Controlling Gene Controlling Gene ExpressionExpression

Control MechanismsControl MechanismsDetermine when to make more

proteins and when to stop making more

Cell has mechanisms to control transcription and translation

Housekeeping genes are genes that are always needed, and are constantly synthesizing proteins (switched on)

There are five levels of There are five levels of control:control:

◦Pre-transcriptional ◦transcriptional (controls

transcription from DNA to mRNA)◦posttranscriptional (controls the

removal of introns)◦translational (controls rate that

mRNA is activated through ribosomes)

◦posttranslational (affects the rate proteins can leave the cell)

The The lac Operonlac OperonAn example of control

mechanism is the lac operon Operon: a cluster of genes under

the control of one promoter and one operator

Operator: regulatory sequence of DNA to which a repressor protein binds

The lac Operon in action!The lac Operon in action!http://www.youtube.com/watch?v

=oBwtxdI1zvkLactose: * disaccharide found in milk, broken down by E. coli•B-galactosidase: enzyme responsible for the degradation of lactose

The lac operon is a cluster of three genes that code for the metabolism of lactose: lacZ codes for B-galactosidaselacY codes for B-galactosidase permease which lets lactose cross the cell membranelacA codes for transacetylase; unknown function.

Repressed StateRepressed State

Repressed StateRepressed StateLacI protein (blue block) is a repressor protein that blocks the transcription of lacZ; binds to the lactose operator and inhibits RNA polymerase by covering the promoter site (switch is of when there is no lactose in the system).

Repressed StateRepressed StateIf there is no lactose present, lac

operon genes are not transcribed or translated. Lactose is an effector/inducer; if it is present, it induces the removal of the repressor and allows the lac genes to be transcribed and translated. (switch is on if there is lactose in the system)

Induced StateInduced StateWhen lactose binds to LacI

protein, it changes and the new complex cannot bind to the operator of the lac operon. This results in RNA polymerase being able to bind to the DNA and start protein synthesis.

Induced StateInduced State

Gene Regulation in Gene Regulation in EukaryotesEukaryotesTranscriptional Regulation:Transcription factors

◦ Interact with the gene promotor in order to allow RNA polymerase to initiate

Activators◦ Attach to transcription factors, RNA

polymerase, and enhancers

Gene Regulation in Gene Regulation in EukaryotesEukaryotes

Post-transcriptional and Translational Control

Modifications of mRNA ◦ do not process the mRNA

Therefore not transported out of the nucleus or may be degraded

RNA interference - small RNA◦ Interact with protein complexes and either

promote mRNA cleavage or inhibit translation itself

Gene Regulation in Gene Regulation in EukaryotesEukaryotes