Transcript Viruses, Bacteria, and the Immune System

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Viruses are parasites
of cells.
They consist of:
Nucleic Acid (DNA or
RNA but not both)
Capsid/Protein Coat
Some also are
surrounded by an
envelope
A Bacteriophage—a virus that attacks
bacteria cells
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A typical virus
penetrates a cell, takes
over the cell’s
machinery and
assembles hundreds of
new viruses.
In the process, the host
cell is usually
destroyed.
Viruses are specific for
the kinds of cells they
attack.
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In the lysogenic cycle, the
viral DNA is temporarily
incorporated into the
DNA of the host cell.
A virus in this dormant
state is called a provirus.
While in this stage, it will
be reproduced every time
a cell divides just like the
normal DNA.
The virus remains inactive
until some trigger (often
an external environmental
stimulus) causes the virus
to begin the lytic cycle.
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The nucleic acid of a
virus could be doublestranded DNA
(dsDNA) or singlestranded DNA
(ssDNA)
Or: it could be doublestranded RNA
(dsRNA) or singlestranded RNA
(ssRNA)
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Retroviruses are ssRNA
viruses that use an
enzyme called reverse
transcriptase to make a
DNA complement of
their RNA.
The DNA complement
can then be transcribed
immediately to
manufacture mRNA, or
it can begin the
lysogenic cycle.
HIV is a retrovirus
Eukaryotic DNA
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A bacterial chromosome is
often called a naked
chromosome because it
lacks the histones and
other proteins associated
with eukaryotic
chromosomes.
Bacteria have one large
circular strand of DNA
and may also have one or
more smaller circles of
DNA called plasmids.
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Bacterial cells
reproduce by binary
fission.
The chromosome
replicates and the cell
divides into two cells,
each cell with one
chromosome.
There are no
centrioles, spindles, or
microtubules.
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Plasmids carry genes
that are beneficial but
not normally essential
to the survival of the
bacterium.
Plasmids replicate
independently of the
chromosome.
Some plasmids, called
episomes, can become
incorporated into the
bacterial chromosome.
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Genetic variation in
bacteria can be
introduced in a
number of ways.
Conjugation
Transduction
Transformation
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Conjugation is a process of
DNA exchange between
bacteria.
A donor bacterium produces
a tube, or pilus, that
connects to a recipient
bacterium.
Through the pilus, the donor
bacterium sends
chromosomal or plasmid
DNA to the recipient.
In some cases, copies of large
portions of a donor’s
chromosomes are sent,
allowing recombination with
the recipient’s chromosome.
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One plasmid, called the
F plasmid, contains the
genes that enable a
bacterium to produce
pili.
When a recipient
bacterium receives the F
plasmid, it too can
become a donor cell.
Another group of
plasmids, called the R
plasmids, provide
bacteria with resistance
against antibiotics.
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Transduction is the transfer of host DNA from
one cell to another by a virus. When a virus is
assembled during a lytic cycle, it is sometimes
assembled with some bacterial DNA.
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Transformation occurs when bacteria absorb DNA
from their surroundings and incorporate it into their
genome. Specialized proteins on the cell membranes of
some bacteria facilitate this kind of DNA uptake.
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Gene expression is the way in which cells with
identical DNA become different by
transcribing only certain genes at a time.
In bacterial cells, certain segments of their
DNA act as Operons. The operon acts like a
switch that can turn several genes on or off at
the same time.
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Among prokaryotes, an
operon is a unit of DNA
that controls the
transcription of a gene.
It contains the following
components:
Promoter region
Operator region
Structural genes
Regulatory genes
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The promoter region is a sequence of DNA to which
the RNA polymerase attaches to begin transcription.
The operator region can block the action of the RNA
polymerase if this region is occupied by a repressor
protein.
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The structural genes contain DNA sequences that code
for enzymes and proteins the cell needs for its normal
daily activities.
A regulatory gene, lying outside the operon region,
produces repressor or activator proteins.
