lactose operon

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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

    http://www.wikipedia/lactose_operonhttp://www.wikipedia/lactose_operon
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    Thank you