Chapter 5 Viruses

I. What are viruses?
    A virus is a particle that contains a single type of nucleic acid protected by protein that may be enclosed in a membranous envelope. It reproduces only in a particular type of living cell by taking over cellular functions and redirecting them to produce identical particles capable of entering and reproducing in the same type of cell.
A.Unique Features
    1. Viruses exist at a level between living things and nonliving molecules.
    2. They consist of a capsid and nucleic acid, either DNA or RNA, not both.
    3. Lack metabolism and respiratory enzymes.
    4. Multiply inside host cells using the assembly line of the host's synthetic machinery,
    5. Are genetic parasites.
    6. Are host-specific.
    7. Are ultramicroscopic and crystallizable.
    8. Some viruses are enveloped, others are naked.
    9. Some have unique nucleic acid structure (single-stranded DNA and double-stranded RNA).
    10.Some animal viruses can become latent.
B. Importance of Viruses
    1. 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.
    2. Agricultural: Hundreds of cultivated plants and domestic animals are susceptible to viral infections, often with adverse economic and ecologic repercussions.
    3. Research: Because of their simplicity, viruses have become an invaluable tool for studying basic genetic principles.

II. Morphology and structure of viruses
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).

III. Types of viruses and related multiplication cycles
A. Classification: Viruses are known to parasitize all types of ceils, including bacteria, algae, fungi, protozoa, animals, and plants. Each viral type is limited in its host range to a single species or group, mostly due to specificity of adsorption of virus to specific host receptors. Viruses are classified first by being separated into major host range groups and thereafter by means of virion characteristics, such as presence of envelope, type of nucleic acid, nucleocapsid morphology, virion size，and antigenic similarities. 
B. Bacteriophages are viruses that attack bacteria. Most bacterial viruses, or bacteriophage, are nonenveloped and have double-stranded DNA.

    1. attachment, or adsorption, to a host cell; they must have special binding molecules for receptors on the bacteria cell;
    2. penetration, they penetrate by injecting their nucleic acid into bacteria cell;
    3. biosynthesis of viral parts such as proteins and nucleic acid under control of virus genome;
    4. maturation, directing assembly of these parts into new virus particles;
    5. releasing complete, mature viruses; they are released as virulent phage upon lysis of the cell.
    Some viruses go into a latent or lysogenic phase in which they integrate into the DNA of the host cell.

C. Vertebrate viruses:
    1. There are approximately 20 families of vertebrate viruses separated into four major groups: (1) DNA-containing, nonenveloped viruses, (2) DNA-containing, envelopedviruses, (3)RNA-containing, nonenveloped viruses, and (4) RNA-containing, enveloped viruses. The single largestgroup consists of the RNA-containing, enveloped viruses.
    2. There are five major phases in the cycle of interaction of vertebrate viruses with their host cells: attachment, penetration, replication, maturation, and release.
Vertebrate viruses penetrate by being engulfed into a vesicle，or membrane fusion，or direct entrance. They are released by budding off with an envelope.
    3. Once inside their host cells, some viruses can persist for prolonged periods. Persistence usually involves integration of the viral DNA into the cellular DNA as a provirus. A few viruses cause diseases that lead to death over a period of months to years. 
    Several vertebrate viruses cause cancer. These viruses have special genes, called oncogenes, that transform normal cells into cancer cells.

V. Method of cultivation
    The need for an intracellular habitat makes it necessary to grow viruses in living cells, either in the intact host animal, in bird embryos, or in isolated cultures of host cells (cell culture).

VI. Identification
    Viruses are identified by means of cytopathie effects in host cells, direct examination of viruses or their components in samples, analyzing blood for antibodies against viruses, performing genetic analysis of samples to detect virus nucleic acid, growing viruses in culture, and symptoms. 
    Viruses were discovered in the 1890s and were distinguished by their small size. They were recognized as agents of disease so small that they could pass through filters that would retain bacteria and could not be seen with a microscope.

· The acellular viral particle is called the virion. All virions have a nucleocapsid containing either DNA or RNA surrounded by a protective protein coat called a capsid. Withmany viruses, the nucleocapsid is enclosed within a membranous envelope.

· There are three morphological forms of nucleocapsids: helical, icosahedral, and complex or undefined.

· Viruses are classified first by being separated into major host range groups and thereafter by means of virion characteristics, such as presence of envelope, type of nucleic acid, nucleocapsid morphology, and virion size.

· Most bacterial viruses, or bacteriophage, are nonenveloped and have double-stranded DNA.

· Most plant viruses are nonenveloped and contain single stranded RNA.

· There are approximately 20 families of vertebrate viruses separated into four major groups: (1) DNA-containing, nonenveloped viruses, (2) DNA-containing, envelopedviruses, (3)RNA-containing, nonenveloped viruses, and (4) RNA-containing, enveloped viruses. The single largestgroup consists of the RNA-containing, enveloped viruses.

· There are five major phases in the cycle of interaction of viruses with their host cells: attachment, penetration, replication, maturation, and release.

· Once inside their host cells, some viruses can persist for prolonged periods. Persistence usually involves integration of the viral DNA into the cellular DNA as a provirus.A few viruses cause diseases that lead to death over a period of months to years.

· Several vertebrate viruses cause cancer. These viruses have special genes, called oncogenes, that transform normal cells into cancer cells.

· Viruses can be propagated in the laboratory using individual host cells, cell (tissue) culture, and animal or plant inoculations.

There are four types of subviral agents that reproduce in cells and cause disease: viroids, prions, satellites, and deltavirus. Viroids are very small pieces of single-stranded RNA that replicate in plant cells. Prions are proteins that propagate in cells. 

Review questions:

1. Viruses were first detected because they are filterable. What do we mean by the term filterable， and how could this property have helped their detection before invention of the electron microscope？

2. Why do we classify viruses as obligatory intracellular parasites？

3. List the four properties that define a virus. What is a virion？

4. Describe the three morphological classes of viruses， then diagram and give an example of each.

5. Describe how bacteriophages are detected and enumerated by the plaque method.

6. Describe the multiplication of a T-even bacteriophage. Be sure to include the essential features of attachment， penetration， biosynthesis， maturation， and release.

7. Recall from Chapter 1 that Koch's postulates are used todetermine the etiology of a disease. Why is it difficult to determine the etiology of a viral infection such as influenza？

8. Assume that this strand of RNA is the nucleic acid for an RNA-containing animal virus： UAGUCAAGGU.

　　（a） Describe the steps of RNA replication for a virus that contains a strand of RNA.

　　（b） Describe the steps of RNA replication for a virus that contains a - strand of RNA.

　　（c） Describe the steps of RNA replication for a virus that contains double-stranded RNA.

　　（d） Describe the steps of RNA replication for a virus that contains reverse transcriptase.

9. In some viruses， capsomeres function as enzymes as well as structural supports. Of what advantage is this to the virus.

10. Prophages and proviruses have been described as being similar to bacterial plasmids. What similar properties do they exhibit？ How are they different？