Lac Operon - Practice Questions & MCQ

Operon:

• An operon is a unit of bacterial gene expression and regulation, including structural genes and control elements in-DNA recognized by regulator gene product(s).
• In an operon, a polycistronic structural gene is regulated by a common promoter and regulatory genes. 
• The elucidation of the lac operon was also a result of a close association between a geneticist, Francois Jacob and a biochemist, Jacque Monod. 
• They were the first to elucidate a transcriptionally regulated system. 

Lac operon:

• In the operon model of gene regulation, four types of genes are involved:
  • one regulatory gene or i gene (the i gene – here the term i does not refer to inducer, rather it is derived from the word inhibitor. It encodes a protein called the lac repressor.
  • three structural genes (z, y, and a)
  • one operator gene: The main function of operator gene is to control the function of structural genes.
  • one promoter gene: In the region, RNA polymerase binds and prepares to initiate transcription. The main function of promoter gene is to initiate mRNA transcription.
• In addition to these genes, repressor, co-repressor, and inducer molecules are also involved.
• The following enzymes are involved in the metabolism of lactose encoded by the lac operon:
  • The beta-galactosidase encoded by lac z gene catalyzes the breakdown of lactose into glucose and galactose.
  • The galactosidase permease encoded by lac y gene permits entry of lactose from the medium into the bacterial cell.
  • The enzyme transacetylase encoded by lac a gene transfers an acetyl group from acetyl coenzyme A to beta-galactosidase.

Functioning of Lac operon when lactose is absent: 

• The lac I gene is transcribed [constitutively i.e. continuously] and the mRNA is translated, producing the lac repressor. 
• The lac repressor has two functional domains or regions: one that binds to the DNA of the operator region, and one that binds to lactose.
• The repressor binds to the operator and blocks RNA polymerase.
• When RNA polymerase is blocked, there is no transcription.
• Thus the enzymes for lactose metabolism are not synthesized, because there is no lactose to metabolize. 
• Thus when lactose is absent, lactose-metabolizing enzymes are not produced.
• Regulation of lac operon by a repressor is referred to as negative regulation. 

Functioning of Lac operon when lactose is present:

• When lactose is present in the environment, the events occur in a different way. 
• A small amount of the lactose enters into the cell and affects the regulation of the operon. 
• Lactose binds to the repressor and the repressor undergoes a conformational change (change of shape).
• With this change, the lac repressor is unable to bind to the operator region. 
• Hence RNA polymerase is not blocked and is able to transcribe the genes of the operon.
• The enzymes encoded by those genes are produced. 
• The lac permease transports more lactose into the cell and beta-galactosidase cleaves the lactose into glucose and galactose. 
• This can be further metabolized by other enzymes, producing energy for the cell.
• Lactose, therefore, is able to induce the synthesis of the enzymes necessary for its metabolism (by preventing the action of the repressor). 
• As such, lactose is the inducer of the lac operon.