Medical Microbiology: Chapter 29

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Medical Microbiology: Chapter 29
2014-08-24 10:07:04
Medical Microbiology

Class reading assignment.
Show Answers:

  1. What is a Capsid?
    The protein shell, or coat, that encloses the nucleic acid genome.
  2. What is a Capsomere?
    Morphologic units seen in the electron microscope on the surface of icosahedral virus particles. Capsomeres represent clusters of polypeptides, but the morphologic units do not necessarily correspond to the chemically defined structural units.
  3. What is a Defective Virus?
    A virus particle that is functionally deficient in some aspect of replication.
  4. What is a an Envelope?
    A lipid-containing membrane that surrounds some virus particles. It is acquired during viral maturation by a budding process through a cellular membrane. Virus-encoded glycoproteins are exposed on the surface of the envelope. These projections are called peplomers.
  5. What is a Nucleocapsid?
    The protein–nucleic acid complex representing the packaged form of the viral genome. The term is commonly used in cases in which the nucleocapsid is a substructure of a more complex virus particle.
  6. What are structural units?
    The basic protein building blocks of the coat. They are usually a collection of more than one nonidentical protein subunit. The structural unit is often referred to as a protomer.
  7. What is a subunit?
    A single folded viral polypeptide chain.
  8. What is a Virion
    The complete virus particle. In some instances (eg, papillomaviruses, picornaviruses), the virion is identical with the nucleocapsid. In more complex virions (herpesviruses, orthomyxoviruses), this includes the nucleocapsid plus a surrounding envelope. This structure, the virion, serves to transfer the viral nucleic acid from one cell to another.
  9. What is the first theory of viral origin?
    Viruses may be derived from DNA or RNA nucleic acid components of host cells that became able to replicate autonomously and evolve independently. They resemble genes that have acquired the capacity to exist independently of the cell. Some viral sequences are related to portions of cellular genes encoding protein functional domains. It seems likely that at least some viruses evolved in this fashion.
  10. What is the second theory of viral origin?
    Viruses may be degenerate forms of intracellular parasites. There is no evidence that viruses evolved from bacteria, although other obligately intracellular organisms (eg, rickettsiae and chlamydiae) presumably did so. However, poxviruses are so large and complex that they might represent evolutionary products of some cellular ancestor.
  11. What are the 7 characteristics that are used to classify viruses?
    1. Virion morphology, including size, shape, type of symmetry, presence or absence of peplomers, and presence or absence of membranes.

    2. Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome in kilobases (kb) or kilobase pairs (kbp), strandedness (single or double), whether linear or circular, sense (positive, negative, ambisense), segments (number, size), nucleotide sequence, G + C content, and presence of special features (repetitive elements, isomerization, 5′-terminal cap, 5′-terminal covalently linked protein, 3′-terminal poly(A) tract).

    3. Genome organization and replication, including gene order, number and position of open reading frames, strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release).

    4. Virus protein properties, including number, size, and functional activities of structural and nonstructural proteins, amino acid sequence, modifications (glycosylation, phosphorylation, myristylation), and special functional activities (transcriptase,reverse transcriptase, neuraminidase, fusion activities).

    5. Antigenic properties.

    6. Physicochemical properties of the virion, including molecular mass, buoyant density, pH stability, thermal stability, and susceptibility to physical and chemical agents, especially ether and detergents.

    7. Biologic properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms, and pathology.
  12. Viruses are separated into major groupings called _____. 

    Virus _____ names hace the suffix - _____.


  13. Genus names carry the suffix ____?
  14. What are Viroids?
    Viroids are small infectious agents that cause diseases of plants. Viroids are agents that do not fit the definition of classic viruses. They are nucleic acid molecules without a protein coat. Plant viroids are single-stranded, covalently closed circular RNA molecules consisting of about 360 nucleotides and with a highly base-paired rodlike structure. Viroids replicate by an entirely novel mechanism. Viroid RNA does not encode any protein products; the devastating plant diseases induced by viroids occur by an unknown mechanism. To date, viroids have been detected only in plants; none have been demonstrated to exist in animals or humans.
  15. What are prions?
    Prions are infectious particles composed solely of protein with no detectable nucleic acid. They are highly resistant to inactivation by heat, formaldehyde, and ultraviolet light that inactivate viruses. The prion protein is encoded by a single cellular gene. Prion diseases, called "transmissible spongiform encephalopathies," include scrapie in sheep, mad cow disease in cattle, and kuru and Creutzfeldt-Jakob disease in humans. Prions do not appear to be viruses.
  16. List the different structural classifications of virons.
    morphogenesis and release from infected cells

    transmission to new hosts

    attachment, penetration, and uncoating in newly infected cells.
  17. What methods are used to resolve fine differences in the basic morphology of viruses?
    Electron microscopy