Action of a
repressor protein.
This protein was
made by a
regulatory gene that
is not shown here.
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The lac operon in E. coli cells controls the breakdown of
lactose (a disaccharide). The lac operon is an example
of an inducible operon because the structural genes are
normally inactive. They are activated when lactose is
present.
http://www.sumanasin
c.com/webcontent/anim
ations/content/lacopero
n.html
http://www.youtube.com
/watch?v=oBwtxdI1zvk
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When the operator region is occupied by the
repressor, RNA polymerase is unable to
transcribe several structural genes that code for
enzymes that control the uptake and
breakdown of lactose.
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When lactose (or its’ isomer allolactose) is
available, however, some of the lactose
combines with the repressor to make it
inactive. Then, the RNA polymerase can attach
and make the enzymes needed to break down
the lactose.
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The trp operon in E. coli produces enzymes for the
synthesis of the amino acid tryptophan.
A regulatory gene produces an inactive repressor that
does not bind to the promoter.
As a result, the RNA polymerase proceeds to transcribe
the genes needed to make tryptophan.
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However, when tryptophan is available to
E.coli from the surrounding environment, the
bacterium no longer needs to make its own.
In this case, rising levels of tryptophan induce
some tryptophan to react with the inactive
repressor and make it active.
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The active repressor now binds to the operator
region, which prevents the transcription of the
structural genes that would make more
tryptophan.
Since these structural
genes stop producing
enzymes
only in the presence of
an active repressor, they
are called repressible
enzymes.
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The internal environment of animals provides
attractive conditions for the growth of bacteria,
viruses and other organisms. To protect against
these, humans possess three levels of defense:
Skin
Internal defenses
The immune response
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Skin—a physical and
hostile barrier covered
with oily and acidic
secretions from sweat
glands
Cilia line the lungs to
sweep invaders out
Gastric juice of the
stomach kills most
microbes
Symbiotic bacteria found
in the digestive tract and
the vagina outcompete
many other organisms.
Healthy skin provides a good
barrier against invasion by foreign
microbes.
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1. Phagocytes—white
blood cells (WBCs) that
engulf pathogens by
phagocytosis.
They include: neutrophils
and monocytes.
Monocytes enlarge into
large phagocytic cells
called macrophages.
Other WBCs called
natural killer cells attack
abnormal body cells such
as tumors or pathogeninfected body cells.
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Complement is a
group of about 20
proteins that
“complement”
defense reactions.
These proteins help
attract phagocytes to
foreign cells and help
destroy foreign cells
by promoting cell
lysis (breaking open
the cells).
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Interferons are
substances secreted by
cells invaded by
viruses that stimulate
neighboring cells to
produce proteins that
help them defend
against viruses.
Therefore, the
neighboring cells are
less likely to be
invaded by the virus.
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The inflammatory
Response is a series of
nonspecific events
that occur in response
to pathogens.
When skin is
damaged:
Histamine is secreted
by basophils, WBCs
found in connective
tissues.
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The histamine
stimulates the dilation
of blood vessels
(vasodilation). This
increases the blood
supply to the
damaged area and
allows for easier
movement of WBCs
through blood vessel
walls.
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The dilation of the blood
vessels also causes
redness, an increase of
temperature, and
swelling. The increase in
temperature, like a fever,
may stimulate WBCs, and
they may make it
inhospitable to pathogens.
Phagocytes, attracted to
the injury, arrive and
engulf pathogens and
damaged cells.
http://www.youtube.com/watch?
v=_bNN95sA6-8
1.Tissue injury:
release of chemical
signals such as
histamine
2. Dilation and
increased leakiness of
local blood vessels;
migration of phagocytes
to the area
3. Phagocytes
(macrophages and
neutrophils) consume
bacteria and cell debris;
tissue heals
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This differs from the inflammatory response
because it targets specific antigens.
Antigens are any molecules (usually a protein)
that can be identified as foreign.
