
To determine:
The lac genotype by using plus sign (+) to indicate the synthesis of
Introduction:
An operon is defined as a group of genes that have a common promoter and regulator and also transcribed as a single unit. This transcription results in the production of single

Explanation of Solution
Tabular representation: Table 1: Transcription of a gene in the present or absent of lactose.
Genotype of strain | Lactose absent | Lactose present | ||
Permease | Permease | |||
- | - | + | + | |
+ | + | + | + | |
+ | + | + | + | |
+ | - | + | - | |
- | - | - | - | |
- | - | + | + | |
+ | + | + | + | |
- | - | + | - | |
- | - | + | - | |
- | - | + | + | |
- | - | - | - | |
- | - | - | - |
Table 1: Transcription of a gene in the present or absent of lactose.
Lac operon is defined as an operon which is required for lactose metabolism and its transport. Lac operon is an example of the negative inducible operon. This operon contains three types of a structural gene which are designated as lac Z, lac Y, and lac A, that contain a common promoter and an operator region. Lac operon also contains regulator gene which is designated as lac I. Lac I gene encodes a repressor protein and inhibit transcription of a gene. Mutation in any genes presents in operon cause the change in gene expression.
The presence and absence of an enzyme in the given strain genotypes are listed below:
(i) In wild-type bacterial strain, the regulation of the lac operon gene is dependent on the presence and absence of lactose (substrate). If lactose is present, then the transcription of the structural gene will turn on and the synthesis of an enzyme
(ii) Mutation in any structural gene results in a change of their gene expression. Suppose, a mutation in
(iii) If a mutation occurs at the site of an operator, then the complex of repressor and corepressor protein will not bind to the operator and transcription will continuously take place. In this case, repressor protein would not able to turn off the gene expression. Thus, in third strain, the gene for
(iv) Mutation in regulator gene prevents the synthesis of the repressor protein. Thus, the transcription of the structural gene will continuously turn on. But in the given fourth strain gene for synthesis of permease enzyme is also mutated so, permease enzyme will not synthesize in both conditions (presence of lactose and absence of lactose).
(v) If a mutation occurs at regulator region, then the regulator gene unable to synthesize the repressor protein this result in the constitutive transcription of a structural gene. But in the given fifth strain promoter region is also mutated which prevent the binding of RNA polymerase to operator and enzyme fail to start transcription of a structural gene. Thus, in fifth strain, no enzyme will produce in both conditions (presence of lactose and absence of lactose).
(vi) If a mutation occurs in regulator gene present in one DNA strand and other DNA strand regulator gene is not mutated
(vii) The mutation in the operator gene represented by
(viii) As the regulator gene is trans-acting, so if a mutation occurs in one DNA strand of regulator gene will not repress the transcription of other DNA gene. But in this case, permease gene is also mutated, so the synthesis of permease enzyme will not occur either in the presence and absence of lactose. Thus, in this case, only synthesis of
(ix) In the ninth strain mutation in operator gene is dominant, thus repressor protein will not repress the transcription. But in this case gene for the synthesis of permease enzyme is also mutated. So, enzyme permease will not form in both conditions (presence of lactose and absence of lactose).
(x) The tenth strain is wild type; hence transcription of a structural gene would occur only in presence of substrate. In absence of substrate, no enzyme will be produced.
(xi) The mutation represented by
(xii) The mutation represented by
The enzyme synthesized by the structural gene will depend on the presence or absence of a substrate. If lactose is present, then the gene for lac operon will turn on and if lactose is absent then gene for lac operon will turn off. Mutation in any of the lac operon gene will change the gene expression.
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Chapter 16 Solutions
Genetics: A Conceptual Approach
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