    Cryoelectron microscopy

    X-ray Diffraction
  18. How do you use an electron microscope to study viral symmetry?
    You use a heavy metal to emphasize the surface structure. The metal permeates the virus and brings out the surface structure by "negative staining".
  19. How do you use cryoelectron microscopy to study viral structures?
    You quick freeze the virus in vitreous ice. This preserves fine structural features and it avoids the use of negative staining. Three-dimensional structural information can be obtained by using computers to process the image.
  20. Viral architecture can be grouped into three types based on the arrangement of morphologic subunits: ___,___,___.
    Cubic Symmetry

    Helical Symmetry

    Complex Structures
  21. All cubic symmetry observed with animal viruses is of the  _____ pattern, which is the _____ arrangement for subunits in a closed shell.

    Most efficient
  22. There are _____ identical subunits on the surface of an icosahedron.
  23. To build a particle size adequate to encapsidate viral genomes, viral chells are composed of multiples of _____ structural units.
  24. Most viruses that have icosahedral symmetry do not have Icosahedral shape, rather, they appear to be _____.
  25. _____ or _____ are involved in condensation of the nucleic acid into a form suitable for packaging.
    virus-encoded core proteins

    cellular histones
  26. Icosahedral capsids are formed _____ of nuclei acid.
  27. True or False: Both DNA and RNA viral groups exhibit examples of cubic symmetry.
  28. Is it possible for empty icosahedral capsids to from?
    Yes, because they form independently of nucleic acids.
  29. Describe the structure of helical symmetry.
    the nucleocapsid is coiled inside a lipid-containing envelop.
  30. All known ecamples of animal viruses with helical symmetry contain _____ and have flexible _____ that are wound into a ball inside the envelop.
    RNA Genomes

  31. What are the two most common methods of measuring the size of a virus?
    Electron microscopy and ultracentrifuge
  32. Viruses range in diameter from about ___ to ___.
    20 nm

  33. What are the functions of structural proteins of viruses?
    Facilitate transfer of viral nucleic acid from host cell to another.

    Protect the viral genome against inactivation by nucleases.

    Participate in the attachment of the virus particle to a susceptible cell.

    Provide the structural symmetry of the virus particle.
  34. Describe how proteins or glycoproteins determine the antigenic characteristics of the virus.
    The host's protective immune response is directed against antigenic determinants of proteins or glycoproteins exposed on the surface of the virus particle.
  35. Describe the role of enzymes found inside some virons.
    Although they are probably not important in the structure of viruses, they are essential for the initiation of the viral replicative cycle when the viron enters a host cell.
  36. What are some examples of enzymes found in some virons?
    RNA polymerase

    Reverse transctiptase
  37. What techniques permit the study of transcription of the viral genome within the infected cell?
    PCR and molecular hybridization
  38. What is a primary cell culture?
    These are made by dispersing cells (Usually with trypsin) from freshly removed host tissue. They are unable to grow for more than a few passages in culture usually.
  39. What are secondary cell cultures?
    Diploid cell lines that have undergone a change that allows their limited culture (50 passages) but that retain their normal chromosome pattern.
  40. What are tertiary cell cultures?
    Continuous cell lines are cultures capable of more prolonged growth that have been derived from diploid cell lines or from malignant tissues. They invariably have altered and irregular numbers of chromosomes.
  41. Describe the was that multiplication of a virus can be monitored.
    1. Development of cytopathic effects (ie, morphologic changes in the cells). Types of virus-induced cytopathic effects include cell lysis or necrosis, inclusion formation, giant cell formation, and cytoplasmic vacuolization (Figure 29-4A, B, and C). Most viruses produce some obvious cytopathic effect in infected cells.

    2. Appearance of a virus-encoded protein, such as the hemagglutinin of influenza virus. Specific antisera can be used to detect the synthesis of viral proteins in infected cells.

    3. Detection of virus-specific nucleic acid. Molecular-based assays such as polymerase chain reaction provide rapid, sensitive, and specific methods of detection.

    4. Adsorption of erythrocytes to infected cells, called hemadsorption, caused by the presence of virus-encoded hemagglutinin (parainfluenza, influenza) in cellular membranes. This reaction becomes positive before cytopathic changes are visible and in some cases occurs in the absence of cytopathic effects.

    5. Viral growth in an embryonated chick egg may result in death of the embryo (eg, encephalitis viruses), production of pocks or plaques on the chorioallantoic membrane (eg, herpes, smallpox, vaccinia), or development of hemagglutinins in the embryonic fluids or tissues (eg, influenza).
  42. Describe Inclusion Bodies.
    Virus specific structures that become far larger than an individual virus particle. They can be situated in the nucleus, cytoplasm, or both. These bodies can be the site of development for virons.
  43. True or False: All viruses are destroyed in basic conditions.