Question

Draw the following genes in the absence of any small effector molecule. Include promoter, coding regions, terminator site and any other control sites and control proteins. Constitutive monogenic Positive control - three proteins Inducible - three proteins Repressible - three proteins Catabolite repression - three proteins

Answer 1

For example we take here lac operon genre

Which contain

Lac I gene for synthesis of repressor.

Lac P promoter region.

Lac O operator region.

Lac Z , lacY, lacA for the synthesis of enzymes.. Beta galactocydase, permease etc.

Repressible -

When lac I gene active it synthesises repressors which bind on operator overlapping with some part of promoter region in the absence of lactose, inhibits RNA polymerase binding on promoter region hence transcription. This is known as Repressible.

Inducible -

In the presence of lactose

Lactose bind with repressors and repressors no more able to bind on operator.

So Promoter region is free for RNA polymerase binding and for transcription. This is known as inducible.

Constitutive -

If operator region of lac operon deleted then there are no more repression by repressors because repressors are not able to bind hence inhibiting the transcription.

Means operon are constitutive.

Positive control-

If any molecule Inhances the expression of gene called as positive regulators.

As in lac operon CAP-CRP dimer are positive regulators which bind on upstream to the promoter region and anhances the synthesis of beta galactocydase, lac operon.

Catabolite repression-

When catabolic products inhibits the expression of gene is called catabolite repression.

Lac operon is a catabolic operon which breaks lactose into glucose and galactose.

Hence in the presence of glucose lac operon expression decreases.