Bmsc210 m2 p5

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Bmsc210 m2 p5
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  1. Virus:
    genetic element that cannot replicateindependently of a living (host) cell
  2. Virology:
    the study of viruses
  3. Virus particle (virion):
    • extracellular form of a virus
    • – Exists outside host and facilitates transmission from one host cell to another
    • – Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material
  4. Viral Genomes (Figure 9.1)
    • – Either DNA or RNA genomes
    • – Some circular, but most linear
  5. – Most viruses are smaller than prokaryotic cells; range from ___ to ___
    0.02 to 0.3 μm
  6. -Most viral genomes are ___er than those of cells
    smaller
  7. Capsid:
    • the protein shell that surrounds the genome of a virus particle (Figure 9.2)
    • • Composed of a number of protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid
  8. Capsomere:
    • -subunit of the capsid
    • • Smallest morphological unit visible with an electron microscope
  9. Nucleocapsid:
    complete complex of nucleic acid and protein packaged in the virion (Figure 9.3)
  10. Enveloped virus:
    • – Have membrane surrounding nucleocapsid
    • • Lipid bilayer with embedded proteins
    • – Envelope makes initial contact with host cell
  11. Helical symmetry:
    length defined by:
    width defined by:
    • rod-shaped viruses (e.g.,tobacco mosaic virus)
    • • Length of virus determined by length of nucleic acid
    • • Width of virus determined by size and packaging of protein subunits
  12. Complex Viruses (Figure 9.5b)
    • – Virions composed of several parts, each with separate shapes and symmetries
    • – Bacterial viruses contain complicated structures
    • • Icosahedral heads and helical tails
  13. The Virus Host
    Viruses replicate only in certain types of cells or in whole organisms and have varying degrees of difficulty to grow
    Bacterial:
    Animal:
    Plant:
    Bacterial viruses are easiest to grow; modelsystems

    - Animal viruses (and some plant viruses) can becultivated in tissue or cell cultures

    - Plant viruses typically are most difficult because study often requires growth of whole plant
  14. The icosohedral shape has ___ triangular sides with a minumum of __ protein units per side
    • 20
    • 3
  15. Titer:
    number of infectious units per volume of fluid
  16. Plaque assay:
    analogous to the bacterial colony; one way to measure virus infectivity (Figure 9.6)
  17. Plaques
    • -clear zones that develop on lawns of host cells
    • • Lawn can be bacterial or tissue culture (Figure 9.7)
    • • Each plaque results from infection by a single virus particle
  18. cytopathic effects
    • -degenerative changes in cells associated with the multiplication of certain viruses.
    • -lysis is not observed
  19. Efficiency of plating
    • -used in quantitative virology
    • – The number of plaque-forming units is almost always lower than direct counts by electron microscopy due to
    • • Inactive virions
    • • Conditions not appropriate for infectivity
  20. Intact Animal Methods
    • – Some viruses do not show recognizable changes in cell cultures yet cause death or disease in whole animals
    • – Virus is diluted
    • – Animals are infected with viral dilution
    • – End point is calculated (LD50 or ID50)
  21. Phases of Viral Replication
    (5)
    – Attachment (adsorption) of the virus to asusceptible host cell

    – Entry (penetration) of the virion or its nucleic acid

    – Synthesis of virus nucleic acid and protein by cellmetabolism as redirected by virus

    – Assembly of capsids and packaging of viralgenomes into new virions (maturation)

