To solve the question regarding lactose and its role in the regulation of the lac operon, we can break it down into the following steps:
### Step-by-Step Solution:
1. **Understanding the Lac Operon**:
- The lac operon is a set of genes in bacteria that are involved in the metabolism of lactose. It consists of structural genes (Z, Y, A), a promoter, an operator, and a regulatory gene (I).
**Hint**: Recall the components of the lac operon and their functions.
2. **Role of the Repressor**:
- The repressor protein, produced by the regulatory gene (I), binds to the operator region of the operon when lactose is absent. This binding prevents transcription of the structural genes.
**Hint**: Think about what happens when lactose is not present in the environment.
3. **Effect of Lactose Presence**:
- When lactose is present, it binds to the repressor protein. This binding changes the shape of the repressor, making it inactive. As a result, the repressor cannot bind to the operator, allowing transcription of the structural genes to occur.
**Hint**: Consider how the presence of a substrate can affect enzyme activity and gene expression.
4. **Induction of Gene Expression**:
- The structural genes (Z, Y, A) are then transcribed to produce mRNA, which is translated into proteins (β-galactosidase, permease, and transacetylase) that help in the breakdown of lactose.
**Hint**: Remember the concept of induction in operons, where the presence of a substrate activates gene expression.
5. **Conclusion**:
- Since lactose is responsible for the activation of the lac operon by inactivating the repressor, it is termed an "inducer." An inducer is a molecule that initiates gene expression.
**Hint**: Reflect on the definition of an inducer in the context of operon theory.
### Final Answer:
Lactose is termed as an **inducer** because it regulates the switching on and off of the lac operon by inactivating the repressor protein.
