Controls Over Genes

More on Transcription zPromoters are regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA zCalled the TATA box zTranscription factors are also involved (proteins that mediate the binding of RNA polymerase) zSpecific base sequences act as signals to stop zCalled the termination signal

mRNA Processing zAfter the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional zIntrons, non-functional segments of DNA are snipped out of the chain (RNA splicing)

mRNA Editing zExons, segments of DNA that code for proteins, are then rejoined by the enzyme ligase zA guanine triphosphate cap is added to the 5” end of the newly copied mRNA zA poly A tail is added to the 3’ end of the RNA zThe newly processed mRNA can then leave the nucleus

CAP Tail New Transcript Result of Transcription

mRNA Transcript mRNA leaves the nucleus through its pores and goes to the ribosomes

Why Control Gene Expression? zSome genes are “on” (being transcribed) almost all the time yCalled housekeeping genes yExamples: ribosome components, enzyme for basic metabolic pathways zMany genes are only turned on when they are needed

Why Control? zTranscribing genes that are not needed is a waste of energy and may interfere with the status of the cell

Regulation zRespond to a range of stimuli yProkaryotes respond to external stimuli (food, enzymes turned on) yEukaryotes also respond to internal stimuli (hormones, growth factors)

Regulation zDevelopmentally regulated yMulticellular organisms progress through developmental stages yDifferent genes expressed at different times during development zCell specialization yDifferent genes expressed in different cells

The strategy behind regulation.. zGene control is control over amount of gene produced (RNA or protein) in cell zMultiple ways to control the amount of gene product in a cell

Controlling gene product amount 1.Rate of transcription – rate mRNA is produced; faster produced = more product 2.mRNA degradation – rate mRNA is broken down; faster broken down = less product

Controlling gene product amount 3.mRNA processing – capping, splicing; slower processing = less product 4.Translation – rate of translation or # of ribosomes translating; fast/more = more product Although control probably involves all of these, the most understood are changes in the rate of transcription

Gene Control – lac operon zLac operon is a gene in bacteria zBacteria have 3 genes in a row (operon) that involve breaking down lactose for energy zIn order to be efficient, these genes should not be expressed unless lactose is present

Lac Operon - vocab zRegulatory protein – control transcription, translation, and gene products by interacting with DNA, RNA, or proteins zRepressor – protein that binds with an operator on prokaryotic DNA to prevent transcription zOperator – short base sequence between a promoter and genes; a binding site for repressors

Lac operon – vocab. zPromoter – piece of DNA where RNA polymerase can bind and start transcription zNegative control – regulatory protein that slows down gene activity zPositive control – regulatory protein that enhances gene activity

Lac operon vocab. zOperon – a promoter and a pair of operators that control a bacterial gene zActivator – protein that exerts positive control over an operon

gene 1gene 2gene 3 lactose operon regulatory gene transcription, translation operator promoter repressor protein Figure 15.3a Page 241

Lac operon zGoal 1 – transcription low when lactose is absent zLac I (gene upstream from operon) produces a repressor which binds to promoter region zBinding of repressor prevents RNA polymerase from binding and transcribing genes

Lac operon zGoal 2 – increase transcription when lactose is present zAllolactose will bind to the repressor, changing its conformation and causing it to fall off the promoter site zPromoter site now available for RNA polymerase to bind; transcription of lac genes begins

Lac operon zGoal 3 – turn off transcription when lactose is used up zAllolactose metabolizes, freeing up the repressor zThe free repressor is available to bind the promoter site and stop transcription

Control of lac operon zNegative control – glucose present - repressor inactivates the lac operon zPositive control – lactose present – activator protein (called CAP) makes promoter more favorable for RNA polymerase to bind and begin transcription

Low Lactose zRepressor binds to operator zBinding blocks promoter zTranscription is blocked Figure 15.3b Page 241

High Lactose gene 1 operator promoter mRNA RNA polymerase lactose allolactose Figure 15.3c Page 241

Most Genes Are Turned Off zCells of a multicelled organism rarely use more than 5-10 percent of their genes at any given time zThe remaining genes are selectively expressed

Homeotic Genes zOccur in all eukaryotes zMaster genes that control development of body parts zEncode homeodomains (regulatory proteins) zHomeobox sequence can bind to promoters and enhancers

X Chromosome Inactivation zIn female mammals, in all cells one of the two X chromosomes is completely inactivated zInactivation is random zInactivated chromosome can be observed in the interphase nucleus as Barr body zGenes on the inactivated chromosome are not expressed