small acellular infectious particles that cause many human, animal and plant diseases. These infectious particles are NOT cells. They cannot metabolize, grow, respond to the environment or reproduce independently of the cells they infect.
What is the leading cause of infectious disease in developed countries?
Many viral pathogens can escape host immune response and there is no antiviral drug to treat them. It is hard to develop antiviral drugs that are non-toxic to host cells because viruses use host enzymes and host synthetic machinery to replicate
What are the characteristics of a virus?
Viruses are small acellular particles that:
1. contain one or several pieces of a single type of nucleic acid (DNA or RNA)
2. have a protein coat around their nucleic acid, and sometimes a lipid envelope around the protein coat
More virus characteristics
3. Replicate inside of a living cell using the metabolic machinery of the host cell
4. Cause their host cell to assemble and release new copies of the virus, which can infect new host cells.
obligate intracellular parasites. They have few or no enzymes of their own and no cytoplasm. They cannot metabolize substrates, cannot synthesize rib
What is the extracellular state of a virus?
A virion. Virions are complete, infectious virus particles that can be trasmitted from one host to another.
What mediates the attachment of virions to its host cell and entrance to the cell?
the protein coat of naked viruses and the lipid envelope of enveloped viruses, mediate the attachment of a virion to its host cell and entrance into the cell
What does the intracellular state of a virus consist of?
The intracellular state of a virus consists of its nucleic acid only. The protein coat are removed when the virus enters its host cell
How do viruses differ from each other?
1. The type of genetic material they contain
2. the kinds of cells they infect
3. Their size and shape
4. The nature of their protein coat and whether they have a lipid envelope
What is the genetic material of viruses?
Can be either DNA or RNA but not both.
What is the host range of a virus?
refers to the spectrum of hosts and cells in which a virus can multiply. Most viruses infect a specific type of cell in one host species.
What is the virus host range determined by?
The presence of a specific attachment site called a receptor on the surface of the host cell and the presence of cellular factors in the cytoplasm of the host cell that are required for replication of the virus
What is a viral capsid?
The protein coat that surrounds a virus' nucleic acid. The nucleic acid and capsid together are called the nucleocapsid. Capsids are large structures composed of protein subunits called capsomeres
What 3 shapes are capsids classified into?
polyhedral, helical and complex
What are polyhedral capsids?
Many-sided geodesic dome-like structures. Icosahedrom is the most common
What are helical capsids?
helical rod-like structures that are formed by capsomeres polymerizing into a helical cylinder around a virus' nucleic acid. They can be rigid or flexible
What are complex capsids?
Capsids that are not helical or polyhedral in shape.
What is the shape of a virus determined by?
The shape of its capsid and whether it has a lipid envelope.
What is a naked virus?
A virus that does not have a lipid envelope.
What are the 3 basic shapes of enveloped viruses?
Enveloped helical viruses; enveloped polyhedral viruses; and enveloped complex viruses
What is the composition of a lipid envelope?
Lipids, glycoproteins and carbohydrates
What mediates the attachment of lipid enveloped viruses to receptors of a host cell?
Animal virus classification is based upon:
1. The type of nucleic acid in the virus' genome
2. the mechanism by which the virus replicates
3. The shape and size of the virus
4. Whether or not the virus has a lipid envelope
Animal virus family names end in:
Animal virus genus names are:
italicized and end in virus. (e.g. lentivirus)
What is a virus species?
A group of viruses that share the same genetic information and ecological niche
Why are viruses difficult to isolate, cultivate and identify?
They must be grown in a suitable host. This is especially true for animal viruses because they must be maintained in living animals or animal tissue culture cells which is expensive, time consuming and often difficult
What are the easiest viruses to grow?
Bacteriophages, viruses that infect bacteria
How are bacteriophages grown?
Bacteriophages are grown in their host bacterium in either broth cultures or on solid agar cultures.
how are solid agar cultures used to quantitate bacteriophages by the plaque method? 1/3
1. Serial dilutions of the phage sample are prepared. Each phage dilution is mixed with a sample of the host bacterium and 0.1 ml of the phage-bacterium mixture spread over a hardened agar growth medium
how are solid agar cultures used to quantitate bacteriophages by the plaque method? 2/3
The growth medium is incubated at 37 degrees C for 1-2 days. Bacteria not infected with a phage multiply and form a bacterial lawn on the surface of the agar.
how are solid agar cultures used to quantitate bacteriophages by the plaque method? 3/3
Where bacteriophages are present in the bacterial lawn, the phages infect the bacteria multiply in them and lyse them, leaving a clear area in the bacterial lawn called a plaque
At low phage dilutions, each plaque in the lawn originates from how many viruses?
A single virus
Why are living animals used to culture viruses?
Some animal viruses can be grown only in live animals; living virus infected animals are used to study the immune response to a viral infection; sometimes animals are used to detect the presence of a pathogenic animal virus in a clinical specimen
What is an animal cell culture?
The growing of animal cells in a tissue culture medium in the laboratory.
What are the 3 major types of tissue culture cell lines?
Primary cell line that are derived from normal tissue; Embryonic diploid cell lines are derived from embryonic tissue; Continuous cell lines derived from cancer tissue or virus transformed cells.
What are cytopathic effects?
Deteriorations that occur in virus-infected cells. They include:
1. Inhibition of protein synthesis, DNA synthesis and mitosis
2. Cell death and cell lysis, which are cytocidal effects
Cytopathic effects 2
3 Formation of inclusion bodies in the nucleus and/or cytoplasm
4 Induction of cell to cell fusion to form giant cells, which are called syncytia
5 Antigenic changes to the surface antigens on infected cells
Cytopathic effects 3
6 Changes in the chromosomes of infected cells
7. Cell transformation and loss of contact inhibition
What is the most common method for identifying animal viruses?
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