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Regulation of Gene Expression Prokaryotes and Eukaryotes Unit 2 AP Biology Regulation of Gene Expression • A cell contains the entire genome of an organism– ALL the DNA. • Gene expression = transcribing and translating the gene • Regulation allows an organism to selectively transcribe (and then translate) only the genes it needs to. • Genes expressed depend on – the type of cell – the particular needs of the cell at that time. Regulation of Gene Expression • Up-regulation = increased expression of gene • Down-regulation = decreased expression of gene • Each of these can be due to a variety of factors (to be discussed later) Image taken without premission from http://www.curatedchildrensbooks.com/category/beginning-readers Gene Regulation in Prokaryotes • Prokaryotes organize their genome into operons • Operon = a group of related genes – One promoter sequence at the very beginning – All of the genes will be transcribed together (in one long strand of RNA). Question… • What is the benefit of organizing the genome into operons? – It’s more efficient – transcribe everything you need for a process at once. Repressible Operon: Trp Operon • Repressible Operon = Operon that is usually “ON” but can be inhibited • The Trp Operon – example of a repressible operon – Genes that code for enzymes needed to make the amino acid tryptophan TrpR Gene • TrpR gene is the regulatory gene for the Trp operon – Found somewhere else on the genome – NOT part of the Trp operon – TrpR gene codes for a protein = TrpR repressor – TrpR gene is transcribed and translated separately from the Trp operon genes. TrpR Repressor • Repressor protein is translated in an inactive form • Tryptophan is called a corepressor – When tryptophan binds to the TrpR repressor, it changes it into the active form Operator Region • There is also an operator region of DNA in the Trp Operon – Just after the promoter region – The TrpR Repressor can bind to the operator if it’s in the active form Trp 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. Trp Operon • Transcription is “OFF” – Occurs when tryptophan is available – Tryptophan binds to the TrpR repressor converts it to active form – TrpR protein binds to operator blocks RNA Polymerase no transcription Question… • 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 Inducible Operon: Lac Operon • Inducible operon = operon is usually “OFF” but can be stimulated/activated • Lac Operon – Example of an inducible operon – Genes code for enzymes that break down lactose LacI gene • LacI gene is the regulatory gene for the lac operon – Found somewhere else on the genome – NOT part of the lac operon – LacI gene codes for a protein = lacI repressor – LacI gene is transcribed and translated separately from the lac operon genes. LacI Repressor • The lacI repressor protein is translated into an active form • When the lacI repressor is bound by lactose (also called allolactose) it becomes inactive – Lactose is the inducer Lac Operon • Transcription is “OFF” – When there is no lactose that needs to be digested – lacI repressor is in active form binds to operator blocks RNA Polymerase no transcription Lac Operon • Transcription is “ON” – When there is lactose that needs to be digested – Lactose binds to lacI repressor inactivates it – RNA Polymerase is able to bind to promoter transcribe genes Do all operons have operator regions? • NO • There 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 Positive 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 around • When glucose levels are low, level of cAMP molecule builds up cAMP and CAP • CAP = regulatory protein that binds to cAMP • CAP is inactive unless cAMP binds to it Positive gene regulation • If there isn’t much glucose high levels of cAMP • CAP and cAMP bind CAP can bind to the promoter stimulates RNA Polymerase to bind Positive gene regulation • When glucose levels rise again, cAMP levels will drop no longer bound to CAP • CAP can’t bind to promoter transcription slows down Positive gene regulation • The lac operon is controlled on 2 levels: – Presence of lactose determines if transcription can occur – CAP in the active form determines how fast transcription occurs Gene Regulation in Eukaryotes • Eukaryotes have large genomes • Other molecules have to help RNA Polymerase find the promoter and start transcription – Transcription factors – Enhancer sequences Transcription Factors • Series of proteins that bind to the DNA to control transcription – Activators: increase expression (ex. bind to promoter to help RNA Polymerase bind) – Repressors: decrease expression of gene • RNA Polymerase also has to bind certain transcription factors in order to be able to start transcription. Question… • How might binding transcription factors help RNA Polymerase bind? – Creates an area that chemically attracts RNA Polymerase more Enhancer sequences • Sequences of DNA that are far away from the gene they help transcribe • Process: – Activator molecules bind to the Enhancer sequence – Enhancer loops around so that the activators can also bind to the transcription factors – Together with RNA polymerase they all cause transcription to start Cell-specific Regulation • Each cell has the DNA to transcribe any gene • Different activators and transcription factors in specific cells will determine which genes are transcribed which proteins are translated