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Microbiology: A Clinical Approach © Garland Science
CHAPTER 12THE STRUCTURE AND INFECTION CYCLE OF VIRUSES
© Eye of Science / Science Photo Library
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OVERVIEW
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VirusesViruses:• are noncellular or Acellular infectious agentsVirology:• study of viruses
Virologists:• scientists who study viruses
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General features of VirusesViruses are infectious agents with both living
and nonliving characteristics.
1. Living characteristics of viruses a. They reproduce at a fantastic rate, but only
in living host cells.
b. They can mutate.
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…General features of Viruses2. Nonliving characteristics of viruses
They are acellular, that is, they contain no cytoplasm or cellular organelles.
They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery.
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Virus particles contains either DNA or RNA (not both)
Nucleic Acid is surrounded or coated by a protein shell (capsid)
Some viruses possess a membrane-like envelope surrounding the particle
…General features of Viruses
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…General Properties of viruses
Consists of 1 molecule of DNA or RNA enclosed in coat of protein
May have additional layersCannot reproduce independent of living
cells nor carry out cell division as procaryotes and eucaryotes do
An intact viral particle is called a virion.
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The Size and Morphology of Selected Viruses
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Generalized Structure of Viruses
Viral componentsNucleic acids
Capsid
Envelope
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…Generalized Structure of Viruses
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The Structure of Viruses
Virion size range is ~10-400 nmAll virions contain a nucleocapsid which is
composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
Some viruses consist only of a nucleocapsid, others have additional components
Envelopes virions having envelopes = enveloped
viruses virions lacking envelopes = naked viruses
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VIRAL ENVELOPES
• Many viruses that infect humans and other animals are enveloped.
• Envelopes form when viral glycoproteins and oligosaccharides associate with the plasma membrane of the host cell.
• All envelopes have a phospholipid bilayer.
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VIRAL ENVELOPESEnvelopes vary in:
SizeMorphologyComplexityComposition
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ENVELOPE GLYCOPROTEINS• They are firmly embedded in the envelope
bilayer.• This is facilitated by domains of host
membrane proteins called spanners.• They can form spikes or other structures on the
outside of the virion.• These can be used to attach to a host cell.
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CapsidsCapsids are large macromolecular structures
which serve as protein coat of virusProtect viral genetic material and aid in its
transfer between host cellsMade of protein subunits called protomers
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GENOMIC PACKAGING• Genome packaging has an important role in the
infection. • Viral genomes are packaged in one of three
ways:• Directly in the capsid-inner side of the
protein coat• Enclosed in special proteins-nucleic acid
binding protein• Enclosed in proteins from the host cell
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VIRUS CLASSIFICATION
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Helical Capsids-Tobacco Mosaic VirusTMV are shaped like hollow tubes with
protein walls
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Influenza Virus – an Enveloped Virus with a Helical Nucleocapsid
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HELICAL VIRUSES
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ICOSAHEDRAL VIRUSESTheir shape is derived from 20 triangular faces
that make up the capsid.The capsid ‘has’ 12 points of symmetry.
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Icosahedral capsids
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THE INFECTION CYCLE• The infection cycle was first worked out in
bacteriophages (bacterial viruses).• Animal virus infections can be either lytic or
lysogenic.
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LYTIC VERSUS LYSOGENIC INFECTION• In a lytic infection, the host cells fills with
virions and bursts.• The result is cell death.
• Lysogenic infections are also known as latent infections.• The viral genome becomes incorporated
into the host cell’s DNA.• It can remain this way for an extended
period.• The host cell lives.
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LYTIC VERSUS LYSOGENIC INFECTION
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LYTIC INFECTIONFor animal viruses, there are six steps in lytic
infection:• Attachment• Penetration• Uncoating• Biosynthesis• Maturation• Release
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AttachmentReceptor sites:
Specific surface structures on host to which viruses attach
Specific for each virusCan be proteins, lipopolysaccharides,
techoic acids, etc.
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TYPES OF RECEPTOR BINDING• Non-enveloped viruses
• Binding takes place between viral capsid and receptor.• Enveloped viruses
• Binding takes place between viral envelope proteins and receptor.
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Entry into the HostMost bacterial viruses (bacteriophages) inject
their nucleic acid into hostEucaryotic viruses usually enter the cytoplasm
with the genome still enclosed.
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Bacteriophages
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Viruslike AgentsPrions
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AssignmentHomework: Important group of RNA
viruses
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Viral Diseases (Next chapter)
A. InfluenzaB. Diseases Caused by the Herpes FamilyC. Viral HepatitisD. Human Immunodeficiency VirusE. Miscellaneous Viral Diseases
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BibliographyMicrobiology, A clinical Approach -Danielle
Moszyk-Strelkauskas-Garland Science 2010.http://en.wikipedia.org/wiki/Scientific_meth
odhttps://files.kennesaw.edu/faculty/
jhendrix/bio 2261/home.html
http://www.cdc.gov/cmv/