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Gene Regulation
Gene Regulation in Prokaryotes – the
Jacob-Monad Model
certain genes are transcribed all the time
– constitutive genes
synthesis of some proteins is regulated
and are produced only when needed
under special conditions
The Lac Operon:
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Jacob and Monad demonstrated how genes
that code for enzymes that metabolize lactose
are regulated
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An operon consists of three elements:
the genes that it controls
 a promotor region where RNA polymerase first
binds
 an operator region between the promotor and
the first gene which acts as an “on-off switch”.
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Intestinal bacteria (E. coli) are able to
absorb the disaccharide, lactose, and break
it down to glucose and galactose (E. coli
will only make these enzymes when grown
in the presence of lactose)
Requires the production of 3 enzymes:
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 - galactosidase – breaks down the lactose to
glucose and galactose
galactose permease – needed to transport
lactose efficiently across bacterial cell
membrane
galactoside transacetylase – function is not
clear
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Production of these enzymes is controlled
by three structural genes and some closely
linked DNA sequences responsible for
controlling the structural genes – entire
gene complex is called the operon
Structural genes of the lactose operon:
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lacZ – codes for  - galactosidase
lacY – codes for galactose permease
lacA – codes for galactoside transacetylase
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Next to the structural genes are 2
overlapping regulatory regions:
promotor – region to which RNA polymerase
binds to initiate transcription
operator – region of DNA that acts as the
switch that controls mRNA synthesis;
sequence of bases that overlaps part of the
promotor region
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when lactose is absent, a repressor protein (in
this case the lactose repressor) binds to the
operator region – repressor protein is large
enough to cover part of the promotor sequence,
too, and blocks RNA polymerase from attaching
to promotor – transcription is blocked
when lactose is present, it acts as an inducer and
“turns on” the transcription of the lactose operon
lactose binds to repressor protein, inactivates it,
and unblocks the promotor region allowing RNA
polymerase to attach and begin transcription
Gene Regulation in Eukaryotes
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most cells in a multicellular organism contain the
same DNA but they don’t all use the DNA all the
time
individual cells express only a small fraction of
their genes – those genes that are appropriate to
the function of that particular cell type
transcription of a cell’s DNA must be regulated
factors such pregnancy may affect gene
expression (genes for milk production are not
used all the time)
the environment may affect which genes are
transcribed (length of day may increase a
change in size of sex organs affecting the
production of sex hormones in birds)
Gene expression may be regulated by:
1. the rate of transcription of genes
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the cell may regulate the transcription of individual
genes through regulatory molecules (ex. steroids
may stimulate the production of certain proteins)
certain parts of eukaryotic chromosomes are in a
highly condensed, compact state making it
inaccessible to RNA polymerase
some of these areas are structural and don’t contain
genes
other of these regions are functional genes that are
not currently being transcribed
entire chromosomes may be inactivated
 ex – Female mammals have two X chromosomes
in each cell but only one is available for
transcription – the other chromosome is
condensed into a tight mass called a Barr body
2. mRNAs may be translated at different
rates
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mRNAs vary in stability (how long they last
before they are degraded) and in the rate at
which they are translated into protein
3. Proteins may require modification before
they can carry out their functions in a cell
4. The rate of enzyme activity may be
regulated (previously discussed in organic
chemistry)