It may be a toxin (such as venom), a part of the
protein coat of a virus, or a molecule unique to
the pathogen.
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The MHC is the
mechanism by which
the immune system is
able to differentiate
between self and
nonself cells.
It is a collection of
glycoproteins that exist
on the membranes of all
body cells.
The proteins of a single
individual are unique.
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The primary agents of
the immune response
are Lymphocytes.
These are white blood
cells that originate in
the bone marrow (like
all blood cells), but
concentrate in
lymphatic tissues such
as the lymph nodes,
the thymus gland, and
the spleen.
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B cells are
lymphocytes that
originate and mature in
the bone marrow.
B cells respond to
antigens.
The plasma
membrane surface of
B cells is characterized
by specialized antigen
receptors called
antibodies.
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Each antibody is a variation of a
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basic Y-shaped protein that consists
of constant regions and variable
regions. The variable regions are
sequences of amino acids that differ
among antibodies and give them
specificity to antigens.
Antibodies are proteins
that are specific to a
particular antigen.
There are 5 classes of
antibodies (also called
immunoglobulins): IgA,
IgD, IgE, IgG, IgM
Antibodies inactivate
antigens by binding to
them. Inactivation is
followed by macrophage
phagocytosis.
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Plasma Cells—these
release specific
antibodies that circulate
throughout the body,
binding to antigens
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Memory Cells—these
are long-lived B cells
that do not release their
antibodies in response
to the immediate
invasion. Instead, they
circulate in the body
and respond quickly to
eliminate any new
invasion by the same
antigen.
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T Cells are lymphocytes
that originate in the
bone marrow but mature
in the thymus gland.
T cells also have antigen
receptors. However,
these receptors are not
antibodies, but
recognition sites for
molecules displayed by
non-self cells.
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When a body cell is
invaded by a virus (or
any other foreign cell),
it displays a
combination of self and
non-self markers. T-cells
interpret this display as
non-self.
Cancer cells or tissue
transplants are also
recognized as non-self
cells by T-cells
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When T-cells encounter nonself cells, they divide and
produce two kinds of cells:
Cytotoxic T Cells (or Killer T
Cells)—these recognize and
destroy non-self cells by
puncturing and lysing them
Helper T Cells—these
stimulate the proliferation of
B-cells and more Killer T
Cells
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When an antigen binds
to a B-cell or when a
non-self cell binds to a
T-cell, the B-cell or Tcell begins to divide,
producing numerous
daughter cells, all
identical copies of the
parent cell.
This produces
numerous cells that will
only attack a particular
antigen
Clonal Selection: only the B or Tcell that bears the particular
effective receptor is “selected” and
reproduces to make clones, or
identical copies of itself.
The CellMediated
Immune
Response:
The Cell-Mediated
Immune Response
uses mostly T-cells
and responds to any
non-self cell, including
cells invaded by
pathogens.
When a non-self cell
binds to a T-cell, the Tcell undergoes clonal
selection.
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This response involves most cells and responds
to antigens or pathogens that are circulating in
the lymph or blood.
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1. B-cells produce
plasma cells. The
plasma cells release
antibodies that bind
with antigens or
antigen-bearing
pathogens.
2. B-cells produce
memory cells which
provide future
immunity.
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3. Macrophage and
Helper T-cells stimulate
B-cell production. Most
often, the antigen or
pathogen is engulfed by
a macrophage. T-cells
then bind to the
macrophage in a cellmediated response.
Interleukins secreted by
the helper T-cells
stimulate the
production of B-cells.
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1. Antibiotics—chemical derived from bacteria or
fungi that are harmful to other microorganisms
2. Vaccines—substances (usually inactivated viruses or
fragments of viruses or bacteria) that stimulate the
production of memory cells.
Passive Immunity—obtained by transferring
antibodies from an individual who previously had a
disease to a newly infected individual. Newborn
infants are protected by passive immunity through the
transfer of antibodies across the placenta and in breast
milk.