Transcript Understanding Our Environment

Viruses
Chapter 33
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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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The Nature of Viruses
•
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Viral Structure - Core of nucleic acid
surrounded by protein coat (capsid).
 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.
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The Nature of Viruses
•
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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.
Viral Shape
 Helical (Rodlike or threadlike)
 Isometric (Icosahedron)
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Bacteriophages
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Bacteriophages (phages)
are 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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
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 Cycle
 Lysogenic
(Temperate)
Viruses
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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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
AIDS
•
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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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
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.
- RNA and reverse transcriptase left
floating in cytoplasm.
 Double strand of DNA,complementary
to RNA, produced.
 Viruses released via exocytosis.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
The Future of HIV Treatment
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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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Disease Viruses
•
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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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Disease Viruses
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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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Disease Viruses
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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.
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Prions and Viroids
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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.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies