Biology 9700 · AS & A Level · Gene control

Gene control — practice question

The lac operon occurs in the genome of the bacterium Escherichia coli. If glucose is unavailable, lactose in the extracellular environment causes the genes of the lac operon to be expressed. This increases the uptake and metabolism of lactose. If glucose and lactose are both present in the extracellular medium, lactose is stopped from entering the bacterial cell. Fig. 3.1 illustrates what occurs when glucose enters a bacterial cell. • Glucose enters the bacterial cell via transport protein, A. • B is an enzyme located in the cytoplasm of the bacterial cell. • B catalyses the phosphorylation of glucose to form glucose 6-phosphate.
(a(i))[3]

Using Fig. 3.1, suggest and explain how glucose in the extracellular environment prevents lactose from entering the bacterial cell.

(a(ii))[2]

Suggest the advantages for the bacterial cell in preventing the entry of lactose when glucose is present in the extracellular environment.

(a(iii))[3]

The glucose 6-phosphate in the cytoplasm of the bacterial cell is converted to produce fructose 1,6-bisphosphate. Fructose 1,6-bisphosphate takes part in glycolysis. Outline the events that take place in glycolysis after fructose 1,6-bisphosphate has been produced.

(b(i))[2]

With reference to Fig. 3.2, describe the effect of the different diets on the percentage of E. coli that are $\text{lac}^{+}$.

(b(ii))[2]

The scientists concluded that $\text{lac}^{+}$ E. coli had a selective advantage while colonising the gut, but only when lactose was present. Suggest why the presence of lactose gave $\text{lac}^{+}$ E. coli a selective advantage.

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