Transcript Lecture Chpt. 18 Gene Regulation in Prokaryotic Organisms

GENE REGULATION
QuickTime™ and a
decompressor
are needed to see this picture.
VS.
QuickTime™ and a
decompressor
are needed to see this picture.
aka. when to turn
genes on!
Consider the difference:
 Prokaryotic cells exist AS
ONE CELL
 That’s right…
QuickTime™ and a
decompressor
are needed to see this picture.
ONE
CELL
DOES IT
ALL!
Consider the difference:
QuickTime™ and a
decompressor
are needed to see this picture.
Prokaryote
genes are
regulated via.
Transcriptional
control
This means that genes are
turned on and off in
response to the need of a
particular gene product(s)
at a particular time
QuickTime™ and a
decompressor
are needed to see this picture.
This is different than
EUKARYOTES
QuickTime™ and a
decompressor
are needed to see this picture.
 Eukaryotic cells have
a long life span,
during which they
may need to respond
repeatedly to many
different stimuli.
 New enzymes are not
synthesized each time
the cells respond to a
stimulus
EUKARYOTES
QuickTime™ and a
decompressor
are needed to see this picture.
 Most enzymes &
proteins are
transformed from
an inactive state to
an active state,
there is a
RESERVE.
 Some eukaryotic
cells have a large
store of inactive
mRNA
THINK ABOUT IT…
»RBC’s produce
hemoglobin
(O2 transporting
protein)
QuickTime™ and a
decompressor
are needed to see this picture.
»Muscle cells
produce
myoglobin
(O2 storing
protein)
THINK ABOUT IT…
QuickTime™ and a
decompressor
are needed to see this picture.
»BOTH CELLS
HAVE GENES
THAT WILL
NEVER BE
USED.. IT
WOULD BE
WASTEFUL TO
MAKE
HEMOGLOBIN
IF YOU WERE A
MUSCLE CELL
QuickTime™ and a
decompressor
are needed to see this picture.
It has 4288 genes that code for
proteins… some are always
needed (ex. glycolysis enzymes)
QuickTime™ and a
decompressor
are needed to see this picture.
Some are
needed only
when there
are certain
environmental
conditions
An E. coli living in an adult cow
intestine is not normally exposed
to LACTOSE (disaccharide)
HOWEVER,
if you were in the colon of a calf…
lactose would be a primary energy
source.
QuickTime™ and a
decompressor
are needed to see this picture.
Should the E. coli invest
energy and materials to
produce lactosemetabolizing enzymes just
IN CASE it ends up in the
digestive system of a calf?
HOW can an individual
bacterium, locked into
the genome that it has
inherited…
cope with the ever
“changing”
environment?????
Turns out, the three genes to
produce enzymes for lactose
metabolism are found together
in a complex
QuickTime™ and a
decompressor
are needed to see this picture.
These three genes, turns out,
are linked by a common control
mechanism
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Operon
A
gene complex,
consisting of a group of
genes w/ related
functions
& DNA sequences that
control them.
Operon
This is the mechanism
by which bacteria
control gene expression
Operon Model
Jacob
and Monod
(1965 Nobel Prize for
physiology or medicine) - for
their discovery of the
Prokaryotic model of gene
control.
Always
on the national AP Biology
So, if you remember… to use
lactose as an energy source…
QuickTime™ and a
decompressor
are needed to see this picture.
The lactose is cleaved by enzyme
galactosidase…
Another enzyme then converts
galactose to glucose
QuickTime™ and a
decompressor
are needed to see this picture.
And still a third enzyme (function not clear)
is needed to complete the process for
glucose to move across the cell
membrane
QuickTime™ and a
decompressor
are needed to see this picture.
E. Coli growing on glucose
produces very little
galactosidase… NO NEED!!
QuickTime™ and a
decompressor
are needed to see this picture.
However, when grown on lactose, there
are SEVERAL THOUSAND
galactosidase molecules
QuickTime™ and a
decompressor
are needed to see this picture.
