The Lac Operon

Why are several genes for different products involved in the same pathway located next to one another in the genome? The lab operon contains DNA sequences coding for three enzymes involved in the fermentation of lactose. Once transcription is initiated the mRNA for all three enzymes is transcribed as a unit. If one protein is made then all three are made. This is only possible because the DNA sequences for all three genes are located next to one another. Once RNA polymerase activity is initiated it can transcribe the mRNA for all three. If the three genes were in different locations the transcription of each one would have to be controlled separately.

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


1	The lac repressor binds to what site within the lac operon?

		A)	lacZ
		B)	lacO
		C)	lacA
		D)	lacl
		E)	lacP

2	Which of the following is not part of the lac operon?

		A)	lacO
		B)	lacZ
		C)	lacA
		D)	lacl
		E)	lacP

3	The lac repressor is inactivated by binding to which of the following?

		A)	lactose
		B)	betagalactosidase
		C)	transcription factors
		D)	glucose
		E)	allolactose

4	The polycistronic mRNA is transcribed from the lac operon when lactose is present in the cell.

		A)	True
		B)	False

5	The lac repressor protein is active in the absence of lactose within the cell.

		A)	True
		B)	False

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