1. Viruses
Chapter 26

2. 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

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

4. Viral Structure

5. 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

6. Bacterial Virus

7. The Nature of Viruses
Viral replication - Viruses can reproduce only when they enter cells and utilize the host’s cellular machinery.
genes translated into proteins by the cell’s genetic machinery
Viral shape
helical - rodlike
isometric - spiral
icosahedron

8. Bacteriophages
Bacteriophages - viruses that infect bacteria
some named as members of a “T” series
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

9. Bacteriophages
Lysogenic cycle
Many bacteriophages integrate their nucleic acid into the genome of the infected host cell (prophage).
The integration of a virus into a cellular genome is termed lysogeny.
lysogenic cycle
lysogenic (temperate) viruses

11. 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

12. 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)
Infection cycle
In normal individuals, white blood cells patrol the bloodstream and attack invading bacteria or viruses.

13. 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 (gp120) on its surface that precisely fits a cell-surface marker protein (CD4) on surfaces of macrophages and T cells.

14. AIDS
After docking onto the macrophage CD4 receptor, HIV requires a second macrophage receptor (CCR5) to cross the cell membrane.
Once inside the macrophage, the HIV particle sheds its protective coat.
RNA and reverse transcriptase left floating in cytoplasm
double strand of DNA, complementary to RNA, produced
viruses released via exocytosis

16. The Future of HIV Treatment
Combination drug therapy
AZT and protease inhibitors
keeps disease in check
Vaccine therapy
may reduce reproductive capability of HIV
Blocking replication
chemokines bind to and block receptors
CAF prevents viral replication
Disabling receptors

18. Disease Viruses
Many human diseases are caused by viruses:
influenza, smallpox , chicken pox, herpes
Viruses may also play a 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

19. 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

20. Disease Viruses
Emerging viruses
viruses that originate in one organism and then pass to another and cause disease
Ebola
Severe Acute Respiratory Syndrome (SARS)
Viruses and cancer
Viruses are capable of altering growth properties of human cells they infect by triggering oncogene expression.

21. Prions and Viroids
Prions
infectious proteins that some believe may be responsible for transmissible spongiform encephalopathies
mad cow disease
Viroids
tiny, naked molecules of RNA that are an important infectious disease agent in plants

22. How Prions Arise

23. Summary
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