Microbiology: A Clinical Approach © Garland Science

CHAPTER 12THE STRUCTURE AND INFECTION CYCLE OF VIRUSES

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OVERVIEW

VirusesViruses:• are noncellular or Acellular infectious agentsVirology:• study of viruses

Virologists:• scientists who study viruses

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.

…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.

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 layersCannot 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 componentsNucleic acids

Capsid

Envelope

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…Generalized Structure of Viruses

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

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 ENVELOPESEnvelopes vary in:

SizeMorphologyComplexityComposition

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.

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

VIRUS CLASSIFICATION

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

ICOSAHEDRAL VIRUSESTheir 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 INFECTIONFor animal viruses, there are six steps in lytic

infection:• Attachment• Penetration• Uncoating• Biosynthesis• Maturation• Release

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.

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

Viral Diseases (Next chapter)

A. InfluenzaB. Diseases Caused by the Herpes FamilyC. Viral HepatitisD. Human Immunodeficiency VirusE. Miscellaneous Viral Diseases

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/