    – Release of mature virions from host cell
  22. Virus replication
    • typically characterized by aone-step growth curve (Figure 9.9)
    • 􀅖 Latent period: eclipse + maturation
  23. Burst size:
    number of virions released
  24. Attachment of virion to host cell is highly specific
    (3points)
    • – Requires complementary receptors on thesurface of a susceptible host and its infectingvirus
    • – Receptors on host cell carry out normal functionsfor cell (e.g., uptake proteins, cell to cellinteraction)
    • – Receptors include proteins, carbohydrates,glycoproteins, lipids, lipoproteins, or complexes
  25. What effects does the attachment of a virus to its host cell have on both parties
    The attachment of a virus to its host cell results in changes to both virus and cellsurface that facilitate penetration
  26. Bacteriophage T4:
    (4steps)
    • -virus of E. coli; one of the mostc omplex penetration mechanisms (Figure 9.10)
    • – Virions attach to cells via tail fibers that interact with polysaccharides on E. coli cell envelope
    • – Tail fibers retract and tail core makes contact withE. coli cell wall
    • – Lysozyme-like enzyme forms small pore inpeptidoglycan
    • – Tail sheath contracts and viral DNA passes intocytoplasm
  27. Restriction-modification systems
    – DNA destruction system; only effective against double-stranded DNA viruses
  28. – Restriction enzymes (restriction endonucleases)
    • -cleave DNA at specific sequences
    • – Modification of host’s own DNA at restriction enzyme recognition sites prevents cleavage of own DNA
  29. Viral mechanisms to evade bacterial restriction systems
    (2)
    • – Chemical modification of viral DNA (glycosylation or methylation)
    • – Production of proteins that inhibit host cell restriction system
  30. Bacteriophage T4 glucosylates its cytosines converting them to _-___-___
    why?
    • 5-hydroxymethyl-cytosine
    • to avoid digestion by bacteria
  31. David Baltimore, Howard Temin, and Renato Dulbecco
    • discovered retroviruses andreverse transcriptase
    • – Shared 1975 Nobel Prize for Physiology or Medicine
    • -Baltimore developed classification scheme for viruses based on relationship of viral genome to its mRNA
  32. The Baltimore Classification Scheme (Figure 9.11)
    (6) d s ds ss+ ss- retro dsD
    • – Class I are double-stranded (ds) DNA viruses
    • – Class II are single-stranded (ss) DNA viruses
    • – Class III are dsRNA
    • – Class IV and V are ssRNA (+ or )
    • – Class VI are retroviruses
    • – Class VII are dsDNA viruses that replicate through an RNA intermediate
  33. Class 2 expression
    converted to dsDna and then uses Rna polymerase to synthesize mRna
  34. Class 3 expression
    use of RNA dependant RNA polymerase to replicate and create mRNA
  35. Class 4/5 expression
  36. Once a host has been infected, new copies of the viral ____ must be made and virus-specific ______ synthesized in order for the virus toreplicate
    • genome
    • proteins
  37. Production of Viral Nucleic Acid and Protein
    Generation of ___ ___ occurs first
    messenger RNA (mRNA)
  38. Viral genome serves as template for viral _____
    mRNA
  39. In some RNA viruses, ___ ___ itself is the mRNA
    viral RNA
  40. In some cases essential transcriptional enzymes are contained in the _____
    virion
  41. Nomenclature used to describe mRNA is used to describe the:
    configuration of the genome of a single-stranded DNA or RNA virus (mRNA is said to be in plus (+) configuration; its complement is in minus () configuration)
  42. Positive-strand RNA virus:
    single-stranded RNAgenome with same orientation as its mRNA
  43. Negative-strand RNA virus:
    single-stranded RNA genome with orientation complementary to its mRNA
  44. Viral Proteins
    – Production follows synthesis of viral _____
    mRNA
  45. • Early proteins
    (4points)
    • – synthesized soon after infection
    • – necessary for replication of virus nucleic acid– typically act catalytically
    • – synthesized in smaller amounts
  46. Late proteins
    • • Synthesized later
    • • Include proteins of virus coat
    • • Typically structural components
    • • Synthesized in larger amounts
  47. Best-studied bacteriophages infect ____
    • entericbacteria
    • – Examples of hosts: E. coli, Salmonella enterica
