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Microbiology: A Clinical Approach © Garland Science
CHAPTER 12
THE STRUCTURE AND INFECTION CYCLE OF
VIRUSES
© Eye of Science / Science Photo Library
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.
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
…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
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
8
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... Companies, Inc Permission required for reproduction or display 19 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... 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 Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 15 GENOMIC PACKAGING • Genome packaging has an important role in the infection • Viral genomes are packaged in one of three ways:... 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... 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 Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 27 TYPES OF. .. reproduction or display 27 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 Copyright... 29 Bacteriophages 30 Viruslike Agents Prions 31 Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 32 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... required for reproduction or display 15 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 . © Garland Science
CHAPTER 12
THE STRUCTURE AND INFECTION CYCLE OF
VIRUSES
© Eye of Science / Science Photo Library
OVERVIEW
Viruses
Viruses:
•
are noncellular. 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
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