Functioning of structural gene is controlled by:

Options

A. Operator
B. Promoter
C. Ligase
D. Regulator gene

Answer

To answer the question "Functioning of structural gene is controlled by:", we can follow these steps:

### Step 1: Identify the Structural Gene
The structural gene in question is often associated with specific functions in the cell. For example, in the lac operon, the structural genes include lacZ, lacY, and lacA, which are responsible for the metabolism of lactose.

### Step 2: Understand the Role of the Promoter
The promoter is a region of DNA where RNA polymerase binds to initiate transcription. It is crucial for the functioning of structural genes because it allows for the synthesis of mRNA from the DNA template.

### Step 3: Identify the Operator's Function
The operator is a segment of DNA that regulates the transcription of the structural genes. It acts as a switch that can turn the transcription of these genes on or off, depending on the presence or absence of specific molecules (like lactose in the lac operon).

### Step 4: Analyze the Conditions for Gene Functioning
When lactose is present, it binds to the repressor protein, causing it to release from the operator. This allows RNA polymerase to bind to the promoter and transcribe the structural genes. Conversely, when lactose is absent, the repressor binds to the operator, blocking RNA polymerase and preventing transcription.

### Step 5: Conclusion
The functioning of the structural gene is primarily controlled by the operator, as it determines whether the RNA polymerase can access the promoter and transcribe the structural genes.

### Final Answer
The functioning of the structural gene is controlled by the **operator**.

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