viruses introduction 1

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Microbiology: A Clin ical 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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Viruses

Viruses:

are noncellular or Acellular

infectious agents

Virology:study of viruses

Virologists:scientists who study viruses


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General features of VirusesViruses are infectious agents with both living

and nonlivingcharacteristics.

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 orRNA (not both)

Nucleic Acid is surrounded or coated by aprotein shell (capsid)

Some viruses possess a membrane-likeenvelope 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 proteinMay have additional layers

Cannot reproduce independent of living cellsnor carry out cell division as procaryotes andeucaryotes 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 components Nucleic 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 nm

All virions contain a nucleocapsidwhich iscomposed 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 = envelopedviruses

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 glycoproteinsand oligosaccharides associate with the

plasma membrane of the host cell.

All envelopes have a phospholipidbilayer.


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VIRAL ENVELOPES

Envelopes vary in:

Size

MorphologyComplexity

Composition


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ENVELOPE GLYCOPROTEINS They are firmly embedded in the envelope

bilayer.

This is facilitated by domains of host

membrane proteins calledspanners.

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 virus

Protect viral genetic material and aid in itstransfer 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


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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 Virusan 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 orlysogenic.

LYTIC VERSUS LYSOGENIC


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INFECTION In a lyticinfection, the host cells fills with

virions and bursts.

The result is cell death.

Lysogenic infections are also known as latentinfections.

The viral genome becomes incorporated

into the host cells DNA.

It can remain this way for an extended

period.

The host cell lives.


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INFECTION


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LYTIC INFECTION

For animal viruses, there are six steps in lyticinfection:

Attachment

Penetration

Uncoating

Biosynthesis

Maturation

Release


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AttachmentReceptor sites:

Specific surface structures on host to which

viruses attachSpecific 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.


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Entry into the HostMost bacterial viruses (bacteriophages) inject

their nucleic acid into host

Eucaryotic viruses usually enter the cytoplasmwith the genome still enclosed.



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Bacteriophages

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Fusion with Host Membrane



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Viral Diseases (Next chapter)

A. Influenza

B. Diseases Caused by the Herpes Family

C. Viral Hepatitis

D. Human Immunodeficiency Virus

E. Miscellaneous Viral Diseases


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Bibliography Microbiology, A clinical Approach -Danielle

Moszyk-Strelkauskas-Garland Science 2010.

http://en.wikipedia.org/wiki/Scientific_method

https://files.kennesaw.edu/faculty/jhendrix/bio2261/home.html

http://www.cdc.gov/cmv/
http://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Scientific_method

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