Chapter 12, Problem 36P

Summary Introduction

To analyze:

For the lac operon partial diploids mentioned in the question, determine whether the synthesis of lac Z mRNA is constitutive, inducible, or uninducible, and indicate whether the partial diploid is ${\text{Lac}}^{+}$ or ${\text{Lac}}^{-}$ (capable or not to utilize lactose).

Introduction:

An operon is a stretch of DNA which consists of a cluster of functionally related genes. It contains an operator, promoter, and related genes. It is present only in prokaryotes. It was first characterized in E. coli. Operon can be inducible or repressible.

Inducible operon: In the presence of a substrate, transcription of structural gene occurs, and in

the absence of a substrate, operator region is occupied by the repressor. So, RNA polymerase is unable to bind at the promoter region, and transcription cannot be initiated. In inducible operon

transcription in inhibited in the absence of a substrate.

Repressible operon: In a repressible operon, the substrate acts as a co-repressor which facilitates the conformational change in the repressor of the operon, and transcription is inhibited in the absence of a co-repressor. The conformation of the original repressor does not allow it to bind the operator region of the operon. Thus, the operator region is free, and RNA polymerase can bind at the promoter region, and the transcription of genes can be initiated.