5: Viruses Flashcards
state the contributions of Ivanowski, d’Herelle, and Reed to the discovery of viruses
Dimitri Ivanovsky - Russian botanist, studying the tobacco mosaic virus, removed all the bacteria using a filter to discover viruses - first discovered viruses as an infectious agent
Félix d’Herelle - first discovered viruses that infect bacteria - bacteriophage - plaques
Walter Reed - used military volunteers and exposed them to mosquitoes to confirm the transmission of Yellow Fever
describe the structure of viruses, including size, genome, capsule, and envelope
Typically 10-100 nm in diameter - smaller than bacteria
Genome - few thousand-200,000 base pairs (a few genes to 200 genes)
Capsid - protein shell that encircles its genome, found in all viruses
Envelope - found in some viruses, lipid bilayer usually obtained from the cytoplasmic membrane of the host cell
list the steps of viral replication
Attach Enter Uncoat Synthesize Assemble Exit
distinguish between viral cell entry mechanisms of enveloped and non-enveloped viruses and contrast these with bacterial and plant viruses
Enveloped Viruses:
1- Membrane Fusion (recognizes receptor on the plasma membrane, fusing with the membrane of their host. CD4 proteins on T cells are receptors for viruses. i.e. HIV)
2- Endocytosis (recognizes a receptor on the plasma membrane, begins endocytosis into the cytoplasm, membrane fusion is initiated by acidification of the endosome. i.e. Influenza, releasing many nucleocapsids)
Non-enveloped Viruses:
Endocytosis (recognizes a surface receptor on the cell membrane, initiates endocytosis, released from the formed endosome and uncoats to release its genome)
Plants:
Require physical damage to their cells wall to expose their cells to viruses
Bacteria:
Tail fibers bind to receptors on the cell wall, causing the capsid shape, core tube passes through the plasma membrane. Never enters the cell
distinguish between lytic and lysogenic replication cycles of viruses
Lytic = immediately kills host cell Lysogenic = phage integrates its genome into the hose cell genome, stable virus genome in host genome = prophage
describe the contrasting hypotheses for the origins of viruses
Coevolution Theory: coevolve with their hosts over time
Regressive Theory: viruses were original living cellular organisms, lost genes they no longer needed
Progressive Theory: combination of genetic material
differentiate between methods for cultivating, purifying, and quantifying viruses
Cultivating Bacteriophages: Mixing viruses with susceptible bacteria, mixed in with agar, poured onto a nutrient agar base and grown
Cultivating Animal Viruses: use duck and chicken eggs
Purifying -
Filtration: larger particles of debris will be filtered out
Differential Centrifugation: 3 successively faster centrifuge speeds, allows for concentration
Gradient Centrifugation: layering a solute at decreasing concentrations from bottom to top, putting the suspension of viruses, will find their density with centrifuging
Quantifying -
contrast the different methods of viral classification
International Committee on Taxonomy of Viruses: uses the Linnaean classification scheme of other forms of life, order names all end in “‑virales,” families end in “‑viridae, as well as morphology, genome structure and biological features
Baltimore Classification System: based on genome type and structure
describe viroids and prions
viroid = RNA with some spectacular secondary structure, making it resistant to ribonuclease degradation. Contain less than 400 nucleotides, no protein or envelope and do not code for any genes prions = misfolded proteins that are contagious
explain the basic mechanism of the CRISPR/Cas system and its potential for genome engineering
locus on the end of bacterial gene that provides a record of encountered viruses
Describe the Attach step of virus replication
recognizes specific cell-surface proteins, sugar modified proteins or polysaccharides
Describe the Attach step of virus replication
recognizes specific cell-surface proteins, sugar modified proteins or polysaccharides
differentiate between methods for quantifying viruses
Direct Count: using electron microscopy. Very easy to do. But expensive equipment and doesn’t differentiate from infectious and noninfectious particles
Hemagglutination Assay: viruses that recognize the sailic acid residues on RBC will stick to them, creating rafts. On a titer plate will have positive, shields, or negative, buttons, depending on the quantity of viruses in the dilutions. PRO Cheap and easy method. CON only works on certain viruses (most enveloped, animal viruses) and doesn’t tell if the particles or viable or the exact quantity
Plaque Assay: plating series of dilutions of virus samples, individually count plaques and then determine the number of plaque-forming unit (PFU’s)
differentiate between methods for quantifying viruses
Direct Count: using electron microscopy. Very easy to do. But expensive equipment and doesn’t differentiate from infectious and noninfectious particles
Hemagglutination Assay: viruses that recognize the sailic acid residues on RBC will stick to them, creating rafts. On a titer plate will have positive, shields, or negative, buttons, depending on the quantity of viruses in the dilutions. PRO Cheap and easy method. CON only works on certain viruses (most enveloped, animal viruses) and doesn’t tell if the particles or viable or the exact quantity
Plaque Assay: plating series of dilutions of virus samples, individually count plaques and then determine the number of plaque-forming unit (PFU’s). Counts only the infectious viruses
Endpoint Assay: Using serial dilutions of animal viruses, estimate the concentration of viruses by identifying the dilution that produced 50% of cultured cells showing cytopathic effects (cell pathogenesis effects) - tissue infectious dose 50 (TCID50) or lethal dose 50 (LD50)
