SAT Subject Biology Practice Question 464: Answer and Explanation

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Question: 464

14. β-galactosidase (β-gal) is a bacterial enzyme used in the metabolism of lactose. Three different groups of human intestinal bacteria were cultured in a glucose-based medium. At Time 1, the bacteria were transferred to media containing lactose as a nutrient source instead of glucose. At Time 2, the bacteria were returned to the glucose-based medium. Samples of bacteria were removed at five-minute intervals and tested for the presence of β-gal. The results are presented below.

What could account for the delay in β-gal production after Time 1 seen in Group 1 ?

A. An inability to synthesize β-gal
B. Transcription of β-gal mRNA
C. Continued use of glucose as a nutrient
D. Replication of the bacterial DNA
E. Infection by a virus

Correct Answer: B

Explanation:

B Choice A is false; clearly the bacteria are able to produce β-gal as shown in the figure. Choice C is also false. The bacteria cannot continue to use glucose because they were switched to a lactose-based medium. There is no reason to assume that the switch to lactose-based medium would induce DNA replication (eliminated choice D), and there has been no infection by a virus (eliminate choice E). It takes a little bit of time for the enzyme β-gal (which is a protein) to be produced, because the mRNA for this enzyme is not normally present. The mRNA is transcribed when the enzyme is needed (such as when the bacteria are in lactose), and the time it takes the mRNA to be transcribed is the delay seen in production of β-gal.

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