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