Question

With respect to lac- operon explain the following terms:-

1. regulator gene
2. promoter gene
3. structural gene
4. inducer

Answer 1

• Regulator gene:

1. This gene controls the operator gene in cooperation with an inducer present in the cytoplasm.
2. The regulator gene precedes the promoter gene. It may not be present immediately adjacent to the operator gene.
3. The regulator gene produces a protein called repressor protein.
4. Repressor binds with operator gene and represses (stops) its action. Therefore, it is called regulator protein.

• Promoter gene:

1. This gene precedes the operator gene. It is present adjacent to the operator gene.
2. RNA polymerase enzyme binds to the promoter gene.
3. Promoter gene base sequence determines which strand of DNA acts as a template.
4. When the operator gene is turned on, the enzyme moves over the operator gene and transcription of structural genes starts.

• Structural gene:

1. When lactose is added to the E. coli culture, the structural genes produce mRNA which in turn produces polypeptides, on the ribosomes.
2. The polypeptides formed, act as enzymes to metabolize lactose in the cell.
3. There are 3 structural genes in the sequence lacZ, lacY, and lace.
4. Enzymes produced by these genes are β-galactosidase, permease, and transacetylase respectively.

• Inducer:

1. It is a chemical in the cytoplasm (allolactose) that inactivates the repressor.
2. When lac operon is switched on, inducer joins with repressor protein preventing the binding of repressor to the operator gene.
3. As operator gene is free, enzyme RNA polymerase can move from promoter to structural genes via operator gene.