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E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because:

  1. A
    In the presence of glucose, E. coli cells do not utilize lactose
  2. B
    They cannot transport lactose from the medium into the cell
  3. C
    The lac operon is constitutively active in these cells
  4. D
    They cannot synthesize functional β-galactosidase

Solution & Step-by-step Explanation

E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because they cannot synthesize functional β-galactosidase. The Z gene of the lac operon encodes the β-galactosidase enzyme, which is necessary for the breakdown of lactose. So, the correct answer is option D. Notice that the other options are not correct. The presence of glucose does not affect the ability of E. coli cells to utilize lactose. The lac operon is not constitutively active in these cells, and the mutation in the Z gene does not affect the transport of lactose into the cell.

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E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because:
A
In the presence of glucose, E. coli cells do not utilize lactose
B
They cannot transport lactose from the medium into the cell
C
The lac operon is constitutively active in these cells
D
They cannot synthesize functional β-galactosidase

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