Transcript operon - TeacherWeb

A.P. Biology Mr. Tesoro
Mar. 3, 2016
Homework Reminder:
Due Fri. Mar. 4, 2016 Answer Ch. 15
objectives (31,32,33)
Do Now: How is the genome of
bacteria (prokaryotes) different from
the genome of eukaryotes?
Aim: What is the operon model
of gene induction in prokaryotes?
What is an operon?
In prokaryotes, all the
genes that code for
related enzymes lie
close together on the
bacterial chromosome.
These ‘structural genes’
and the the ‘operator
region’ that turns them
“on and off” are called
an OPERON . The
operon has a promoter
site upstream.
Operon Structure
An operon consists of:
1) the genes that it
controls (lac X,Y, Z)
2) a promotor region
where RNA polymerase
first binds
3) an operator region
between the promotor
and the first gene which
acts as an “on-off
switch”. (yellow area)
How is an operon regulated?
Remote from the operon is
another gene called the
regulator.
The regulator gene produces
a small protein called the
repressor.
Ordinarily, the repressor binds
to the operator and keeps the
RNA polymerase from
transcribing the DNA.
The operon is “off”. No mRNA
is made.
How is an operon regulated? (2)
An inducer is a
substance that
removes the
repressor so that the
operon can become
active and
transcription can
occur.
An example of an inducible operon
In 1961, Francois Jacob and Jacques Monod
(Pasteur Institute in France) proposed the
operon model for the control of gene
expression in bacteria.
They received the Nobel Prize in 1965 for
their efforts.
They studied the lactose operon (lac operon),
an operon that is normally “off” because a
repressor binds to the operator site.
The lac operon, an inducible operon
Lactose is a type of complex sugar. An energy source.
Usually the cell uses glucose for its energy needs.
However, when it ingests lactose, it must metabolize it to
make glucose.
An inducer, allolactose (an isomer) binds to the repressor,
changes it conformation and causes it to fall off the operon.
The operon has been turned “on.”
The structural enzymes can now digest lactose. When its
concentration is greatly reduced, the repressor can once
again bind to the operator and turn the lac operon “off”.
The lac operon: a summary
Bacteria keep their lac operon “off” unless the
cell gets a supply of lactose to digest.
Lactose (allolactose) acts as its own inducer and
turns on the genes that will digest it.
Once it is digested, the repressor once again
turns the operon “off”
This is an example of negative feedback. An
event occurs (lactose is added) that reverses the
original (normal) condition. A second event
occurs (repressor returns) to reverse the first
event (lactose).
Gene off  gene on  gene off
A repressible operon: the trp
operon
Many operons are
continuously “on”
unless they are
turned “off” by a corepressor substance.
A co-repressor binds
to an inactive
repressor molecule
and activates it.
The tryptophan (trp) operon
The trp operon contains 5
structural genes that are
designed to make
tryptophan, an essential
amino acid.
If the cell contains too
much tryptophan, the
tryptophan acts as a corepressor and binds to the
inactive repressor,
activating it.
The active repressor binds
to the operator site and
turns the operon off.
The trp operon: a summary
The trp operon is always on because the
bacterial cell must always have a supply of
tryptophan.
To conserve energy, the cell turns this operon
“off” when it has a good supply of
tryptophan.
When it needs to make more tryptophan, the
repressor falls off and the operon is once
again turned “on”.
This is also a case of negative feedback.
Gene “on”  gene “off”  gene “on”