Controlling Gene Controlling Gene Expression Expression
Jan 18, 2016
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