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Lecture 20

Lecture 20

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Biological Sciences
Aarti Ashok

Lecture #20 th Date: Thursday, July 15 /2010 Control of gene expression: prokaryotes - Bacteria often need to turn genes on and off rapidly in response to changing environmental o e.g. presence of certain nutrients - addition of inducer; e.g. lactose o it is a waste to make --galactose when there is no lactose ? transcription is stopped for that gene - polycistronic Æ multiple proteins - control of promoter controls entire pathway in response to environment change The regulation of operons in bacteria - operator Æ encodes within promoter - repressor Ærepresses transcription o e.g. lactose The Lac operon: an inducible operon - bacteria = glucose lovers - make sure lactose present and cAMP levels low for high levels of lac operon transcription - the concentration of cAMP is inversely proportional to the concentration of glucose: o glucose high Æ cAMP low o glucose low Æ cAMP high } inverse proportionality - cAMP-CRP binding complex binds to the lac promoter and enhances attachment of RNA pol o enhances lac operon transcription - low glucose & lactose available: high (constitutive) level of expression - high glucose & lactose available: low (basal) level of expression o chosen b/w the two: glucose & lactose; ?}ZL[šÁLšš}šŒLZ Œ]o }‰Œ}L o b/c cAMP-CRP enhances leÀo}ƉŒZZ]}L8µš]š[ZL}ššZŒ The Trp operon: a repressible operon - Tryptophan = corepressor Control of gene expression: eukaryotes - Control of eukaryotic gene expression is most complex and occurs primarily at 3 levels: o Transcriptional-level control: whether or not a gene is transcribed o Processing-level control: the mechanism controlling conversion of pre-mRNA to mRNA o Translational-level control: whether or not an mRNA is translated; how often and at what rate Transcriptional-level control - Transcription factors; modifications: methylation/deacetylation - Different growth conditions: in different environments, different genes are expressed ?mRNAs are also different - Different stimuli (signals) Æmake different TFs (transcription factors) Æ bind different DNA site (sequence) Æ regulate genes An aside: DNA microarrays - Yeast are grown in 2 different conditions: o Glucose-rich media vs. o Ethanol rich media - They turn on or turn off genes that are needed or not needed in the different environment in order to survive - Sometimes the fluorescence emitted by this type of interaction is yellow: o Meaning both red and green cDNAs present; both yeasts express this gene o e.g. actin or tubulin - If there is no colour: that gene is transcribed under neither growth condition - Certain genes are expressed only under certain conditions Structure of TFs - TFs are proteins that contain multiple domains: o DNA binding domain o Activation domain o Dimerization-domain: Æbinds another protein of identical structure Æforms a dimer; function in both DNA binding and activation - Each domain folds independently; have different functions DNA binding domains of TFs - Related structures that interact with DNA are called ^motifs_ - An example includes HMG box = high mobility group - Other examples: o Zinc-finger motif ƒ Zinc fingers occur in tandem (e.g. 3,5,9 repeats in a row) ƒ TFIIIA = transcription of 5S rRNA o Helix-loop-helix motif ƒ Have a basic amino acid region (i.e. lysine, arginine, histodine etc.); involved in interacting with DNA ƒ Transcription factors associated with this motif are dimers (hetero- or homo- dimers); interacting together of two similar structures = dimerization ƒ MyoD J muscle differentiation = not turned on at all times; only certain stages of development o Leucine zipper motif ƒ Also contains dimmers (2 alpha helices)
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