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Viruses
Say Hello to My Little Friend
What is a virus?
• nucleic acids enclosed
in a protein coat
• Very tiny-smaller than
the smallest bacteria
What do they look
like?
• A virus has an inner core of
nucleic acid, either RNA or
DNA, and an outer protein
coat called a capsid.
• Some relatively large
viruses, such as human flu
viruses, may have an
additional layer, called an
envelope, surrounding
their capsids.
Bacteria have viruses
too
A virus that infects a
bacterium is called a
bacteriophage or phage
for short.
A phage has a tail
structure as well as a
capsid.
Viruses have DNA
• Viral nucleic acid is
either DNA or RNA and
contains instructions
for making copies of
the virus.
• Some viruses have only
four genes, while
others have hundreds.
Infection
Before a virus can replicate,
it must enter a host cell.
A virus recognizes and
attaches to a host cell when
one of its proteins interlocks
with a molecular shape that
is the receptor site on the
host cell’s plasma
membrane.
Attachment to host
cell
A protein in the tail fibers
of the bacteriophage T4
recognizes and attaches
the T4 to its bacterial
host cell.
In other viruses, the
attachment protein is in
the capsid or in the
envelope.
Attachment is a
specific process
Attachment is a specific process
• Each virus has a specifically shaped
attachment protein. Therefore,
each virus can usually attach to
only a few kinds of cells.
• In general, viruses are species
specific, and some also are celltype specific. For example, polio
viruses normally infect only
intestinal and nerve cells.
Species Specific
The species specific
characteristic of viruses is
significant for controlling
the spread of viral
diseases.
Can you think of a virus
that can cross species?
Some examples of
viruses:
•
•
•
•
•
•
•
Influenza
Rabies
Ebola
HIV
Polio
Measles
HPV
Is a virus a living
thing?
Most biologists consider viruses
to be nonliving because they
don’t exhibit all the criteria for
life.
• They don’t carry out
respiration, grow, or develop.
• All viruses can do is
replicate—make copies of
themselves—and they can’t
even do that without the help
of living cells.
Viruses can not
reproduce
Viruses need to attach to
and infect a host cell in
order to replicate. They
do not have the
machinery required, so
they must take over the
host cell and force it to
make copies of the virus
How do they get in the
cell?
The virus attaches to the
outside of the cell and
then either injects it’s
genetic material into the
cell or is enveloped by
the cell and absorbed
into the cell in a vacuole
which is then burst,
releasing the virus.
Two cycles of
Replication
Lytic and Lysogenic
Lytic cycle
The host cell uses its own
enzymes, raw materials,
and energy to make
copies of viral genes that
along with viral proteins
are assembled into new
viruses, which burst from
the host cell, killing it.
Lytic Cycle
• The new viruses can then infect and kill
other host cells.
Lysogenic Cycle
Not all viruses kill the
cells they infect. Some
viruses go through a
lysogenic cycle, a
replication cycle in which
the virus’s nucleic acid is
integrated into the host
cell’s chromosome.
Cycles combined
Therefore, every cell that
originates from an
infected host cell has a
copy of the virus. The
lysogenic phase can
continue for many years.
However, at any time, the
provirus can be activated
and enter a lytic cycle.
Examples of viruses
that have a lysogenic
cycle
Herpes
(simplex I and simplex II)
Chicken pox
Release of Virus from
the host cell
Either lysis, the bursting of a cell, or
exocytosis, the active transport
process by which materials are
expelled from a cell, release new
viruses from the host cell.
The plasma membrane surrounds
the virus, enclosing it in a vacuole
that then fuses with the host cell’s
plasma membrane. Then, the
viruses are released to the outside.
VACCINES
The only reason most of us are alive right now
We’ve previously
discussed some
diseases that are
vaccine preventable:
Measles
Mumps
Rubella
Polio
Influenza HPV
Hepatitis B Tetanus
Pertussis
Diptheria
Starting from the Beginning:
Edward Jenner:
He is credited with developing
the first smallpox inoculation.
