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5 Virus characteristics
- 1. Obligate intracellular parasites
- 2. Filterable, smaller than bacteria
- 3. Contain DNA or RNA as genetic material 4. Reproduce by assembly only inside another cell
- 5. Lack enzymes to produce ATP
•Protein coat or capsid
•Genome = Nucleic acids with or without associated polymerase enzymes
•With or without surrounding lipid membrane (Enveloped or naked)
Classification of virus
- •Nucleic acid type
- –DNA, RNA, ds versus ss (+ve or -ve sense), circular or linear, single chromosome or segmented
- –E.g. Capsid structure, size, presence or absence of envelope
- •Family, (genus), species
DNA Organizational Structure
RNA Organizational Structure
Steps for Virus Reproduction
- 1. Attachment/absorption
- 2. Entry/penetration
- 3. Uncoating (only in animal viruses)
- 4. Synthesis
- 5. Reassembly
- 6. Release from cell
Cycle of Infection and Replication
How does Lysogenic cycle differ from lytic cycle?
- 1. Recombination of viral nucleic acids into the chromosome of the host acting like a transposon (prophage).
- 2. Production of repressors for the synthesis of viral sequences during lysogeny.
- 3. Lytic cycle switch with cell stress and damage e.g. UV or chemical exposure
How are viruses identified in the lab?
- •Electron microscopic morphology
- •Physical properties
- •Antigenic properties: Antibodies to specific proteins of different viruses
- •Genetic techniques
- –Hybridization with specific nucleic acid probes
- –Restriction digest profiles
What determines the Host range of viruses?
- •Presence of specific proteins or glycoproteins on the surface of the virus particle
- •Binds specific receptors on susceptible host cells
- •Cellular enzymes that support viral reactions
How do you culture of viruses?
- •Grow in living cells in culture
- –Cell lines generated from different kinds of tissues adapted to growth in culture
- •Living animals
- –Chicken embryos or other lab animals
How is the reproduction of animal virus different from that of bacteriophage?
- •1. Entry of animal virus include capsid. Just the nucleic acid in phages
- –Entry methods include
- •2. Uncoating of nucleic acid is necessary for the animal viruses
- –In the cytoplasm (RNA viruses) or nucleus (DNA viruses)
What determines the differences if the pathways of viral multiplication?
- •Multiplication cycle depends of the genetic material of the virus
- •1. Type of nucleic acids
- –DNA or RNA,
- •2. Single stranded (ss) or double stranded (ds)3. If ssRNA, positive or negative sense
What is the cell damage by viruses?
- •1. Cell death and lysis
- •2. None
- –Persistent production of enveloped virus
- –Latent prophage/provirus
- •3. Transformation - altered behavior of cells
5 Types of infection by viruses
- •Slow infections
How can some virus cause cancer?
- •Integration of DNA at randoms sites on host genome
- •Encode oncogenes
- –Such as tumor suppressors (cell cycle inhibitors)
- –Mutated forms of cell cycle proteins or cell division activator proteins
What is the Major Immune Defense against viruses?
- –Neutralization of virus attachment and hence reduce spread
- –Activation of destruction by phagocytes and complement (enveloped) or NK cells
- • 2. Tc or Td killing of virus infected cells - main process of virus clearance
-Prions contain protein only
-Examples of prion-caused human diseases are Kuru, Creutzfeldt-Jakob disease and Gerstmann-Straussler syndrome.
-Prions also cause scrapie in sheep.
- -Bovine spongiform encephalopathy (BSE)
- or Mad Cow
- -RNA only
- -Does not code for any proteins
- -Replicated in cells to large amounts overwhelming host (RNA polymerase)
- -Silencing RNA?
Hepatitis D >delta agent
- -only known human viroid disease
- -enclosed in a hepatitis B virus capsid.
- -simultaneous infection with both the hepatitis B virus and the hepatitis D viroid
- -D viroid causes liver cell death via sequestering this 7S RNA and/or cleaving it.