  48. Most phages contain _____ genomes
    dsDNA
  49. Viral Life Cycles 
    Virulent mode:
    viruses lyse host cells after infection
  50. Temperate mode:
    • viruses replicate their genomes in tandem with host genome and without killing host
    • • Virus can also be lytic
  51. Virulent Bacteriophages
    • – First viruses studied in detail contained linear, dsDNA genomes that infect enteric bacteria
    • – Examples include T1, T2, …, T7
    • • T2, T4, and T6 are closely related viruses; T4 most extensively studied
  52. T4
    • has a dsDNA genome that is circularly permuted and terminally redundant (Figure 9.13)
    • – Both factors affect genome packagingDNA contains the modified base5-hydroxymethylcytosine (Figure 9.14)
    • – DNA is resistant to virtually all knownrestriction enzymes
  53. T4 genome can be divided into three parts:___, ____ and ___ proteins
    early, middle, and late
  54. – Early and middle proteins:
    enzymes needed for DNA replication and transcription
  55. – Late proteins:
     head and tail proteins and enzymes required to liberate mature phage particles
  56. Temperate viruses:
    • can undergo a stable genetic relationship within the host (Figure 9.16)
    • – But can also kill cells through lytic cycle
  57. Lysogeny:
    state where most virus genes are not expressed and virus genome (prophage) isreplicated in synchrony with host chromosome
  58. Lysogen:
    • - a bacterium containing a prophage 
    • - Under certain conditions lysogenic viruses may revert to the lytic pathway and begin to produce virions
  59. Bacteriophage lambda (Figure 9.17)
    (5points)
    • – Linear, dsDNA genome
    • – Complementary, single-stranded regions 12 nucleotides long at the 5' terminus of each strand(Figure 9.18)
    • – Upon penetration, DNA ends base-pair, forming the cos site, and the DNA ligates and forms double-stranded circle
    • – When lambda is lysogenic, its DNA integrates into E. coli chromosome at the lambda attachment site (att)
    • – When it enters lytic pathway, lambdasynthesizes long, linear concatamers of DNAby rolling circle replication (Figure 9.19)
  60. Regulation of lytic vs. lysogenic events inlambda is controlled by a complex genetic switch (Figure 9.20)
    – Key elements are two repressor proteins:
    • • cI protein (the lambda repressor): causes repression of lambda lytic events
    • • Cro repressor: controls activation of lytic events
  61. Entire virion ___ the animal cell, unlike in prokaryotes
    enters
  62. Eukaryotic cells contain a ___, the site of replication for many animal viruses
    nucleus
  63. Animal viruses contain all known ___ of viral genome replication (Figure 9.21)
    modes
  64. Many more kinds of enveloped ___ viruses than enveloped ___ viruses exist
    • animal
    • bacterial
    • – As animal viruses leave host cell, they canremove part of host cell’s lipid bilayer for theirenvelope
  65. Persistent infections:
    • release of virions from host cell does not result in cell lysis
    • • Infected cell remains alive and continues to produce virus
  66. Latent infections:
    delay between infection by the virus and lytic events
  67. Transformation:
    conversion of normal cell into tumor cell
  68. Cell fusion:
    two or more cells become one cell with many nuclei
  69. Retroviruses:
    • RNA viruses that replicatethrough a DNA intermediate
    • – Enveloped viruses (Figure 9.23a)
    • – Contain a reverse transcriptase (copiesinformation from its RNA genome into DNA),integrase, and protease
    • – Virion contains specific tRNA molecules
  70. Retroviruses have a unique genome
    – Two identical ssRNA molecules of the plus (+)orientation
  71. Retroviruses Contain 3 specific genes
    • • gag: encode structural proteins
    • • pol: encode reverse transcriptase andintegrase
    • • env: encode envelope proteins
  72. Process of Replication of a Retrovirus (Figure9.24)
    • – Entrance into the cell
    • – Removal of virion envelope at the membrane
    • – Reverse transcription of one of the two RNAgenomes
    • – Integration of retroviral DNA into host genome
    • – Transcription of retroviral DNA
    • – Assembly and packaging of genomic RNA
    • – Budding of enveloped virions; release from cell

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