Concept explainers
Even though the lac Z, Y, and A structural genes are transcribed as a single polycistronic mRNA, each gene contains the initiation and termination signals essential for translation. Predict what will happen when a cell growing in the presence of lactose contains a deletion of one
(a)
To determine: The fate of a cell growing in the presence of lactose and containing a deletion in one of the nucleotides presents early in the Z gene.
Introduction: The lac operon contains its promoter, regulator, and three structural genes designated as Z, Y, and A. The Z gene encodes β-galactosidase aids in cleaving the disaccharide lactose into its monomers glucose and galactose.
Explanation of Solution
If there is a deletion of one nucleotide in the early Z gene, then there will be a frameshift of all the reading frames present downstream from the deletion, which alters many amino acids. As a result of mutation, either premature chain termination of the translation will occur, or the normal chain termination will be ignored. If such a cell is placed in a lactose medium, it will be incapable of growing because of the unavailability of the β-galactosidase gene.
Thus, the cell with a deletion mutation in the early Z gene is incapable of growing in lactose medium because of the absence of the β-galactosidase gene.
(b)
To determine: The fate of a cell growing in the presence of lactose and containing a deletion in one of the nucleotides present early in the A gene
Introduction: The lac operon contains three structural genes known as lacZ, lacY, and lacA. The three genes are transcribed in the form of a single mRNA under the control of one lac promoter. The lacA codes for transacetylase that transfer acetyl groups from acetyl-coenzyme A to β-galactoside.
Explanation of Solution
When there is a deletion of one nucleotide in the early A gene, the expected function of A gene product is impaired, but the use of lactose as a carbon source is not influenced. In this situation, if a cell is placed in a lactose-containing medium, then the cell will grow normally.
Thus, the cell with a deletion mutation in the early A gene has impaired gene A product, and it can grow normally in the lactose-containing medium.
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Chapter 16 Solutions
Concepts of Genetics
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