Transcript Section 18.1 Summary – pages 475-483

Discuss with the people around you the difference between
these three words:
-Pathogen
-Infection
-Disease
-Epidemic
-Pandemic
Viruses are classified by:
• What type of genetic material they contain.
• Type of capsid that surrounds the nucleic acid.
• Presence of an envelope
• What types of cells or species it infects.
A pathogen is a disease-causing agent.
Once a pathogen, like a virus, begins multiplying
inside cells or tissues, we call this an infection.
A disease occurs when defenses cannot be mobilized
fast enough to keep the pathogen’s activities from
interfering with body functions.
An epidemic is when a disease spreads fast through
part of a population for a limited time, then subsides.
A pandemic is when an epidemic breaks out in several
countries at the same time.
AIDS is a pandemic.
The first case reported in 1981,
it has been estimated that 1.1 million people are
currently living with HIV in the U.S. today
(56,000 new REPORTED cases last year)
Currently, worldwide it is estimated 40.3 million are
HIV positive.
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With a healthy immune system: 1000 to 1500 CD4 cells (Helper T cells)
●
Once the CD4 count is below 200, the person is considered to have AIDS.
HIV - AIDS
• Lysogenic Cycle
• RNA retro-virus
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Infects helper T-cells
Has a protective envelope
Mutates at a high rate
10 different HIV strains, 1 super strain
HIGHLY Preventable.
What is a retrovirus?
• Once inside a host cell, the retrovirus makes DNA
from its RNA.
• To do this, it uses reverse transcriptase, an enzyme it
carries inside its capsid.
• This enzyme helps produce double-stranded DNA
from the viral RNA.
• Then the double-stranded viral DNA is integrated into
the host cell’s chromosome and becomes a provirus.
Retrovirus
RNA
Retrovirus
RNA
DNA is made from
the viral RNA.
DNA
Reverse
transcriptase
Entering
cell
Provirus in
host chromosome
mRNA
Retrovirus Cycle
New virus parts
Exiting
cell
New virus
forming
• The species specific characteristic of
viruses is significant for controlling the
spread of viral diseases.
For example, smallpox was easier to eradicate because
it only affects humans and since it is a DNA virus does
not mutate very often.
(unlike the bird flu and West Nile that affect several
types of animals.)
Small pox
Cancer and Viruses
• Some viruses have been linked to certain cancers in
humans and animals.
• These viruses disrupt the normal growth and division
of cells in a host, causing abnormal growth and
creating tumors.
HPV
• The first virus to be identified was a plant virus, called
tobacco mosaic virus, that causes disease in tobacco
plants.
Tobacco mosaic virus
causes yellow spots on
tobacco leaves, making
them unmarketable.
Prions
• Prions are abnormal neuron-proteins that behave like
viruses, but do not carry genetic information.
• Prions are thought to cause other neuron-proteins to
fold themselves incorrectly, resulting in improper
functioning.
Prions
• Prions are responsible for many fatal degenerative
diseases that first affect neural functioning, such as
mad cow disease, its human equivalent: CreutzfeldtJakob disease, and kuru.
Prions
• Scrapie, a prion disease, causes sheep to by crazy
and rub up against trees and posts until the scrape off
their wool.
Viroids
• Viroids are virus-like agents composed of a single
circular strand of RNA with no protein coat.
• The amount of viroid RNA is much less than the
amount found in viruses.
• Viroids have been shown to cause
infectious diseases in potatoes, citrus
and other crop plants each year- causing
millions of dollars in damage.