Concept 33 Genes can be turned on and off.

HI! You just learned about how the lac operon works. Let's review some of the key components of the system. When glucose and lactose are present, which diagram illustrates the state of the lac operon? Lactose removes the inhibitor from the operator, but glucose is present and prevents cAMP production. The cAMP-CRP complex doesn't bind to the lac operon. In which of the following cases will b-gal be produced? The inhibitor must be removed by lactose, and cAMP-CRP must bind to the lac operon to activate transcription. cAMP level is regulated by glucose and is high when glucose is absent. Jacob and Monod did another experiment where the female bacteria received a plasmid with a working copy of the operator from the male. The plasmid carried nothing else. Inhibitor binds to the working operator and turns off the b-gal gene. (No, the working operator on the plasmid does not affect the female b-gal gene.) The working operator produces inhibitor; inhibitor turns off b-gal. (No, the operator does not produce inhibitor.) The working operator turns on b-gal. (No, b-gal is already on.) The inhibitor binds to the working operator, but can't turn off b-gal. (That is correct) The female has a mutated operator (O-) and inhibitor can't bind to it. What happens when a working operator (O+) is donated by the male? The inhibitor physically blocks mRNA transcription when it binds to the operator. Therefore, the operator and the gene are on the same piece of DNA. A working operator on a different piece of DNA cannot fix a mutated operator. CONGRATULATIONS!!! YOU'RE SO SMART!