Noticing that milkmaids who had
caught cowpox did not catch the
smallpox virus, he would apply a
small amount of pus from a sore of
someone who had the virus to the
arm of an uninfected person,
allowing their body to develop an
immunity to the virus. Coined the
term VACCINE from the name of
cowpox: Variolae vaccinae
Jonas Salk
Developed the first
inactivated polio vaccine.
Polio was a worldwide
epidemic, with about
20,000 cases/yr in the US.
Refused to patent the
vaccine-received no profit
from it. “There is no
patent-could you patent
the sun?”
So how do they work?
A vaccine works by exposing the
body’s immune system to an antigen.
Antigens stimulate the production of
antibodies by the immune system.
The antibodies kill off the virus, and
“keep a record” of the antigen that
they had to fight off.
If the immune system is exposed to
the antigen again, it will recognize and
destroy the virus with it’s ready and
waiting antibodies.
Antigens and Antibodies
• Each antibody can usually only work on one
antigen
• Once the immune system has been exposed
to an antigen, it will have a supply of
antibodies specifically to fight it
• Once a person has been exposed to a virus,
they will develop an active immunity to it,
meaning their body is producing antibodies
to fight that particular antigen. This is a
strong immunity and can often be
permanent
• There is also passive immunity-where a
person gains antibodies produced by
another human or animal, for example:
unborn babies from their mother. This type
is weaker and not as long lasting as active
immunity
But won’t it make you
sick?!
Vaccines do have side effects,
yes, but they are designed in
such a way to be safe.
There are multiple different
kinds of vaccines, and all of
them involve weakening,
killing, modifying or isolating
parts of the virus.
Due to these alterations, the
virus in the vaccine is not able
to cause the disease that it will
cause the immune system to
develop antibodies for.
Types of vaccines
http://www.vaccines.gov/more_info/types/i
ndex.html
• Live attenuated
• Inactivated Vaccines
• Subunit Vaccines
• Toxoid Vaccines
• Conjugate Vaccines
• DNA Vaccines
• Recombinant Vector
Vaccines
Live Attenuated
Vaccines
• Contain a weakened form of the virus,
usually through growing them in
different host cells
• Are the best “teachers” of the immune
system
• Force a strong immune response and can
give lifelong immunity to the disease
• Cons: people with compromised immune
systems can not receive live attenuated
vaccines
• Live attenuated vaccines need to be
refrigerated
• Very remote chance of mutating to
virulent form
Inactivated Vaccines
• The virus is “inactivated” or “killed”
by using heat, radiation, or chemicals
• Safer than live attenuated vaccinesno threat of mutation to virulent
form
• Does not need to be refrigerated,
can usually be transported freeze
dried
• Cons: Forces a weaker immune
response, causing the need for
booster shots
Subunit Vaccines
• Contain only the antigens of the
virus
• Antigens are the proteins that are
recognized by the immune system
that illicit a response
• Because this vaccine does not
contain the entire microbe, adverse
reactions to it are less likely
• Contain only the antigens that get
the best immune response
• Either grown in a lab and then
“broken apart” with chemicals, OR
antigens produced from the viral
DNA (known as recombinant subunit
vaccines)
Other types of
vaccines
Toxoid: if there is a toxic secretion,
it is neutralized with formaldehyde
and sterilized water-this “detoxified
toxin” raises an immune response
Conjugate: Polysaccharides may
“disguise” the antigens, therefore,
conjugate vaccines are made to
recognize the polysaccharides on
the microbe, not the antigens
Experimental: DNA and
Recombinant Vector vaccines-insert
genetic material directly into host,
or recombine it into a harmless
bacteria
Herd Immunity
AKA: Community Immunity
• A situation in which a sufficient
proportion of a population is
immune to an infectious disease
(through vaccination and/or
prior illness) to make its spread
from person to person unlikely.
• Even individuals not vaccinated
(such as newborns and those
with chronic illnesses) are
offered some protection
because the disease has little
opportunity to spread within the
community.