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LACTOSE
OPERON
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PREPARED BY:
NORERLYDA BINTI HAMDAN
(2009364241)
NUR SABARIAH BINTI ADNAN
(2009946651)
NUR NABILAH BINTI ABU BAKAR
(2009977639)
ERNIE SUHAIZA BINTI RADZI
(2009301039)
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The control of gene expression
Each cell in the human contains all the geneticmaterial for the growth and development of ahuman
Some of these genes will be need to beexpressed all the time
These are the genes that are involved in ofvital biochemical processes such as respiration
Other genes are not expressed all the time They are switched on an off at need
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Operon
An operon is a group ofgenes that aretranscribed at the
same time. They usually control an
important biochemicalprocess.
They are only found inprokaryotes.
Jacob, Monod & Lwoff NobelPrize.org
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The Lactose Operon
Consists ofthree genes (Z, Y, A) each involved
in processing the sugar lactose
One of them is the gene for the enzyme -
galactosidase (Z)
This enzyme hydrolyses lactose into glucose and
galactose
Formation of lac inducer (allolactose)
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Lac Genes function
The lac Z gene which is responsible for making -galactosidase -galactosidase is the enzyme that hydrolyzes (breaks)
the bond between glucose and galactose to make the
disaccharide lactose Lac Y that makes the enzymepermease
Permease is a transport protein that carries lactoseinto the cell
Lac A gene which is responsible for makingtransacetylase Scientists are not sure oftransacetylases function
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i
Operon
RegulatoryGene
p o z y a DNA
m-RNA
-Galactosidase
Permease
Transacetylase
Protein
repressor
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Promoter: where RNA polymerase attaches
to DNA sequence for transcription Operator: where repressor attaches to DNA
sequence to block transcription
Inducer: inactivates the repressor andremoves it from the DNA (lactose is the
inducer)
Repressor: attaches to the operator andblocks RNA polymerase from transcribing
mRNA
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How does Lactose Operon work?
Glucose not available, so cells need other
source of energy such as lactose.
LETS WATCH THE VIDEO
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Two forms of regulation
1) cis regulation of transcription (cis = adjacent)
refers to DNA sequences that regulate the transcription of
downstream genes, through interaction with DNA binding
proteins
Promoter:
RNA polymerase II binds to promoter
Initiates transcription of the lac genes
Operator Binds a repressor which inhibits DNA synthesis
Overlaps with the promoter by about 9 bp
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CAP binding sites
Within the promoter sequence
Activated CAP binds to this sequence and results in
more efficient binding of RNA polymerase II
Turns the operon ON
Without CAP bound there is a very small amount of thelac genes producedstill referred to as OFF
The promoter CAP binding sites are considered to bepositive
cis elements.
Function in activating the operon. The operator is a negative cis element.
Function in deactivating the operon
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2) Trans regulation of transcription (trans = across)
Proteins that regulate transcription by binding to cis
elements
a) Repressor
Has two binding sites
One is a DNA binding site, associates with operator asa tetramer
Inhibits RNA synthesis by blocking the action of RNA
polymerase II
Forms a large complex next to the promoter anddoesnt allow enough space for the polymerase to
bind called STERIC HINDERANCE
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When the inducer (allolactose) binds the
repressor (lacI) it undergoes a conformational
change and can no longer bind the operator as a
tetramer. Allows RNA polymerase II to bind.
Modification of conformation due to the
binding of a small moleculeALLOSTERICREGULATION
b) RNA polymerase - Transcribes the RNA
c) CAP - Increases RNA polymerase binding efficiency
& requires cyclic AMP to be active
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Negative control of the lacoperon
The igene encodes a repressor which, in the absence of lactose (top), binds to the
operator (o) and blocks transcription of the three structural genes (z, -
galactosidase; y, permease; and a, transacetylase). Lactose induces expression of the
operon by binding to the repressor (bottom), which prevents the repressor from
binding to the operator. P = promoter; Pol = polymerase
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Positive control of the lacoperonLow levels of glucose activate adenylyl cyclase, which converts ATP to cyclic AMP (cAMP).
Cyclic AMP then binds to the catabolite activator protein (CAP) and stimulates its binding
to regulatory sequences of various operons concerned with the metabolism of
alternative sugars, such as lactose. CAP interacts with the subunit of RNA polymerase
to activate transcription
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Carbohydrates Activatorprotein Repressorprotein RNApolymerase lac Operon
+ GLUCOSE
+ LACTOSE Not boundto DNA Lifted offoperator siteKeeps falling
off promoter
siteNo
transcription+ GLUCOSE
- LACTOSE Not boundto DNA Bound tooperator site Blocked bythe repressor Notranscription- GLUCOSE
- LACTOSE Bound to
DNA
Bound to
operator site Blocked by
the repressor No
transcription
- GLUCOSE
+ LACTOSE Bound toDNA Lifted offoperator site Sits on thepromoter site Transcription
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Mutation
Structural genes:
One functional copy of a gene; lactose is metabolized
Two non-functional copies of a gene present in the cell leads to a non-
functional metabolic pathway
Remove lacY lactose cannot enter the cell
Remove lacZ lactose cannot be broken down into glucose and
galactose
Operator:
Normal repressor can bind (O+)
Mutated constitutive expression negative regulation (expressed as O-
or Oc)
Repressor cant bind so the operator is desensitized to the lacI protein
Transcription occurs whether lactose is present or not
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Promoter
Normal RNA polymerase can bind (P+)
Mutated RNA polymerase never binds (P-)
Inhibitor
Normal can bind the operator when no lactose is present.
(I+) Two mutants:
1) Mutated DNA binding domain never binds operator,
operon is not sensitive to inhibition by lacI protein. (I-)
2) Mutated allolactose binding site (Is super suppressor)loses the ability to associate with allolactose; the
tetramer can never be removed from the operator in
response to lactose entering the cell
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References
Principle of Genetics fifth edition by D. Peter
Snustad and Michael J. Simmons, 2010, Wiley
and Sons.
Essentials of Genetics fifth edition by William
S. Klug and Michael R. Cummings, 2005,
Pearson Education Inc.
Website: www.wikipedia.com/lactose_operon
Website: pdf search engine
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Thank you