1. CHAPTER 12 THE STRUCTURE AND INFECTION CYCLE OF VIRUSES
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2. OVERVIEW

3. Viruses Viruses: are noncellular or Acellular infectious agents
Virology:
study of viruses
Virologists:
scientists who study viruses

4. 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.

5. …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.

6. …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

7. …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.

8. The Size and Morphology of Selected Viruses
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9. Generalized Structure of Viruses
Viral components
Nucleic acids
Capsid
Envelope

10. …Generalized Structure of Viruses

11. The Structure of Viruses
Virion size range is ~ 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

12. 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.

13. VIRAL ENVELOPES Envelopes vary in: Size Morphology Complexity
Composition

14. 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.

15. 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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16. 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

17. VIRUS CLASSIFICATION

18. Helical Capsids-Tobacco Mosaic Virus
TMV are shaped like hollow tubes with protein walls
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19. Influenza Virus – an Enveloped Virus with a Helical Nucleocapsid
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20. HELICAL VIRUSES

21. ICOSAHEDRAL VIRUSES
Their shape is derived from 20 triangular faces that make up the capsid.
The capsid ‘has’ 12 points of symmetry.

22. Icosahedral capsids

23. THE INFECTION CYCLE
The infection cycle was first worked out in bacteriophages (bacterial viruses).
Animal virus infections can be either lytic or lysogenic.

24. 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.

25. LYTIC VERSUS LYSOGENIC INFECTION

26. LYTIC INFECTION
For animal viruses, there are six steps in lytic infection:
Attachment
Penetration
Uncoating
Biosynthesis
Maturation
Release

27. Attachment Specific surface structures on host to which viruses attach
Receptor sites:
Specific surface structures on host to which viruses attach
Specific for each virus
Can be proteins, lipopolysaccharides, techoic acids, etc.
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28. 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.

29. 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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30. Bacteriophages

31. Viruslike Agents
Prions

32. Fusion with Host Membrane
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33. Assignment
Homework: Important group of RNA viruses

34. Viral Diseases (Next chapter)
A. Influenza B. Diseases Caused by the Herpes Family C. Viral Hepatitis D. Human Immunodeficiency Virus E. Miscellaneous Viral Diseases

35. Bibliography
Microbiology, A clinical Approach -Danielle Moszyk-Strelkauskas-Garland Science 2010.
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