Transcript Viruses - Francis Howell High School

VIRUSES
• 1897: a Dutch scientist
suggested that something
smaller than a bacteria
could cause disease.
THE
DISCOVERY
• 1935: Wendell Stanley
OF VIRUSES
isolated crystals of tobacco
mosaic virus. Reasoned
that living organisms do
not crystallize, so he
inferred that viruses were
not truly alive.
VIRUSES CAN REPRODUCE
ONLY BY INFECTING
LIVING CELLS.
Structure and
composition
• The protein coat surrounding a
virus is called a capsid.
• Most viruses have proteins on
their surface membrane or
capsid that bind to receptor
protein on the cell
• Viruses must bind
precisely to proteins on
the host cell and then use
the host’s genetic
system.
• Most viruses can only
infect a very specific kind
of cell.
• Viruses that infect
bacteria are called
bacteriophages.
VIRAL
INFECTIONS
WHAT
HAPPENS
AFTER A
VIRUS
INFECTS A
CELL?
• Inside living cells, viruses
use their genetic
information to make
multiple copies of
themselves. Some
viruses replicate
immediately, while other
initially persist in an
inactive state within the
host.
LYTIC
INFECTIONS
Lysogenic Infections
RNA
VIRUSES
• About 70% of viruses contain RNA
rather than DNA.
• Example: Cold viruses—invade a host
cell, make many copies of the viral RNA.
Host cell makes other viral proteins and
capsids.
• Example: HIV– called a retrovirus.
When virus invades host cell, makes a
DNA copy of its RNA which inserts into
the host’s DNA. This DNA may remain
very inactive for many years.
EMERGING
VIRAL
DISEASES
• Caused by new or reappearing infections
agents that typically exist in animal
populations.
• Ex. Ebola—animals living in tropical forests
expose humans clearing forests for homes or
agriculture.
• Ex. Hantavirus, virus is found in deer mice and
other rodents, causes a very dangerous form
of pneumonia in humans.
• Ex. SARS –may have transferred to humans
from civet cats.
• Ex. Avian flu---what is the current situation?
PREVENTION
AND
TREATMENT
• Viral disease is prevented and
treated through vaccination,
vector control and drug
therapy
• Vaccinations: a harmless
versions of the virus,
bacterium or a toxin causes
an immune response.
• Viral vaccines can be made
from inactivated (can’t
replicated in a host), or
attenuated (weakened virus
can’t cause disease)
• Vector control: control animals that
carry the virus, like mosquitos for
yellow-fever, or rabies vaccinations
for pets to prevent humans from
getting rabies.
• Drug Therapy: Some antiviral drugs
interfere with viral nucleic acid
synthesis.
• Difficult to design a drug that blocks
the virus but doesn’t harm cells.