gene expression regulation: the lac operon

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Gene Expression Regulation: The lac Operon. Control Mechanisms. There are 42 000 genes that code for proteins in humans However, not all proteins are required at all times. E.g. Insulin is only required in a cell when glucose levels are high. - PowerPoint PPT Presentation

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  • Control MechanismsThere are 42 000 genes that code for proteins in humansHowever, not all proteins are required at all times.E.g. Insulin is only required in a cell when glucose levels are high.It would be inefficient and wasteful for a cell to transcribe, translate the insulin gene when glucose levels are lowRegulation is therefore vital to an organisms survivalRegulation the turning on or off of specific genes depending on the requirements of an organism

  • Why Turn Genes On and Off?Cell Specializationeach cell of a multicellular eukaryote expresses only a small fraction of its genesDevelopmentdifferent genes needed at different points in life cycle of an organismafterwards need to be turned off permanentlyResponding to organisms needscells of multicellular organisms must continually turn certain genes on & off in response to signals from their external & internal environment

  • The control of gene expression can occur at any step in the pathway from gene to functional protein

    The focus of todays lesson will be on the regulation of gene expression at the transcriptional level.

  • Gene ExpressionGene expression for all genes falls into one of two categories. (prokaryotes and eukaryotes)

    constitutive expression genes which are always turned onknown as housekeeping genes

    induced expression genes which are only turned on as needed

  • lac Operonlac - lactose

    operon several genes in a sequence all controlled by a single promoter (mainly in prokaryotes; some eukaryotes). It also includes an operator which is the on and off switch.promotergene 1gene 2gene 3operator

  • lac OperonCells mainly use glucose as a source of energy.

    The lac operon is only turned on when glucose is absent, but lactose is present.

    lac Operon Animation

  • lac Operon DetailsOperon codes for 3 enzymes found in E. coli

    beta-galactosidase (lacZ gene)enzyme which breaks down lactose

    permease (lacY gene)protein transporter which brings lactose into cell

    transacetylase (lacA gene)adds acetyl group to galactose

  • Repressor Proteinrepressor protein (lacI gene)transcribed by a different gene from the lac operonbinds to the operator portion in the presence of glucoseprevents RNA polymerase from transcribing genes when bound to operatorLactose is not needed as an energy source because glucose is present

    When do you want the repressor to bind / not bind to the operon?

  • When lactose is absent:enzymes are not needed to metabolise lactoserepressor binds to the operator to inhibit transcription

  • When lactose is present:Lactose (or allactose) binds to the repressor proteinlactose-repressor complex cannot bind to the operatortranscription can proceed

  • Effector MoleculesSince lactose is the molecule that determines when the operon is turned on or off, it is known as an effector molecule.

    effector molecule any molecule that can regulate the activity of a protein

    inducer effector molecule that binds repressor protein to cause it to fall off operator

  • lac Operon Animation

    lac Operon Animation

  • trp Operontrp tryptophan

    The genes of the trp operon are used to make the amino acid tryptophan.

    It is turned off when enough tryptophan is in the cell.

    Tryptophan is the effector molecule.

  • trp Operon

    Operon codes for 5 genes found in E. coli

    Five polypeptides combine to make three enzymes.each enzyme participates in a step to make tryptophan

  • Repressor Proteinrepressor protein (trpR)transcribed as a different gene from trp operonbinds to operator when tryptophan is presentprevents RNA polymerase from transcribing genes when bound to operator

    When do you want the repressor to bind / not bind to the trp operon?

  • When tryptophan needs to be made:enzymes are required to make tryptophanrepressor is NOT bound to operatortranscription can proceed

  • When cell has enough tryptophan:tryptophan binds to repressorrepressor can now bind operator to prevent transcriptionRNA Polymerase

  • Effector MoleculeSince tryptophan is the molecule that determines when the operon is turned on or off, it is known as an effector molecule.

    corepressor effector molecule that binds repressor protein to cause it to bind to the operator

  • http://www.youtube.com/watch?v=8aAYtMa3GFU

  • Classwork/HomeworkSection 5.5 Questions pg. 258 #1-6