Bacteria can,
in one
sweep, turn
off or on
genes that
code for the
production
of these
Operon
Model
1. Operon Area
a. Operator
b. Promoter (where RNA
polymerase
binds to
DNA to begin transcription)
c. Structural Genes
2. Regulatory Gene
Operon
Structure
1. Operon Area
a. Operator
b. Promoter
c. Structural Genes
OPERON AREA
OPERATOR ->
PROMOTER ->
GENES THEY
Operator - segment of DNA that
“turns on” the RNA polymerase that
is binding to the gene coding area
Pix of
switch
here
OPERATOR
Segment of DNA
that “turns on”
the ability for
RNA polymerase
Pix of
switch
here
OPERATOR
If blocked, will
NOT permit RNA
polymerase to
pass - preventing
Promotor - area where RNA
polymerase binds to the DNA
OPERATOR OFF
If blocked, will NOT permit
RNA polymerase to pass preventing transcription
RNA
polyme
raseblo
Operator ON
RNA
polymerase
not blocked
Operator ON
RNA polymerase can
bind to promotor
Operator ON
via. RNA polymerase
Genes
transcribed
OPERATOR ->
PROMOTER ->
GENES THEY
CONTROL ->
2. Regulatory Gene - codes
for repressor
molecules
(this is
upstream)
In this case,
continuously made by
the regulatory gene
Repressor
Protein switches OFF
the Operator
and the
OPERON
cannot be
transcribed
Repressor
Protein switches OFF
the operon
RNA polymerase
cannot move
down the operon
Repressor
Protein switches OFF
the operon
RNA polymerase
no mRNA made no enzymes made
(no breakdown of
lactose)
ex. lac Operon - codes for three
genes that function in the
production of an enzyme that
breakdowns the disaccharide lactose
(no breakdown of
lactose)
lac Operon - usually off, only
works when
substrate (in this case: lactose)
is present
Promoter - (remember) area
where RNA polymerase binds to
the DNA to promote transcription
Genes - plural… code for enzymes
that break down lactose
repressor
with no lactose in the system ->
regulatory gene codes for
repressor.
repressor binds to
operator - no RNA polymerase
attachment - no digestive enzyme
inducer
with lactose present - inducer (an
isomer of lactose) binds allosterically
to the repressor
inducer
with lactose present - shape change,
and the DNA no longer recognizes
the repressor
inducer
with lactose present - RNA
polymerase actively transcribes the
structural genes
When
inducer
enough Lactose has been
digested, the Repressor binds
again to the Operator and switches
the Operon "off”…
no digestive enzymes made.
inducer
Repressor usually controls by keeping
the operon off!…. The presence of an
inducer inactivates the repressor,
permitting the genes to be transcribed.
usually involved in genes that BREAK
DOWN molecules to provide energy
Lets do another example…
Pix of intestine here
Think of the
poor bacteria
that live in
your colon…
Pix of intestine here
They are
dependent on
your whims of
what you
decide to eat
for their
nutrients!
Pix of intestine here
They need
tryptophan (an
amino acid) to
survive!!
Pix of intestine here
What if you
don’t eat
anything with
tryptophan in
it??
Pix of intestine here
THEY STILL
NEED the
tryptophan
Turns out…there
are five genes that
code for the
enzymes that
produce
tryptophan.
A single
promoter
serves ALL FIVE
genes
Transcription w/
give rise to one
LONG mRNA that
codes for all five
enzymes in the
pathway
ex. “trp Operon”
usually ON
ex. “trp Operon”
It is the normal state for cell to
make tryptophan
If tryptophan
accumulates b/c
you are eating
it… the bacteria
will shut off its
own synthesis of
tryptophan, by
not making an
enzyme
Tryptophan present
transcription usually
on!!
Tryptophan absent - usual
Repressor usually controls by keeping the
operon on!…. The presence of an inducer
activates the repressor, inhibiting the genes
from being transcribed.
usually involved in genes that PUT
TOGETHER molecules to provide
energy
Summary
• Know Operon theory !
• Know the difference between repressible
and inducible enzymes.