1. Viruses
Chapter 33
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2. The Discovery of Viruses The Nature of Viruses Bacteriophages
Outline
The Discovery of Viruses
The Nature of Viruses
Bacteriophages
Cell Transformation and Phage Conversion
AIDS
The Future of HIV Treatment
Disease Viruses
Prions and Viroids
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3. The Discovery of Viruses
Viruses possess only a portion of the properties of organisms.
Segments of DNA or RNA wrapped in a protein coat.
Must reproduce within cells.
Vary greatly in appearance and size.
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4. The Nature of Viruses
Viral Structure - Core of nucleic acid surrounded by protein.
Classified by nature of genomes.
RNA-based viruses - retroviruses.
Nearly all form a protein sheath or capsid around their nucleic acid core.
Many animal viruses form an envelope around the capsid.
Host Range - Suitable cells for a virus.

5. The Nature of Viruses Viral Shape Helical (Rodlike or threadlike)
Isometric (Icosahedron)
Bacteriophages (phages) are viruses that infect bacteria.
Some named as members of a “T” series.
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6. Raven - Johnson - Biology: 6th Ed
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7. Genes translated into proteins by the cell’s genetic machinery.
The Nature of Viruses
Viral Replication - Viruses can reproduce only when they enter cells and utilize host’s cellular machinery.
Genes translated into proteins by the cell’s genetic machinery.
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8. Bacteriophages Lytic Cycle
Tail fiber contacts lipoproteins of host bacterial cell wall.
Tail contracts and tail tube passes through opening in base plate, piercing bacterial cell wall.
Contents injected into host cytoplasm.
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9. Bacteriophages Lysogenic Cycle
Many bacteriophages integrate their nucleic acid into the genome of the infected host cell (prophage).
Integration of a virus into a cellular genome is termed lysogeny.
Lysogenic (Temperate) Viruses
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10. Cell Transformation and Phage Conversion
Transformation - Genetic alteration of a cell’s genome by the introduction of foreign DNA.
Phage Conversion - Foreign DNA contributed by bacterial virus.
Disease-causing bacteria Vibrio cholerae usually exists in harmless form.
Bacteriophage that infects V. cholerae introduces into the host bacterial cell a gene that codes for the cholera toxin.
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11. Compromising the Immune System
AIDS
Acquired Immunodeficiency Syndrome (AIDS) was first reported in the US in 1981.
Estimated over 33 million people worldwide are infected with Human Immunodeficiency Virus (HIV).
Compromising the Immune System
In normal individuals, white blood cells patrol the bloodstream and attack invading bacteria or viruses.
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12. AIDS
In AIDS patients, the virus hones in on CD4+ T cells, infecting and killing them.
Without T cells, the body cannot defend against invading bacteria or viruses.
Each HIV particle possesses glycoprotein (gp 120) on its surface that precisely fits a cell-surface marker protein (CD4) on surfaces of macrophages and T cells.
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13. AIDS
After docking onto macrophage CD4 receptor, HIV requires second macrophage receptor (CCR5) to cross the cell membrane.
Once inside the macrophage, the HIV particle sheds its protective coat.
viralRNA and reverse transcriptase left floating in cytoplasm.
Double strand of DNA,complementary to RNA, produced.
Viruses are released via exocytosis.
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15. The Future of HIV Treatment
Combination Drug Therapy
AZT and protease inhibitors
Keeps disease in check.
Using A Defective HIV Gene
May reduce reproductive capability of HIV.
Chemokines and CAF
Chemokines bind to and block receptors.
CAF Prevents viral replication.
Disabling Receptors
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16. Disease Viruses Many human diseases are caused by viruses:
Influenza, Smallpox ,Chicken Pox, Herpes
May also play role in autoimmune diseases such as multiple sclerosis and diabetes.
Influenza
Flu viruses are animal retroviruses distinguished by their capsid.
Types A, B, and C
Subtypes differ in protein spikes.
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17. Viral genes are readily re-assorted by genetic recombination.
Disease Viruses
Recombination
Viral genes are readily re-assorted by genetic recombination.
Novel combinations of H and N spikes unrecognizable by human antibodies.
Inability to make perfect vaccines.
Flu Pandemics
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18. Disease Viruses
Emerging Viruses
Viruses that originate in one organism and then pass to another and cause disease.
Ebola
Viruses and Cancer
Viruses are capable of altering growth properties of human cells they infect by triggering oncogene expression.
i.e. – human papilloma virus

19. Prions and Viroids Prions
Infectious proteins that some believe may be responsible for transmissible spongiform encephalopathies.
Viroids
Tiny, naked molecules of RNA that are an important infectious disease agent in plants.
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20. The Discovery of Viruses The Nature of Viruses Bacteriophages
Review
The Discovery of Viruses
The Nature of Viruses
Bacteriophages
Cell transformation and Phage Conversion
AIDS
The Future of HIV treatment
Disease Viruses
Prions and Viroids
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

21. Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies