Biology, Eighth Edition (Raven)

Chapter 16: Control of Gene Expression

The Lac Operon (Induction)

Why do some genes have multiple pathways controlling gene expression? Control of transcription is often a complex process. The presence of one molecule may prevent transcription while the presence of a different molecule may stimulate transcription but only if the first molecule is not present. Multiple transcription factors and complex interactions between the factors allow genes to respond to a range of different conditions. Here is a hypothetical example. If a particular enzyme is needed to break down a the end product of a metabolic pathway but only if that end product is not being used to react with an environmental toxin then the gene producing the enzyme will be most efficient if it can respond to the presence of both the waste product and the toxin.

View the animation below, then complete the quiz to test your knowledge of the concept.



1.

When glucose is present
A)cyclic AMP is high, the catabolite activator protein (CAP) binds to the activator binding site, and transcription of lactose is turned on
B)cyclic AMP is low, CAP binds to the site activator binding site, and transcription of lactose is turned on
C)cyclic AMP is high, CAP does not bind to the activator binding site, and transcription of lactose is turned on
D)cyclic AMP is low, CAP does not bind to the activator binding site, and transcription is turned off
2.

The lac repressor
A)binds to the operator and prevents transcription
B)binds to the CAP site and prevents transcription
C)binds to the CAP site and facilitates transcription
D)binds to the operator and facilitates transcription
3.

When both glucose and lactose are present
A)cyclic AMP is high so transcription occurs
B)the lac repressor binds with the lactose and transcription occurs
C)RNA polymerase is able to bind to the operator so transcription occurs
D)transcription is turned off
4.

In the absence of glucose, when lactose is present it combines with the repressor, allowing RNA polymerase to carry on transcription.
A)True
B)False
5.

When both glucose and lactose are absent, transcription occurs.
A)True
B)False
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