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1. Definition: Obligate intracellular parasite composed of: a. Nucleic acid - either DNA or RNA b. Protein coat
2. Characteristics a. Single type of nucleic acid - DNA or RNA b. Protein coat, or capsid, some have envelopes c. Multiply inside of living cells using the host cell machinery d. Direct the synthesis of structures to transfer viral nucleic acid to other cells
Host Range
1. The specific types of cells a virus can infect in its host species represent the host range of the virus.
2. Usually species specific.
3. Classification a. Animal virus b. Plant virus c. Bacterial virus (bacteriophage)
4. Host range is determined by attachment sites (receptors).
Viral Size
1. Determined by electron microscopy.
2. Ranges from 20 to 14,000 nm in length.
1. Virions are complete, fully developed viral particles composed of nucleic acid surrounded by a coat.
Nucleic acid
1. Viruses contain either DNA or RNA (not both). a. Nucleic acid may be single- or double-stranded. b. Nucleic acid may be circular or linear or separate molecules.
2. Nucleic acid:protein ranges from about 1% - 50%.
Capsid and Envelope
1. Capsid - protein coat a. Capsomeres are subunits of the capsid.

121 b. Protomeres are capsomere subunits.
2. Envelope – outer covering of some viruses, consists of some combination of: a. Lipids b. Proteins c. Carbohydrates d. Spikes are carbohydrate-protein projections from the envelope and may or may not be present i. Influenzavirus - Hemagglutinin and Neuraminidase.
3. Non-enveloped or naked viruses are protected by their capsid alone.
General Morphology
1. Based on capsid architecture. a. Helical viruses b. Polyhedral viruses
– many sides i. Icosahedron (20 triangular faces and 12 corners) c. Enveloped viruses end up being approximately spherical. i. Enveloped helical ii. Enveloped icosahedral d. Complex viruses are, well, complex. i. See bacteriophages.
1. Classification of viruses is based on type of nucleic acid, strategy for replication, and morphology.
2. Virus family names end in –viridae; genus names end in –virus.
3. A viral species is a group of viruses sharing the same genetic information and ecological niche.
4. Families of viruses that affect humans: a. DNA viruses b. RNA viruses
1. Viruses must be grown in living cells.
2. The easiest viruses to grow are bacteriophages.
Growing Bacteriophages In The Laboratory
1. The plaque method: a. Virus, bacteria, and agar mixed, plated and incubated. b. After replication the virus lyses the bacteria, forming plaques, or clear zones. c. Each plaque is assumed to come from a single viral particle. d. The titer of the virus is given in plaque forming units.

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Growing Animal Viruses In The Laboratory
1. Live animal cultures have to be used for some animal viruses.
2. Simian AIDS and feline AIDS provide models for studying human AIDS.
3. Embryonated eggs can serve as substitutes for some viruses. a. Can inoculate membrane that best supports specific virus (allantoic, amniotic, chorioallantoic, or yolk sac).
4. Cell culture is a lot cheaper and easier to work with (contamination can be a problem however). a. Primary cell lines have a short lifespan in culture
– a few generations before reaching senescence. b. Diploid cell lines are derived from embryos and can grow for up to 100 population doublings before senescence. c. Continuous cell lines are derived from transformed cells and grow indefinitely in culture. i. Hela cells – 1 st continuous cell line, derived from Helen Lane
(fictional name), a cancer patient who died in 1951. This is the oldest continuous cell line. d. Viral growth can cause cytopathic effects in the cell culture.
Viral Identification
1. Serological methods a. Western blotting
2. Cytopathic effects
3. Molecular methods include PCR and RFLPs.
1. Viruses do not contain enzymes for energy production or protein synthesis.
2.
For a virus to multiply, it must invade a host cell and direct the host’s metabolic machinery to produce viral enzymes and components.
Multiplication of Bacteriophages
1. The virus may cause lysis or lysogeny.
2. Events of the lytic cycle: a. Attachment or adsorption i. Requires a receptor b. Penetration i. T-evens release lysozyme to break down a portion of the cell wall. ii. The tail sheath contracts and the tail core is driven through the hole in the wall to the plasma membrane. iii. The viral genome is then injected into the bacterium. c. Biosynthesis

