In general, __________ operons are inactive until the substrate of their genes’ polypeptides is present.

Verified step by step guidance

1. Understand the concept of operons in microbiology: Operons are clusters of genes under the control of a single promoter and regulatory elements, allowing coordinated expression of genes involved in a common pathway.
2. Recall that operons can be classified based on their regulation mechanism, such as inducible or repressible operons.
3. Identify that inducible operons are typically inactive (turned off) until a specific substrate or inducer molecule is present in the environment.
4. Recognize that the presence of the substrate acts as an inducer, binding to the repressor protein and causing it to release from the operator region, thereby allowing transcription of the operon's genes.
5. Conclude that the blank in the sentence should be filled with 'inducible' because inducible operons are inactive until the substrate of their gene products is present.

Key Concepts

Inducible Operons

Inducible operons are gene clusters that remain inactive until a specific substrate or inducer molecule is present. When the substrate binds to the repressor protein, it causes a conformational change that releases the repressor from the operator, allowing transcription to proceed.

Repressor Proteins and Operator Sites

Repressor proteins bind to operator sites on DNA to block RNA polymerase and prevent transcription. In inducible operons, the repressor is active by default and only detaches when the inducer (substrate) is present, enabling gene expression.

Substrate as an Inducer

The substrate of the enzymes encoded by the operon acts as an inducer by interacting with the repressor protein. This interaction inactivates the repressor, triggering the operon to become active and produce enzymes needed to metabolize the substrate.