Download Nonliving characteristics of viruses - Lectures For UG-5

Virology
Lec 1
Dr Sadia Anjum
General features of Viruses
Viruses are infectious agents with both living and nonliving
characteristics.
Living characteristics of viruses
a. They reproduce at a fantastic rate, but only in living host cells.
b. They can mutate.
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 called Envelop
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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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
Generalized Structure of Viruses
Viral components
Nucleic acids
Capsid
Envelope
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…Generalized Structure of 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.
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:
1. Directly in the capsid-inner side of the protein coat
2. Enclosed in special proteins-nucleic acid binding protein
3. 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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Inc. Permission required for reproduction
or display.
14
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.
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.
• 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 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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Fusion with Host Membrane
Copyright © The McGraw-Hill Companies,
Inc. Permission required for reproduction
or display.
24
Viral Diseases (Next chapter)
A.
Influenza
B.
Diseases Caused by the Herpes Family
C.
Viral Hepatitis
D.
Human Immunodeficiency Virus
E.
Miscellaneous Viral Diseases
F.
HIV
G.
Enteric viral infection
Bibliography
Microbiology, A clinical Approach -Danielle MoszykStrelkauskas-Garland Science 2010.
https://files.kennesaw.edu/faculty/jhendrix/bio
2261/home.html
http://www.cdc.gov/cmv/