November 14, 2011 Biology 311C Lecture 34 • Some genes are organized in units called operons (one shared promoter, several protein-coding genes, one long mRNA). • Lac operons genes are usually turned off, but turned on in the presence of lactose sugar. • When lactose is present, it is an inducer, the lac operons are turned on, the repressor in active, and the operon is on. • Even though you have a long mRNA molecule, it can code for several proteins. • The active repressor is made by a regulatory gene when it is transcribed to make mRNA. • If there was a frame-shift mutation in the regulatory gene, beta galactosidase would constantly be produced. • The active repressor is not produced so it can’t stop the operator and RNA polymerase can continuously transcribe. • If a substitution mutation in the lac operon promoter (and RNA polymerase could not bind), none of the proteins from the genes will be made. • They all have the same promoter so if there is no RNA polymerase, transcription will
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