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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
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
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
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
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
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
Lac Z Gene
The tryptophane gene is turned
on when there is no tryptophan
in the media
That is when the cell wants to
make its’ own tryptophan
E. coli cells can not make the
sugar lactose
They can only have lactose when
it is present in their environment
Then they turn on genes to beak
down lactose
GENE REGULATION
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
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 polyerase
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
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
The site the repressor binds to is
called the operator
LAC Z GENE
Normally the repressor sits on
the operator repressing
transcription of the lac Z gene
In the presence of lactose the
repressor binds to the sugar
and this allows the polymerase
to move down the lac Z gene
LAC Z GENE
This results in the production
of beta galactosidase which
breaks down the sugar
When there is no sugar left the
repressor will return to its spot
on the chromosome and stop
the transcription of the lac Z
gene
GENE REGULATION
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 one/off switches or
dimmer switches
GENE REGULATION
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
INTRONS AND EXONS
Eukaryotic DNA differs from
prokaryotic DNA it that the coding
sequences along the gene are
interspersed with noncoding
sequences
The coding sequences are called
EXONS
The non coding sequences are
called
INTRONS
INTRONS AND EXONS
After the initial transcript is
produced the introns are spliced
out to form the completed message
ready for translation
Introns can be very large and
numerous, so some genes are
much bigger than the final
processed mRNA
INTRONS AND EXONS
Muscular dystrophy
DMD gene is about 2.5 million base
pairs long
Has more than 70 introns
The final mRNA is only about
17,000 base pairs long
RNA Splicing
Provides a point where the expression of a
gene can be controlled
Exons can be spliced together in different
ways
This allows a variety of different
polypeptides to be assembled from the
same gene
Alternate splicing is common in insects and
vertebrates, where 2 or 3 different proteins
are produced from one gene