CHAPTER 12
THE STRUCTURE AND INFECTION CYCLE OF
VIRUSES
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Microbiology: A Clinical Approach © Garland Science
OVERVIEW
Viruses
Viruses:
• are noncellular or Acellular
infectious agents
Virology:
• study of viruses
Virologists:
• scientists who study viruses
General features of Viruses
 Viruses 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.
…General features of Viruses
2. 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.
…General features of Viruses
 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 Properties of viruses
 Consists of 1 molecule of DNA or RNA
enclosed in coat of protein
 May have additional layers
 Cannot reproduce independent of living cells
nor carry out cell division as procaryotes and
eucaryotes do
 An intact viral particle is called a virion.
The Size and Morphology of
Selected Viruses
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Generalized Structure of Viruses
Viral components
 Nucleic acids
 Capsid
 Envelope
9
…Generalized Structure of Viruses
The Structure of Viruses
 Virion size range is ~10-400 nm
 All 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
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.
VIRAL ENVELOPES
Envelopes vary in:
 Size
 Morphology
 Complexity
 Composition
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.
Capsids
 Capsids are large macromolecular structures
which serve as protein coat of virus
 Protect viral genetic material and aid in its
transfer between host cells
 Made 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
VIRUS CLASSIFICATION
Helical Capsids-Tobacco Mosaic
Virus
 TMV 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
ICOSAHEDRAL VIRUSES
 Their shape is derived from 20 triangular faces
that make up the capsid.
 The capsid ‘has’ 12 points of symmetry.
Icosahedral capsids
THE INFECTION CYCLE
• The infection cycle was first worked out in
bacteriophages (bacterial viruses).
• Animal virus infections can be either lytic or
lysogenic.
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.
LYTIC VERSUS LYSOGENIC
INFECTION
LYTIC INFECTION
 For animal viruses, there are six steps in lytic
infection:
• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Maturation
• Release
Attachment
Receptor sites:
 Specific surface structures on host to which
viruses attach
 Specific for each virus
 Can 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.
•
Entry into the Host
 Most bacterial viruses (bacteriophages) inject
their nucleic acid into host
 Eucaryotic viruses usually enter the cytoplasm
with the genome still enclosed.
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Bacteriophages
30
Viruslike Agents
 Prions
31
Fusion with Host Membrane
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Assignment
 Homework: Important group of RNA viruses
Viral Diseases (Next chapter)
A.
B.
C.
D.
E.
Influenza
Diseases Caused by the Herpes Family
Viral Hepatitis
Human Immunodeficiency Virus
Miscellaneous Viral Diseases
Bibliography
 Microbiology, A clinical Approach -Danielle
Moszyk-Strelkauskas-Garland Science 2010.
 http://en.wikipedia.org/wiki/Scientific_metho
d
 https://files.kennesaw.edu/faculty/jhendrix/bio
2261/home.html
 http://www.cdc.gov/cmv/