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GENE EXPRESSION and the LAC OPERON

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GENE EXPRESSION and the LAC OPERON Powered By Docstoc
					    GENE EXPRESSION and the
         LAC OPERON
We have about 42 000 genes inside our DNA that
codes for proteins. Obviously not all the proteins
are needed at the same time.

Cells have developed methods to control the
transcription and translation of genes, depending on
their needs.
HOUSEKEEPING GENES are always needed and
are therefore constantly being transcribed and
translated.

TRANSCRIPTION FACTORS turn genes on when
required.

Gene regulation is important for organism survival
and it can occur at four levels:
   TRANSCRIPTIONAL : regulates which genes
    are transcribed (DNA to mRNA) or regulates the
    rate of transcription

   POSTTRANSCRIPTIONAL: removing the
    introns of the mRNA before it goes off to
    translation.
   TRANSLATIONAL: controls how often and how
    fast mRNA transcripts are converted into
    proteins.

   POSTTRANSLATIONAL: controlling the
    amount of time before a protein becomes
    functional.
             THE lac OPERON
   Lactose is milk sugar and is broken down by E.
    coli in the intestines of mammals.
    BETA GALACTOSIDASE is the enzyme
    responsible for the breaking down of lactose.
   The bacteria have a NEGATIVE CONTROL
    SYSTEM that blocks the production of beta-
    galactosidase if lactose is NOT present. The
    system works by inhibiting TRANSCRIPTION of
    the gene that codes for the beta-galactosidase
    protein.
   The beta-galactosidase gene is part of an
    OPERON.
   An operon is on a prokaryotic DNA molecule. It
    contains a PROMOTER, OPERATOR and some
    structural genes.
   The promoter and operator do not code for
    protein but are important in the REGULATION
    of the genes' transcription.
   The lac operon consists of a promoter, operator
    and three genes (lac Z, lac Y, and lac A) which
    code for the proteins and enzymes that break
    down lactose.
   A REPRESSOR protein, called lacI, binds to the
    operator and prevents the genes for beta-
    galactosidase from being transcribed (blocks
    RNA polymerase).
   The promoter region and the operator region
    overlap (share nucleotides) so when lacI binds to
    the operator it is essentially blocking the
    promoter site where RNA POLYMERASE wants
    to bind.
   In the lac operon system lactose is known as a
    signal molecule or an INDUCER. When it is
    present it binds to the lacI protein which pulls it
    away from the operator site.
   This frees up the promoter site for RNA
    polymerase and allows for the proper
    TRANSCRIPTION of the genes responsible for
    lactose degredation.
            THE trp OPERON
 Tryptophan is an amino acid used by E. Coli to
make protein.
 The trp operon is another example of co-ordinated

regulation. It contains 5 genes that make the three
enzymes responsible for synthesizing tryptophan.
While the lac operon has transcription induced

when lactose is present (in order to make enzymes
to break it down), the trp operon is repressed when
high levels of trp are present (no need to make any
more).
   The EFFECTOR, the cause of the gene being
    repressed, is trp.
    Trp is called a CO-REPRESSOR.
    When trp levels are high, trp binds to an inactive
    repressor protein making it ACTIVE. This
    repressor-trp COMPLEX then binds to the
    operator site and prevents the gene from being
    transcribed.
   When levels of trp drop it RELEASES from the
    repressor protein and the repressor protein
    disengages from the operator so that the trp genes
    can be transcribed.

				
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posted:4/8/2010
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