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Can events inside a cell control gene expression? Gene expression is controlled by a number of factors. External signals from other parts of the body or from adjacent cells can turn genes on or off. Genes can also respond to events happening inside their own cell. An excess of the product of a metabolic pathway may turn off the genes coding for enzymes in the pathway. An excess of substrate may turn on genes coding for the same enzymes. When gene expression responds to cellular conditions the cell can produce the appropriate materials for the current situation.

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

1In the trpL mRNA, the attenuator is located closest to what stem loop region?
A)region 1
B)region 2
C)region 3
D)region 4

2If transcription is occurring along, what stem loop structures are formed in the trpL mRNA?
A)1-2 and 3-4
B)1-3 only
C)2-4 only
D)1-3 and 2-4

3Which of the following occurs if tryptophan levels are high?
A)regions 2 and 3 form a stem loop structure
B)regions 3 and 4 form a stem loop structure
C)regions 1 and 4 form a stem loop structure
D)no stem loop structures are formed

4The process of attenuation in the trp operon turns off the operon when it is not needed by the cell.

5If regions 1 and 2, and 3 and 4, of the trpL mRNA form stem loops, the trpE gene will be transcribed.

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