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Chapter 13
Characterizing and Classifying Viruses, Viroids, and
Prions
Viruses
" Cause many
infections of humans, animals, plants, and bacteria
" Cannot carry out
any metabolic pathway
" Neither grow nor
respond to the environment
" Cannot reproduce
independently
" Obligate
intracellular parasites
Characteristics of Viruses
" Cause most diseases
that plague industrialized world
" Virus miniscule,
acellular, infectious agent having one or several pieces of either DNA or RNA
" No cytoplasmic
membrane, cytosol, organelles (one exception)
" Have extracellular
and intracellular state
Characteristics of Viruses
" Extracellular
state
" Called
virion
" Protein coat
(capsid) surrounding nucleic acid
" Nucleic acid
and capsid also called nucleocapsid
" Some have
phospholipid envelope
" Outermost
layer provides protection and recognition sites for host cells
" Intracellular state
" Capsid
removed
" Virus exists
as nucleic acid
How Viruses Are Distinguished
" Type of genetic
material they contain
" Kinds of cells
they attack
" Size of virus
" Nature of capsid
coat
" Shape of virus
" Presence or
absence of envelope
Genetic Material of Viruses
" Show more variety
in nature of their genomes than do cells
" May be DNA or RNA;
never both
" Primary way
scientists categorize and classify viruses
" Can be dsDNA,
ssDNA, dsRNA, ssRNA
" May be linear and
composed of several segments or single and circular
" Much smaller than
genomes of cells
Hosts of Viruses
" Most only infect
particular kinds of hosts cells
" Due to
affinity of viral surface proteins or glycoproteins for complementary proteins
or glycoproteins on host cell surface
" May only infect
particular kind of cell in host
" Generalists
infect many kinds of cells in many different hosts
Capsid Morphology
" Capsids protein
coats that provide protection for viral nucleic acid and means of attachment to
hosts cells
" Capsid composed of
proteinaceous subunits called capsomeres
" Come capsids
composed of single type of capsomere; other composed of multiple types
The Viral Envelope
" Acquired from host
cell during viral replication or release; envelope is portion of membrane system of host
" Composed of
phospholipid bilayer and proteins; some proteins are virally-coded
glycoproteins (spikes)
" Envelopes
proteins and glycoproteins often play role in host recognition
Viral Replication-lytic and
lysogeny
" Dependent on
hosts organelles and enzymes to produce new virions
" Replication cycle
usually results in death and lysis of host cell lytic replication
" Stages of lytic
replication cycle
" Attachment
" Entry
" Synthesis
" Assembly
" Release
Replication of Animal Viruses
" Same basic
replication pathway as bacteriophages
" Differences result
from
" Presence of
envelope around some viruses
" Eukaryotic
nature of animal cells
" Lack of cell
wall in animal cells
Attachment of Animal Viruses
" Chemical
attraction
" Animal viruses do
not have tails or tail fibers
" Have glycoprotein
spikes or other attachment molecules that mediate attachment
Synthesis of Animal Viruses
" Each type of
animal virus requires different strategy depending on its nucleic acid
" Must consider
" How mRNA is
synthesized?
" What serves
as template for nucleic acid replication?
Assembly and Release of Animal Viruses
" Number of viruses
produced and released depends on type of virus and size and initial health of
host cell
" Enveloped viruses
cause persistent infections
" Naked viruses released
by exocytosis or may cause lysis and death of host cell
Latency of Animal Viruses
" When animal
viruses remain dormant in host cells
" May be prolonged
for years with no viral activity, signs, or symptoms
" Some latent
viruses do not become incorporated into host chromosome
" When provirus is
incorporated into host DNA, condition is permanent; becomes permanent physical
part of hosts chromosome
The Role of Viruses in Cancer
" Normally, animals
genes dictate that some cells can no longer divide and those that can divide
are prevented from unlimited division
" Genes for cell
division turned off or genes that inhibit division turned on
" Neoplasia
uncontrolled cell division in multicellular animal; mass of neoplastic cells is
tumor
" Benign vs.
malignant tumors
" Metastasis
" Cancers
Factors Involved in Activation of
Oncogenes
" Ultraviolet light
" Radiation
" Carcinogens
" Viruses
How Viruses Cause Cancer
" Some carry copies
of oncogenes as part of their genomes
" Some stimulate
oncogenes already present in host
" Some interfere
with tumor repression when they insert into hosts repressor gene
" Several DNA and
RNA viruses are known to cause ~15% of human cancers
" Burkitts
lymphoma
" Hodgkins
disease
" Kaposis
sarcoma
" Cervical
cancer
Culturing Viruses in the Laboratory
" In Whole Organisms
" Bacteria
" Plants and
Animals
" Embryonated
Chicken Eggs
" In Cell (Tissue
Culture)
Characteristics of Viroids
" Extremely small,
circular pieces of RNA that are infectious and pathogenic in plants
" Similar to RNA
viruses, but lack capsid
" May appear linear
due to H bonding
Characteristics of Prions
" Proteinaceous
infectious agents
" Composed of single
protein PrP
" All mammals
contain gene that codes for primary sequence of amino acids in PrP
" Two stable
tertiary structures of PrP
" Normal
functional structure with α-helices called cellular PrP
" Disease-causing
form with β-sheets called prion PrP
" Prion PrP converts
cellular PrP into prion PrP by inducing conformational change
Characteristics of Prions
" Normally, nearby proteins
and polysaccharides force PrP into cellular shape
" Excess PrP
production or mutations in PrP gene result in initial formation of prion PrP
" When prions
present, they cause newly synthesized cellular PrP to refold into prion PrP
Prion Diseases
" All involve fatal
neurological degeneration, deposition of fibrils in brain, and loss of brain
matter
" Large vacuoles
form in brain; characteristic spongy appearance
" Spongiform
encephalopathies BSE, CJD, kuru
" Only destroyed by
incineration; not cooking or sterilization
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