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Viruses (Replication/Reproduction Process
A 5-step process that uses…
Viruses
Replication/Reproduction Process
A 5-step process that uses the host cell's synthesizing components to assemble complete virion units
Step 3. Duplication/Synthesis
-Copying & expression of the viral genome at the expense of the host's synthetic equipment; results in production of viral components
Step 4. Maturation
-Assembly of the individual viral parts into whole, intact virions
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Step 5. Release
-Escape from host cell by the active, infectious viral particles; done through budding/exocytosis
Step 1. Adsoprtion
-Recognition process between a virus & host cell; results in virus attachment to host cell
Structure -Consists of 2 main parts:
- The protein shell/capsid
- The nucleic acid core
1. Capsid
-Protein coat that encloses & protects the nucleic acid core
-Constructed from identical subunits, called "capsomers"; made of protein molecules called protomers
-2 Types of capsids
Icosahedral
-A 20-sided polygon w/ 12 evenly spaced corners -Vary in no. of capsomers
-Each may be made of 1 or several proteins
-Some are enveloped
-Triangular hexons = Capsomers forming flat faces
-Round pentons = Capsomers forming the corners
Helical
-Rod shaped capsomers that bond together to form a series of hollow discs that resemble a bracelet
2. Core
-Made of nucleic acids
-Contain the virus's genetic material, which houses the instructions for replication/reproduction
-Nucleic acid can be either (double- or single-stranded) DNA or (double- or single-stranded) RNA, but not both
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Cultivation & Diagnosis
Methods for Cultivation
1) Live animal studies in laboratories
2) Bird embryos; birds undergo their embryonic period inside an egg, which is a nearly perfect experimental system for viral propagation
3) Cell culture; host cell cultures are grown in special sterile chambers containing the correct types & proportions of growth factors; appearance of plaque = indication of virus growth in culture
Methods of Diagnosis -Direct Diagnostic Methods:
- Culture studies
- Electron microscopy
- Immunofluorescence microscopy
- PCR technique
-Indirect Diagnostic Methods:
- Patient serum testing (for host antibody reactions to viruses)
Basic Properties
-Obligate intracellular parasites; appropriate hosts to replicate
-Range from 15 nanometers to 200 micrometers
-Not cells; structure is very compact & economical
-Geometric; can form crystal-like masses
-Inactive outside, but active inside host cells
-Molecules on surface have high specificity for attachment to host cell
-Not able to independently fulfill the characteristics of life
-Multiply by taking control of host cell's genetic material & regulating synthesis & assembly of new viruses
-Lack enzymes for most metabolic processes
-Lack machinery for protein synthesis
Cytopathic Effects -Defined as "Virus-induced damage to the cell that alters its microscopic appearance"-Examples include:
- Change in size & shape
- Cytoplasmic inclusion bodies
- Nuclear inclusion bodies
- Cells fusing to form multinucleated cells
- Cell lyisis
- DNA alteration
- Transformation into cancerous cells
Naming
-No taxa above Family (No kingdom, phylum, etc.)
-19 families of animal viruses
-Family name ends in -viridae (Ex. Herpesviridae)
-Genus name ends in -virus (Ex. Simplexvirus)
-"Herpes Simplex Virus 1" (HSV-1)
Nomenclature Order
1) Family - Herpesviridae
2) Genus - Varicellovirus
3) Common Name - Chickenpox virus
4) Disease - Chickenpox
Host Range -Requirements for a cell that a virus can infect
- Cell must have specific structure/receptor on its surface for viral attachment
- Cell has to contain enzymes & materials needed to produce new virions
-May infect one species or many
- HIV (only humans) vs. Rabies (many animals)
-May infect one tissue or many within a host
- Hepatitis (liver only) vs. Polio (Intestinal & nerve cells)
Complex Viruses -Have a more intricate structure than helical & icosahedral viruses-Parts include:
- Capsid head
- Nucleic acid
- Collar
- Sheath
- Tail fibers
- Tail pins
- Base plate
Important Points for Viral Diagnostics -Diagnosing viral diseases is more difficult than other agents-Overall clinical practice must be considered-Appropriate host cell samples must be taken
- Infected cell cultures must be examined for cytopathic effects
- Parts of the virus & immune responses/antibodies must be screened
-Treatment of viral infections is also difficult because the drugs that attack the viral replication cycle also cause serious side effects to the host