Question

Explain the regulation of lac operon in absence and presence of lactose as an inducer.

Answer 1

An operon is defined as a functioning unit of the DNA. It consists of a cluster of genes which are controlled by a single molecule which is known as the promoter. Operons are found in a variety of organisms such as prokaryotes, viruses and also in the eukaryotic organism. The lac operon is found in the bacterium, E. coli.

Complete answer:
An operon consists of some important types of components which are the promoter, operator and structural genes. The promoter is a nucleotide sequence that helps a particular gene to be transcribed. The enzyme RNA polymerase recognizes the promoter and transcription is started. An operator is a segment of DNA which binds to a repressor. A repressor is a DNA or RNA binding protein that inhibits the expression of one or few genes and causes the transcription to stop. In order to start the process of transcription, a small molecule is introduced into the system which is known as the inducer. The inducer displaces the repressor molecule from the operator site and results in the operon to start transcription.

Operons can be regulated by the action of the inducers and the repressors and this enables the organism to regulate the expression of various genes. In the negative control, the repressor binds with the operator site to stop the transcription. When an inducer is added, it causes to change the structural configuration of the repressor and it cannot further bind with the operator. This type of negative control is found in lac operon.

The lac operon is an operon required for the metabolism of lactose. It consists of three structural genes which are lacZ, lacY and lac A which codes for enzymes β-galactosidase, beta-galactoside permease and galactoside acetyltransferase. When no lactose is available, the lac repressor stops the production of enzymes and when lactose is available the transcription process begins. The absence of lactose causes the repressor to bind with the operator and transcription is stopped.

However, the regulator gene for lac operon produces a protein that blocks RNA polymerase to bind to the operator. This binding of the regulator protein is prevented only when allolactose binds to it and causes inactivation of the repressor. Allolactose is formed from lactose and it acts as an inducer. It brings about conformational changes and decreases affinity of the repressor for the operator site and transcription can proceed.

Note: In a lac operon the important components are repressor, operator, promoter site and three structural genes that produce important enzymes are produced that are needed for transcription. The repressor binds with the operator site and stops the production of enzymes which halts the transcription process. But when lactose is present, it binds with the repressor and changes its structure and prevents the repressor to bind with the operator. The transcription process can occur as well as enzymes are also produced. Lactose acts as a negative inducer in the lac operon.