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Some genes are transcribed and translated all the time, e.g. genes for constitutive
enzymes. Other enzymes are produced only when they're required; these are inducible
enzymes. One of these inducible enzymes is -galactosidase, which hydrolyses lactose in
the bacterium Escherichia coli (E.Coli)
The operon consists of:
A promoter region, where RNA polymerase binds to start transcription
An operator region, where an inhibitor binds
Structural genes for -galactosidase, Lactose permease and another enzyme.
In addition there is a regulator gene elsewhere on the bacterial chromosome that
codes for a repressor substance that binds to the promoter region; inhibiting
transcription of the three lac genes z, y and a.
The lac operon
a) High concentration of Glucose, low
concentration of Lactose. As there is
no lactose to inhibit the repressor, it
binds to the operator region preventing
transcription by the RNA polymerase to
b) Low concentration of
Glucose, high concentration of
Lactose. As lactose is present in high
quantities, it inhibts the repressor
causing it to change its active site.
This means the repressor can no
longer bind to the operator region and
so RNA polymerase can bind to the
promoter region and transcription can
take place which leads to the
production of the three proteins.
Protein 1 -galactosidase (used to hydrolyse lactose)
Protein 2 Lactose Permease (Lactose membrane transport protein), and Protein
3 is another enzyme.
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An operon is a length of DNA, made up of structural genes and control
The operator and promoter regions are both genes however they do
not code for polypeptides.
Structural Genes Codes for lac enzymes
Regulator Gene Produces repressor proteins
Promoter Binds to RNA polymerase
Operator Binds to repressor
2. The repressor protein binds to the operator region and prevents RNA polymerase
from transcribing the structural genes. It acts as an inhibitor.