Question

In a theoretical operon, genes \(A, B, C,\) and \(D\) represent the repressor gene, the promoter sequence, the operator gene, and the structural gene, but not necessarily in the order named. This operon is concerned with the metabolism of a theoretical molecule (tm). From the data provided in the accompanying table, first decide whether the operon is inducible or repressible. Then assign \(A, B\) \(C,\) and \(D\) to the four parts of the operon. Explain your rationale. \((\mathrm{AE}=\text { active enzyme; } \mathrm{IE}=\text { inactive enzyme; } \mathrm{NE}=\text { no enzyme. })\) $$\begin{array}{lcc} \text { Genotype } & \text { tm Present } & \text { tm Absent } \\ A^{+} B^{+} C^{+} D^{+} & \text {AE } & \text { NE } \\ A^{-} B^{+} C^{+} D^{+} & \text {AE } & \text { AE } \\ A^{+} B^{-} C^{+} D^{+} & \text {NE } & \text { NE } \end{array}$$ $$\begin{array}{lcc} \text { Genotype } & \text { tm Present } & \text { tm Absent } \\ A^{+} B^{+} C^{-} D^{+} & \text {IE } & \text { NE } \\ A^{+} B^{+} C^{+} D^{-} & \text {AE } & \text { AE } \\ A^{-} B^{+} C^{+} D^{+} / F^{\prime} A^{+} B^{+} C^{+} D^{+} & \text {AE } & \text { AE } \\ A^{+} B^{-} C^{+} D^{+} / F^{\prime} A^{+} B^{+} C^{+} D^{+} & \text {AE } & \text { NE } \\ A^{+} B^{+} C^{-} D^{+} / F^{\prime} A^{+} B^{+} C^{+} D^{+} & A E+I E & N E \\\ A^{+} B^{+} C^{+} D^{\prime} / F^{\prime} A^{+} B^{+} C^{+} D^{+} & A E & N E \end{array}$$

Short answer

Assign genes A, B, C, and D to the repressor gene, promoter sequence, operator gene, and structural gene, and explain the rationale behind your decisions. Answer: The operon is inducible. Gene A is the repressor gene, gene B is the promoter sequence, gene C is the operator gene, and gene D is the structural gene. This conclusion is based on observing enzyme activity in the presence and absence of the theoretical molecule (tm) and comparing genotypes with mutated and unmutated versions of the genes.

Step-by-step solution

Determine if the operon is inducible or repressible

Compare enzyme activity in the presence and absence of tm for each genotype. If the enzyme activity increases in the presence of tm, the operon is inducible, and if it decreases, the operon is repressible. - For the genotype \(A^{+}B^{+}C^{+}D^{+}\), enzyme activity is AE (active enzyme) in the presence of tm and NE (no enzyme) in its absence. This indicates that this genotype represents an inducible operon.

Identify the repressor gene

Compare genotypes with a mutated version of gene A and an unmutated version. Observe the changes in enzyme activity to determine if gene A is the repressor gene. - In the presence of tm: \(A^{-}B^{+}C^{+}D^{+}\) has AE activity, while \(A^{+}B^{+}C^{+}D^{+}\) has AE activity. - In the absence of tm: \(A^{-}B^{+}C^{+}D^{+}\) has AE activity, while \(A^{+}B^{+}C^{+}D^{+}\) has NE activity. The enzyme is active in both situations when gene A is mutated, indicating that gene A is the repressor gene.

Identify the promoter sequence

Compare genotypes with a mutated version of gene B and an unmutated version. Observe the changes in enzyme activity to determine if gene B is the promoter sequence. - In the presence of tm: \(A^{+}B^{-}C^{+}D^{+}\) has NE activity, while \(A^{+}B^{+}C^{+}D^{+}\) has AE activity. - In the absence of tm: \(A^{+}B^{-}C^{+}D^{+}\) has NE activity, while \(A^{+}B^{+}C^{+}D^{+}\) has NE activity. The enzyme is not produced in any situation when gene B is mutated, indicating that gene B is the promoter sequence.

Identify the operator gene

Compare genotypes with a mutated version of gene C and an unmutated version. Observe the changes in enzyme activity to determine if gene C is the operator gene. - In the presence of tm: \(A^{+}B^{+}C^{-}D^{+}\) has IE (inactive enzyme) activity, while \(A^{+}B^{+}C^{+}D^{+}\) has AE activity. - In the absence of tm: \(A^{+}B^{+}C^{-}D^{+}\) has NE activity, while \(A^{+}B^{+}C^{+}D^{+}\) has NE activity. The enzyme is produced but inactive when gene C is mutated, indicating that gene C is the operator gene.

Identify the structural gene

Since we have identified the roles of genes A, B, and C, gene D must represent the remaining part of the operon, the structural gene. To summarize: - The operon is inducible. - Gene A is the repressor gene. - Gene B is the promoter sequence. - Gene C is the operator gene. - Gene D is the structural gene.

Key concepts

Genetics Education

Genetics education plays a crucial role in understanding how traits are passed from one generation to another and how gene expression is regulated within living organisms. The study of genetics has evolved from simple Mendelian inheritance patterns to a more complex picture, where interaction between multiple genes and environmental factors determine an organism's phenotype. Understanding the operon concept is foundational in genetics education as it provides insights into gene regulation mechanisms, which is important for fields such as medicine, biotechnology, and agriculture.

Furthermore, educational approaches such as step-by-step exercises help consolidate knowledge. In the example given, students learn to analyze and interpret genetic information by examining the function of different genes within an operon in a hypothetical scenario. This type of exercise not only reinforces the operon model but also demonstrates how mutations can affect gene function and enzyme production. In creating educational content, emphasizing clear explanations and real-life applications can make genetics more accessible and engaging for students.

Gene Regulation

Gene regulation is a fundamental concept in biology that explains how a cell controls the production of different proteins, which are crucial for the organism's functioning. The operon model, first proposed by Jacob and Monod, serves as a paradigm for gene regulation in prokaryotes. It shows how genes can be turned 'on' or 'off' in response to specific environmental stimuli.

In our exercise, the focus is on understanding whether the theoretical operon is inducible or repressible. Inducible operons, like the one presented, are normally off but can be turned on by the presence of a substrate, while repressible operons are generally on but can be turned off by the product they help create. This sophisticated system ensures that the organism's resources are not wasted producing substances that are not needed at a given time. It is crucial for students to understand that mutations in any component of the operon, such as the repressor gene, promoter sequence, or operator gene, can lead to disruptions in the normal function of the cell, with potentially drastic consequences.

Molecular Genetics

Molecular genetics is the field of biology that studies the structure and function of genes at a molecular level. It involves the exploration of genetic expression, mutation, recombination, and DNA repair. In the context of the problem we analyzed, molecular genetics provides a framework for understanding how different parts of the operon interact at the molecular level to control the expression of structural genes.

The operon itself is a unit of genetic function found in prokaryotes, consisting of a sequence of adjacent genes including a promoter and an operator to which a repressor binds. In our example, this knowledge helps students understand how the genotype affects the phenotype, revealing a direct link between the sequence of DNA and the function of the proteins through a simple operon system. When explaining complex topics like these, it's imperative to break down the process into understandable segments, simplifying the interrelationships among DNA, RNA, and protein synthesis, as they are fundamental to deciphering the molecular basis of gene expression and regulation.