Chapter 12 structure of viruses revised

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
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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 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 span ners.
•
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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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 hollow tubes with protein
walls
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18
Influenza Virus – an Enveloped
Virus with a Helical Nucleocapsid
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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 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