked.

• Some have unique nucleic acid structure (single-stranded DNA and double-stranded RNA).

• Some animal viruses can become latent.