123 i. Viral DNA and proteins are synthesized. ii. Host protein synthesis is stopped by degradation of host DNA, interference with transcription, or repression of translation. iii. The eclipse period is the period when all viral particles are present but before they are assembled. d. During maturation or assembly phage DNA and capsids are assembled into complete viruses. e. Release occurs when phage lysozyme breaks down the cell wall and newly synthesized phage particles are released. f. Burst time is the time from phage adsorption to release. g. Burst size is the number of newly synthesized phages produced from one infected cell.
3. Lysogeny is a cycle in which the phage DNA recombines with the bacterial chromosome. a. The incorporated viral DNA is now a prophage. b. The prophage genes are regulated by a repressor coded for by the prophage, the prophage is replicated each time the host DNA is replicated. c. Exposure to mutagens can lead to excision of the prophage and initiation of the lytic cycle. d. Outcomes of lysogeny i. Immune to reinfection by the same kind of phage ii. Host cell exhibits new properties iii. Specialized transduction
Multiplication of Animal Viruses
1. Attachment or adsorption
2. Penetration a. Endocytosis b. Fusion
3. Uncoating of viral nucleic acid may be accomplished by host or viral enzymes.
4. Biosynthesis a. DNA of most DNA viruses is released into the nucleus of the host cell. b. Transcription and translation of early genes produces enzymes to reproduce viral DNA. c. Transcription and translation of late genes produces capsid proteins in the cytoplasm. d. RNA viruses multiply in the cytoplasm. RNA-dependent RNA polymerase synthesizes a double-stranded RNA. e. Sense strand (+ strand) can act as mRNA directly and as a template for antisense strand (- strand) synthesis. f. Retroviruses use reverse transcriptase (RNA-dependent DNA polymerase) to transcribe DNA from RNA
5. Maturation or Assembly
6. Release

124 a. Rupture
– naked viruses b. Budding – enveloped viruses
1. Rous chicken sarcoma
2. Virus-induced adenocarcinoma in mice
Transformation Of Normal Cells Into Tumor Cells
1. Oncogenes - Activation by mutagenic chemicals
2. Oncogenic viruses are capable of producing tumors
3. Oncogenic viruses integrate into host cell DNA.
4. Transformed cells lose contact inhibition, contain virus-specific antigens (TSTA and T antigen), exhibit chromosome abnormalities, and can produce tumors when injected into susceptible animals.
5. Protein kinases
DNA Oncogenic Viruses
1. Found among: a. Adenoviruses b. Herpesviruses c. Poxviruses d. Papovaviruses e. Hepadnaviruses f. EBV i. Burkitt’s lymphoma ii. Nasopharyngeal carcinoma g. Herpes simplex i. Association with cervical cancer
RNA Oncogenic Viruses
1. Retroviruses a. HIV b. HTLV-1 and HTLV-2 are associated with human leukemia and lymphomas
2. Reverse transcriptase produces viral DNA that integrates into host genome as a provirus.
3. Activation of Oncogenes a. Mutation b. Transduction c. Translocation d. Gene amplification

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1. Cold sores
2. Shingles
1. Usually fatal
2. Progressive encephalitis (rubella virus)
3. Progressive pneumonia (retrovirus)
1. Prions are infectious proteins.
2. Insoluble aggregates of protein with normal primary sequence but exhibit altered folding pattern.
3. Normal protein (PrP) coded for by a gene on chromosome 20
4. Abnormal form found in diseased animals.
5. Abnormally-folded proteins (PrP sc ) cause normal proteins to assume the pathogenic conformation.
6. Diseases (9 animal and 4 human at my last count) a. Scrapie (sheep) b. Transmissible mink encephalopathy c. Bovine spongiform encephalitis (mad cow disease) d. Kuru
– New Guinea, contracted by eating infected brain tissue
(cannabilism was a mourning rite among members of this particular tribe between about 1920 and 1950) e. Creutzfeldt-Jakob disease (CJD) is very similar and has a heritable form and may be passed by contact (neurosurgery, corneal transplants, pituitary-derived GH preparations f. Gerstmann-Straussler-Scheinker syndrome (GSS) similar, inherited
1. Chrysanthemum stunt disease
2. Citrus excortis disease
3. Coconut cadang-cadang
4. Cherry chloratic mottle
5. Cucumber pale fruit disease
6. Viroids are naked pieces of RNA that can cause some plant diseases a. Potato spindle tuber disease
1. Alzheimer’s senile dementia
2. Multiple sclerosis

3.
Parkinson’s disease
4. Amyotrophic lateral sclerosis
5. Diabetes mellitus
6. Rheumatoid arthritis
7. Hepatitis
8. Lupus erythematosus
9. Some neoplastic diseases
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