Chapter 13: Viruses, Viroids, and Prions
What are the two ways that viruses can multiply?
1. Lytic cycle: host cell dies 2. Lysogenic cycle: host cell will remain alive
Viral species
A group of viruses sharing the same genetic information and ecological niche (host range)
Bacteriophages
Also called phages; viruses that infect bacteria
Prion
An infectious protein; causes neurological diseases; spongiform encephalopathies: large vacuoles develop in brain; runs in families, but not purely inherited; mad-cow, kuru
Herpes viridae
Around 100 types; cold sores; herpes
How are viral species designated?
By descriptive common names
How are viruses classified?
By their nucleic acid and by differences in the structures of their coats
True or False? Most viruses can infect many types of cells.
False. Most viruses can only infect specific types of cells of only one host species.
True or False? Viruses contain both DNA and RNA.
False. Viruses only contain a single type of nucelic acid, either DNA or RNA.
Polyhedral viruses
Many-sided virus; shape of an icosahedron; 20 triangular faces, 12 corners
What is the difference between primary cell lines and continuous cell lines?
Primary cell lines are derived from tissue slices and tend to die out after only a few generations. Continuous cell lines occur when viruses are routinely grown in a lab. These transformed cells can be maintained through and indefinite number of generations (immortal cell lines)
Capsid
Protein coat surrounding and protecting the nucleic acid
Capsomeres
Protein subunits that make up the capsid; in some viruses the proteins composing the capsomeres are of a single type, in others they are composed of several different types
Poxviridae
Pus-filled lesions; small pox
How do some viruses escape antibodies?
Regions of their genes that code for their surface proteins are susceptible to mutation
Helical viruses
Resemble long rods; may be rigid or flexible; viral nucleic acid found within a hollow cylindrical capsid
Cytopathic effect (CPE)
Cell deterioration
What are 3 methods commonly used for culturing animal viruses?
1. In living animals 2. In embryonated eggs 3. In cell cultures
What are the stages of multiplication of animal viruses?
1. Attachment 2. Entry 3. Uncoating 4. Transcription and translation of a portion of viral DNA 5. Synthesis of capsid proteins 6. Capsid proteins migrate into nucleus of host cell and maturation occurs 7. Release
What are the 5 steps of the lytic cycle?
1. Attachment: Phage attaches to host cell 2. Penetration: Phage injects its DNA into the host cell 3. Biosythesis: Cut up host DNA, direct synthesis of viral components 4. Maturation: Viral components are assembled into virons 5. Release (lysis): Host cell is lysed and new virons are released to infect other cells
What different variations can a virus's nucleic acid have?
1. DNA or RNA 2. Single-stranded or double-stranded 3. Linear or circular 4. Size - few thousand up to 250,000 nucleotides
What are 3 ways viral growth can be signaled in an embryonated egg?
1. Death of the embryo 2. Embryo cell damage 3. Formation of typical pocks or lesions on the membrane
What are the 4 ways multiplication of animal viruses different from the basic bacteriophage multiplication?
1. Differ in their mechanism of entering the cell 2. Synthesis and assembly of new viral components are different due to the differences between prokaryotic cells and eukaryotic cells 3. Mechanisms of maturation and release are different 4. Effects on the host cell differs
What are 2 reasons why one would want to culture a virus in an embryonated egg?
1. Fairly convenient 2. Inexpensive
Why have cell cultures replaced embryonated eggs as the preferred type of growth medium for many viruses?
1. Generally very homogeneous collections of cells 2. More convenient to work with
What are the 4 kinds of general morphology of viruses?
1. Helical 2. Polyhedral 3. Enveloped 4. Complex
What are the 5 steps of the lysogenic cycle?
1. Phage attaches to host cell 2. Phage DNA penetrates host cell 3. Phage DNA circularizes (at this point it can enter the lytic or lysogenic cycle) 4. Once it has entered the lysogenic cycle it is called a prophage and can stay in this stage for many generations 5. Eventually it will enter back into the lytic cycle
What are 3 reasons one would need to culture a virus in a living animal?
1. Some animal viruses can only be cultured in living animals 2. Most experiments to study the immune system's response to viral infections must be performed in virally infected live animals 3. May be used as a diagnostic procedure for identifying and isolating a virus from a clinical specimen
Virion
A complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside of a host cell, and is a vehicle of transmission from one host cell to another
Plant viruses
Can cause color change, deformed growth, wilting, stunted growth; plant viruses must enter through wounds or be assisted by other plant parasites because plant cells are protected by cell wall; infected plants can spread to other plants by pollen
Enveloped viruses
Capsid covered with an envelope; roughly spherical
Spikes
Carbohydrate-protein complexes that project from the surface of the envelope; these can help some viruses attach to a host
Adenoviridae
Causes acute respiratory diseases, like the common cold
Complex viruses
Complexly structured virus; particularly bacterical viruses; some bacteriophages have capsids to which additional structures are attached
Envelope
Covers the capsid; usually consists of some comnination of lipids, proteins, and carbs; not all viruses have envelopes
Diploid cell lines
Developed from human embryos and can be maintained for about 100 generations; widely used for culturing viruses that require a human host
How does one culture a virus in an embryonated egg?
Drill a hole in the shell, inject a viral suspension or suspected virus-containing tissue into the fluid of the egg; several membranes in egg so virus is injected into the one most appropriate for its growth
Viroid
Infectious RNA; plant disease
Host Range
The spectrum of host cells a virus can infect
What is the structure of the capsid determined by?
The viral nucleic acid
What determines a virus's host range?
The virus's requirements for its specific attachment to the host cell as well as the availability within the potential host of cellular factors required for viral multiplication
How are cell culture lines started?
Treat slice of animal tissue with enzymes that separate the individual cells, suspend cells in solution that provides osmotic pressure, nutrients, and other growth factors, normal cells will adhere to container and reproduce to form a monolayer
True or False? Viruses contain a protein coat that surrounds the nucleic acid.
True.
True or False? The capsid usually makes up most of the mass of the virus
True. Especially in smaller viruses
True or False? Most drugs that interfere with viral multiplication also interfere with the functioning of the host cell.
True. These drugs are toxic for clinical use.
When are viruses considered alive?
Viruses are only considered alive when they are in their host cells.
How do viruses multiply?
Viruses must take over the metabolic machinery of the host cell in order to multiply.
Nonenveloped viruses
Viruses whose capsids are not covered by an envelope; the capsid of a nonenveloped virus is what protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus's attachment to susceptible host cells
Papovaviridae
Warts and tumors; papilloma virus can cause uterine cancer
When a host has been infected by a virus, the host immune system is stimulated to produce ___________, the interaction between the host's ___________ and virus proteins should ____________the virus and stop the infection.
antibodies; antibodies; inactivate
For a virus to infect the host cell, the ____________ of the virus must chemically interact with the specific _____________ on the surface of the cell
outer surface; receptor sites
The receptor site can be ___________ or _____________. For animal viruses, the receptor sites are on the ________________ of the host cell.
part of the cell wall of the host; part of the fimbriae or flagella; plasma membrane of the host cells
Antibodies
proteins that react and potentially inactivate the surface proteins of the virus