Gene Regulation: Negative & Positive Control in Prokaryotes & Eukaryotes, Study notes of Genetics

Download Gene Regulation: Negative & Positive Control in Prokaryotes & Eukaryotes and more Study notes Genetics in PDF only on Docsity! 1 Genetic Regulation • Regulation of transcription –Two forms •negative regulation •positive regulation Negative Regulation • A repressor protein binds to the regulatory region of a gene preventing transcription • Only when an inducer is present to alter the binding of the repressor can transcription take place • Inhibition needs to be overridden for transcription to occur • Common in prokaryotes Positive Regulation • An effector molecule binds with the regulatory region and activates transcription • No inhibition to override for transcription to occur • This type of regulation is common in eukaryotes 2 Genetic Regulation • Sometimes, systems can be both negatively and positively regulated –so not mutually exclusive • Structural genes –genes that code for polypeptides Gene Regulation in Prokaryotes • Structural genes arranged in clusters controlled by regulatory sites are called operons –E. coli has a lac operon • gene cluster + regulatory units involved in controlling lactose metabolism • the genes are regulated and expressed together as a unit lac Operon • 3 proteins, 2 are necessary for the metabolism of lactose – lactose permease (lacY gene) • transporter molecule which allows lactose to enter the cell – β-galactosidase (lacZ gene) • cleaves lactose to yield galactose and glucose 5 lac Operon E.g. Positive Regulation lacI lacPlacO lacZ lacY lacA When lactose is present, no glucose cAMP-CAP (effector) binds to the promoter (lacP), allowing RNA polymerase to bind strongly Transcription: polycistonic mRNAmRNA repressor lactose Tryptophan Operon • Structural genes that code for enzymes that make the amino acid tryptophan (biosynthesis). • When tryptophan is present at high enough levels, the operon is off. • When tryptophan is not Tryptophan Operon mRNA trpR P O L Att trpE trpD trpC trpB trpA E.g. Negative Regulation When trptophan is present Repressor protein Tryptophan binds (co-repressor) Binds to the operator No transcription 6 Tryptophan Operon trpR P O L Att trpE trpD trpC trpB trpA E.g. Negative Regulation When trptophan is not present mRNA Repressor protein Transcription: polycistonic mRNA