GENE REGULATION
Virtually every cell in your body
contains a complete set of genes
But they are not all turned on in
every tissue
Each cell in your body expresses
only a small subset of genes at any
time
During development different cells
express different sets of genes in a
precisely regulated fashion
GENE REGULATION
Gene regulation occurs at the level
of transcription or production of
mRNA
A given cell transcribes only a
specific set of genes and not
others
Example: Insulin is made by
pancreatic cells
CENTRAL DOGMA
Genetic information always goes
from DNA to RNA to protein
Gene regulation has been well
studied in E. coli
When a bacterial cell encounters a
potential food source it will
manufacture the enzymes
necessary to metabolize that food
2 Types of Regulation
Inducible and repressible
Inducible operon, such as lac
operon = normally turned off
Repressible operon, such as the trp
operon = normally turned on
Gene Regulation
In addition to sugars like glucose and
lactose E. coli cells also require
amino acids.
One essential aa is tryptophan.
When E. coli is swimming in
tryptophan (milk & poultry) it will
absorb the amino acids from the
media.
When tryptophan is not present in the
media then the cell must manufacture
its’ own amino acids.
Trp Operon : Model #1
E. coli uses several proteins encoded by
a cluster of 5 genes to manufacture the
amino acid tryptophan.
All 5 genes are transcribed together as a
unit called an operon, which produces a
single long piece of mRNA for all the
genes.
Operon is a segment of DNA that
includes a specific gene sequence and
the promotor/operator region for that
gene.
RNA polymerase binds to a promoter
located at the beginning of the first gene
and proceeds down the DNA transcribing
the genes in sequence
Fig. 16.6
•The tryptophan gene is turned
on when there is no tryptophan
in the environment.
•That is when the cell
wants/needs to make its’ own
tryptophan.
• TRP OPERON Animation
• http://bcs.whfreeman.com/thelifewire/content/
chp13/1302002.html
GENE REGULATION
In addition to amino acids, E.
coli cells also metabolize
sugars in their environment.
In 1959 Jacques Monod and
Fracois Jacob looked at the
ability of E. coli cells to digest
the sugar lactose.
GENE REGULATION
In the presence of the sugar
lactose, E. coli makes an enzyme
called beta galactosidase.
Beta galactosidase breaks down
the sugar lactose so the E. coli can
digest it for food.
It is the LAC Z gene in E coli that
codes for the enzyme beta
galactosidase.
GENE REGULATION
When does an E.coli cell need to
make beta galactidosidase?
They can only have lactose when
it is present in their environment.
Then they turn on genes to beak
down lactose.
The E. coli bacteria only needs
beta galactosidase if there is
lactose in the environment to
digest.
There is no point in making the
enzyme if there is no lactose sugar
to break down.
It is the combination of the
promoter and the DNA that
regulate when a gene will be
transcribed.
GENE REGULATION
There is a region of DNA called
the OPERON.
This combination of a promoter
and a gene is called an
OPERON.
Operon is a cluster of genes
encoding related enzymes that
are regulated together.
GENE REGULATION
Operon consists of
A promoter site where RNA
polymerase binds and begins
transcribing the message
A region that makes a repressor
Repressor sits on the DNA at a spot
between the promoter and the gene to
be transcribed.
This site is called the operator
Model #2: LAC Z GENE
E. coli regulate the production of
Beta Galactocidase by using a
regulatory protein called a
repressor
The repressor binds to the lac Z
gene at a site between the
promotor and the start of the
coding sequence (operator region).
The site the repressor binds to is
called the operator region.
Model #2: LAC Z GENE
Normally the repressor sits on
the operator repressing
transcription of the lac Z gene
(lactose is not present).
In the presence of lactose the
repressor binds to the lactose
and this allows the polymerase
to transcribe the lac Z gene
and thus produce the needed
enzymes.
Model #2: LAC Z GENE
This results in the production
of beta galactosidase which
breaks down the sugar
When there is no lactose left
the repressor will return to its
spot on the chromosome and
stop the transcription of the lac
Z gene
Lac Operon Animation
http://www.sumanasinc.com/webcont
ent/animations/content/lacoperon.html
GENE REGULATION: Eukaryotes
In eukaryotic organisms like
ourselves there are several
methods of regulating protein
production
Most regulatory sequences are
found upstream from the promoter
Genes are controlled by regulatory
elements in the promoter region
that act like on/off switches or
dimmer switches
GENE REGULATION: Eukaryotes
Specific transcription factors bind
to these regulatory elements and
regulate transcription
Regulatory elements may be tissue
specific and will activate their gene
only in one kind of tissue
Sometimes the expression of a
gene requires the function of two
or more different regulatory
elements