This is a definition that our professor mentioned in class $-$the process of turning on the expression of gene in the response to a substance in the enviornment is called induction.


From wikipedia $-$ in genetics and biochemistry the removal of repression, such as of an operon so that gene transcription occurs or is enhanced, with the net result frequently being elevation of the level of a specific enzyme. This effect is called derepression.

In lac operon when Allolactose molecule is present in the medium (in the bacterial cell) it binds to the monomer of repressor and reduces the binding affinity of repressor to the operator site and the RNA polymerase is free to synthesis a polycistronic mRNA. Thus the allolactose molecule turns on the expression of lac operon by removing repression.

So I conclude that in lac operon induction is same as derepression. But does it hold true for all operons or there are operons in which induction is achieved without repressing the repressor?

  • $\begingroup$ what is the purpose of the question? Is there a case when you want to use one of the words or it is just curiosity? This will help with answering $\endgroup$ – aaaaaa Jul 30 '16 at 21:27
  • $\begingroup$ @aaaaaa I am both curious and wish to apply the words to my answers during exam. $\endgroup$ – Tyto alba Jul 31 '16 at 16:35
  • $\begingroup$ @SanjuktaGhosh Don't play with words in your exams. Always use the words that the examiner would understand :P $\endgroup$ – WYSIWYG Jul 31 '16 at 19:33
  • $\begingroup$ @WYSIWYG :) I usually write which is most appropriate without bothering about the examiner. $\endgroup$ – Tyto alba Aug 1 '16 at 5:06

I’d prepared this before I saw that @WYSIWYG had answered in the same vein. So the credit to him. But just to clarify:

Technically you are correct. Although having the same end-result, induction can occur by a direct mechanism, not involving de-repression, the example in the lac operon being the catabolite activator protein (CAP). However, it is quite normal to refer to a de-repressor as an inducer.

If we take a simplistic view and regard the key step in bacterial transcription as the binding of RNA polymerase to the promotor then the situations for de-repression and positive induction are:


The binding of RNA polymerase to the promotor is prevented by the repressor (e.g. the lac repressor) binding to the DNA (at the operator). The de-repression occurs when the so-called inducer (allolactose here) binds to the repressor, reducing its binding affinity for the operator and hence allowing RNA polymerase access to the promoter.

Positive induction

Here the affinity of the RNA polymerase for the promotor is sub-optimal and is enhanced by the inducer. In fact, this generally occurs by a protein–protein interaction — in the case of cAMP induction of transcription of the lac operon it is the binding of the cAMP to the CAP that causes it to interact with RNA polymerase and increase the binding affinity of the latter.

Another on-line resource on this topic is at LibreTexts, and this has diagrams I should perhaps have included here.

In eukaryotes direct induction would seem to be more common than de-repression, with inducers such as steroid hormones binding to specific receptor proteins which then bind to the DNA. However the details of the DNA interactions are much more complex.


Induction is an effect of a stimulus on the cell. During an induction of gene expression a gene is expressed in response to a stimulus.

De-repression is a mechanism of gene regulation which may lead to expression of the corresponding gene. The gene can be expressed via direct activation as well. The action of CAP on lac operon would be an example of direct activation. There are several other examples too.


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