Chapter 13 - Viruses and Prions
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I. General Characteristics of Viruses
A. Viruses were originally described as infectious agents that could not be removed by filtration. While this definition is highly practical it does not preclude some unique bacteria, such as Mycoplasma, which are not viruses.
B. Viruses are not living organisms, but rather nucleic acid constructs designed for facilitated self-replication.
1. While the nucleic acid of viruses encode future copies of themselves, they are obligate intracellular parasites since they utilize the machinery of a cell to execute their coding.
2. Viruses do not possess ribosomes or an ATP-generating metabolism.
C. Contain a single type of nucleic acid, either DNA or RNA.
D. The nucleic acid is surrounded by a protein coat. The protein coat can in turn be surrounded by a lipid, protein, and carbohydrate envelope, but it is not present in all viruses.
E. Host Range – the spectrum of host cells that a virus can infect.
1. Typically viruses show a very selective host range, a single type of cells in a single species
2. Crossing species boundaries is possible, resulting in spread of viruses to new hosts.
F. Viral Size – viruses are smaller than bacteria, in general, ranging from 20-1000 nm in size.
II. Viral Structure
A. Virion – a complete, fully developed, and infectious viral particle.
B. Classification of viruses is based on the differences in nucleic acid structure and coat structure.
C. Nucleic Acid structure
1. Viruses can utilize either DNA or RNA as their genetic material, but only one is used in the virions.
2. The nucleic acid can be either single-stranded or double-stranded
3. The nucleic acid can be either linear or circular
4. The nucleic acid can be in one segment or divided between multiple fragments
5. Total genome size range from around 2000 bp to around 250,000 bp (E.coli runs around 4 million chromosome)
D. Capsid and Envelope
1. The nucleic acid is surrounded by a protein coat called the capsid
2. The arrangement of capsid subunits is characteristic of the particular type of virus.
3. Some viruses cover the capsid with an envelope, typically derived from the host cell plasma membrane.
4. Viruses can be covered by spikes, carbohydrate-protein complexes that project from the surface of the surface of the virion
E. General Morphology – analogous to the morphologies of bacteria
1. Helical
2. Polyhedral
3. Enveloped
4. Complex
III. Taxonomy of Viruses
A. Viral taxonomy is based on three characteristics
1. Nucleic acid type
2. Strategy for replication
3. Morphology
B. Viral species – groups that share the highly conserved genetic sequence and an environmental niche.
C. Specific viral genera will be covered later.
IV. Isolation, Cultivation, and Identification of Viruses
A. Growing bacteriophage in the laboratory
1. Viruses that grow in bacteria are easily grown on lawns of bacteria.
2. Viral growth produces zones of clearing in the lawn called plaques
3. The concentration of bacteriophage are reported as plaque-forming units (PFU)
B. Growing animal viruses in the laboratory
1. In living animals
2. In embryonated eggs
3. In cell cultures
C. Methods of Viral Identification
1. Direct observations by electron microscope
2. Serological methods
3. Molecular methods
V. Animal Viral Multiplication
A. The initial steps of viral infection are shared by both RNA and DNA viruses
1. Attachment – receptors on the surface of the virion bind to carbohydrates and/or proteins on the plasma membrane of the target cell (1st level of viral specificity).
2. Entry
a. Nonenveloped viruses enter by pinocytosis, after attaching to the plasma membrane
b. Enveloped viruses can enter by pinocytosis or by fusing their envelope to the plasma membrane of the cell, thereby depositing the capsid inside the cell.
3. Uncoating – removal of the coat proteins
a. Can be accomplished by host enzymes, or in rare cases (poxviruses) the viral DNA encodes the needed enzyme.
b. Serves to separate the viral nucleic acids from the coat proteins.
B. Biosynthesis of viral nucleic acids can be accomplished through a variety of methods, mostly depending on the type of nucleic material you start with.
1. Cellular enzymes can copy the viral DNA in the nucleus (DNA viruses)
2. Cellular enzymes can copy the viral RNA in the cytoplasm (RNA viruses)
3. Certain viruses possess an enzyme called Reverse Transcriptase, an enzyme that can turn RNA into DNA (i.e. Retroviridae)
C. Maturation and Release
1. Nonenveloped viruses are released by rupturing of the host cell
2. Enveloped viruses extrude out through the host membrane (budding), deriving a portion of their envelope from the host cell.
VI. Viruses and Cancer
A. In the early 1900s it was demonstrated that cancers in chickens and mice can be transmitted between animals by cell-free filtrates containing viruses.
B. Oncogenes are genes present in normal animal cells that can be triggered by a variety of substances to make the cell cancerous. Many of these genes can be transmitted by viruses (though they are derived from animal genes)
C. About 10% of cancers are known to be virus-induced.
D. Many human cancers are linked to viruses
1. Cervical cancer – Human Papillomavirus
2. Burkett’s Lymphoma – Epstein-Barr Virus
3. Liver Cancer – Hepatitis B Virus
4. Adult T-cell Leukemia – HTLV
VII. Latent Viral Infections
A. Latent Infection – a viral infection that remains in equilibrium with the host not becoming symptomatic, eventually being reactivated as the immune system fails.
B. A classic example is the chickenpox infection (Varicellovirus), which can remain dormant for decades and then reactivate in 10 – 20% of cases as a disease called shingles.
VIII. Persistent Viral Infections – viral infections that build gradually over a long time (as compared to the sudden explosion in Latent Infections), these are typically fatal.
IX. Prions
A. Prions are infectious form of a normal animal protein; there are no associated nucleic acids
B. The infectious process appears to be the ability of the prion to catalyze the conversion of the normal form into more of the infectious form.
C. Thought to be a rare process, it is transmissible.
D. The most known example is “Mad Cow Disease” which is originally derived from a sheep disorder called scrapies. The species jump appears to have occurred due to the now unused practice of feeding animal byproducts (including contaminated neural tissue) to livestock.
E. Prions kill by forming plaques of converted proteins in the brain.
F. Prions are highly resistant to destruction (remember the list of antimicrobial sensitivity).