Bmsc210 m2 p5

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Bmsc210 m2 p5
2013-03-10 18:36:45
Bmsc210 m2 p5

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
  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 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
    – 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
    • – 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:
    • -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
    • – 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 _-___-___
    • 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 _____
  39. In some RNA viruses, ___ ___ itself is the mRNA
    viral RNA
  40. In some cases essential transcriptional enzymes are contained in the _____
  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 _____
  45. • Early proteins
    • – 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
  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)
    • – 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
  62. Eukaryotic cells contain a ___, the site of replication for many animal viruses
  63. Animal viruses contain all known ___ of viral genome replication (Figure 9.21)
  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