Question

The operator gene of the lac operon is ‘turned on’ when lactose molecules bind to:
A. mRNA
B. Promoter site
C. Repressor protein
D. Operator gene

Answer 1

Hint: DNA causes gene expressions corresponding to the type of gene present in our DNA. But these must be controlled in order to avoid overexploitation of the genes. Hence, for avoiding the overproduction of the gene products there is the presence of some regulators which regulate the gene expression.

Complete step by step answer: The gene expression can be regulated at several levels. While considering eukaryotes, it can be regulated at the transcriptional level during the formation of the primary transcript, during the process of splicing, during the transport of mRNA from the nucleus to the cytoplasm, or during the process of translation.

Further, in the case of prokaryotes, the gene expression is regulated by the control of the rate of transcriptional initiation. In prokaryotes, a polycistronic gene is regulated by a common promoter and the regulatory genes (which mean there is one common promoter that regulates the genes) and this arrangement is called an operon. Lactose acts as an inducer in lac operon (inducer is the one which is responsible for switching on and switching off of the operon). Lac operon is used by bacteria only when the glucose availability is low and the environment has abundant lactose. It then utilized the lactose to produce energy in absence of glucose. Also, the lac operon is under negative control, that is, it is normally turned off due to the presence of repressor protein in the operator region so that the transcription does not occur when glucose is present in the medium. When there is only lactose present in the medium, then some amount of lactose, usually called allolactose gets into the operon and binds to the repressor protein, and turns on the lac operon. It binds to the repressor protein so that it does not bind to the operator to switch off the operon.
Therefore the correct answer is option C- repressor protein.

Note: The word lac here in lac operon refers to lactose. The lac operon was elucidated by a geneticist, Francis Jacob and a biochemist, Jacque Monod. They were the first to elucidate a transcriptionally regulated system. Few more examples are trp operon, ara operon, his operon, val operon etc.