The Sweetie gene is not expressed during normal growth conditions, but is expres

Question

The Sweetie gene is not expressed during normal growth conditions, but is expressed after treatment of cells with sucrose. In principle, the gene could be turned on by production or activation of a transcriptional activator (positive control) or by inactivation or depletion of a transcriptional repressor (negative control). One way to distinguish between these alternatives is to isolate and characterize mutants in which Sweetie is expressed in the absence of sucrose. Three possible mutants are described below. Explain whether each mutant supports positive or negative control. A. A single base-pair change located 800 base pairs upstream (5’) of the Sweetie coding region. B. A mutation in a coding region distinct from Sweetie that, when present in both copies of the chromosome in a diploid cell, causes expression of Sweetie in the absence of sucrose. However, when a diploid cell has the mutant gene on one chromosome and a normal copy on the other chromosome, Sweetie is expressed normally. (This is a recessive mutation.) C. A mutation in a coding region distinct from Sweetie that causes expression of Sweetie in the absence of sucrose, even when a diploid cell has one normal copy of the gene and one mutant copy. (This is a dominant mutation.)

Explanation / Answer

A. Might be a positive control, because mutation takes place in upstream region where the transcription start site would be located

B. Negative control. Because on one chromosome it is normal and on 2nd chromosome it is mutated. If it is recessive, we cannot see the expression of sweetie.

C. Positive control. Here, mutation is dominant, sweetie can express normally

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