Presentation on theme: "Regulation of Gene Expression"— Presentation transcript:
1Regulation of Gene Expression Prokaryotes and Eukaryotes
2Regulation of Gene Expression A cell contains the entire genome of an organism– ALL the DNA.Gene expression = transcribing and translating the geneRegulation allows an organism to selectively transcribe (and then translate) only the genes it needs to.Genes expressed depend onthe type of cellthe particular needs of the cell at that time.
3Gene Regulation in Prokaryotes Prokaryotes organize their genome into operonsOperon = a group of related genesOne promoter sequence at the very beginningAll of the genes will be transcribed together (in one long strand of RNA.
4Question… What is the benefit of organizing the genome into operons? It’s more efficient – transcribe everything you need for a process at once.
5Repressible Operon: Trp Operon Repressible Operon = Operon that is usually “ON” but can be inhibitedThe Trp Operonexample of a repressible operonGenes that code for enzymes needed to make the amino acid tryptophan
6TrpR Gene TrpR gene is the regulatory gene for the Trp operon Found somewhere else on the genomeNOT part of the Trp operonTrpR gene codes for a protein = TrpR repressorTrpR gene is transcribed and translated separately from the Trp operon genes.
7TrpR Repressor Repressor protein is translated in an inactive form Tryptophan is called a corepressorWhen tryptophan binds to the TrpR repressor, it changes it into the active form
8Operator RegionThere is also an operator region of DNA in the Trp OperonJust after the promoter regionThe TrpR Repressor can bind to the operator if it’s in the active form
9Trp Operon Transcription is “ON” Occurs when there is no tryptophan available to the cell.Repressor is in inactive form (due to the absence of tryptophan)RNA Polymerase is able to bind to promoter and transcribe the genes.
10Trp Operon Transcription is “OFF” Occurs when tryptophan is available Tryptophan binds to the TrpR repressor converts it to active formTrpR protein binds to operator blocks RNA Polymerase no transcription
11Question…Under what conditions would you expect the trp operon to go from “OFF” to “ON” again?When there is no longer tryptophan available– all of it has been used up
12Inducible Operon: Lac Operon Inducible operon = operon is usually “OFF” but can be stimulated/activatedLac OperonExample of an inducible operonGenes code for enzymes that break down lactose
13LacI gene LacI gene is the regulatory gene for the lac operon Found somewhere else on the genomeNOT part of the lac operonLacI gene codes for a protein = lacI repressorLacI gene is transcribed and translated separately from the lac operon genes.
14LacI RepressorThe lacI repressor protein is translated into an active formWhen the lacI repressor is bound by lactose (also called allolactose) it becomes inactiveLactose is the inducer
15Lac Operon Transcription is “OFF” When there is no lactose that needs to be digestedlacI repressor is in active form binds to operator blocks RNA Polymerase no transcription
16Lac Operon Transcription is “ON” When there is lactose that needs to be digestedLactose binds to lacI repressor inactivates itRNA Polymerase is able to bind to promoter transcribe genes
17Do all operons have operator regions? NOThere are some genes that always need to be transcribed they do not need to have operators to regulate them in this manner.Ex. genes that participate in cellular respiration
18Positive Gene Regulation In the lac operon there are other molecules to further stimulate transcription.Lactose will only be digested for energy when there isn’t much glucose aroundWhen glucose levels are low, level of cAMP molecule builds up
19cAMP and CAP CAP = regulatory protein that binds to cAMP CAP is inactive unless cAMP binds to it
20Positive gene regulation If there isn’t much glucose high levels of cAMPCAP and cAMP bind CAP can bind to the promoter stimulates RNA Polymerase to bind
21Positive gene regulation When glucose levels rise again, cAMP levels will drop no longer bound to CAPCAP can’t bind to promoter transcription slows down
22Positive gene regulation The lac operon is controlled on 2 levels:Presence of lactose determines if transcription can occurCAP in the active form determines how fast transcription occurs
23Gene Regulation in Eukaryotes Eukaryotes have large genomesOther molecules have to help RNA Polymerase find the promoter and start transcriptionTranscription factorsEnhancer sequences
24Transcription Factors Series of proteins that bind to the promoter to help RNA Polymerase bindRNA Polymerase also has to bind transcription factors in order to be able to start transcription.Like a sign on the door that helps direct you to a particular room in the school.
25Question…How might binding transcription factors help RNA Polymerase bind?Creates an area that chemically attracts RNA Polymerase more
26Enhancer sequencesSequences of DNA that are far away from the gene they help transcribeProcess:Activator molecules bind to the Enhancer sequenceEnhancer loops around so that the activators can also bind to the transcription factorsTogether with RNA polymerase they all cause transcription to startEnhancers can be up to 20,000 bp away from promoter. Like an even bigger sign to help direct people to a room
27Cell-specific Regulation Each cell has the DNA to transcribe any geneDifferent activators and transcription factors in specific cells will determine which genes are transcribed which proteins are translated