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 ge by smb11581

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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

								
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