Control of Gene Expression

DNA carries all the information required for the survival and propagation of life. However, not all information is required at all times! Therefore, the question is, how do we regulate what information is to be processed at what time? For example, when I have consumed sugar I need an enzyme to break down these sugars. However, this enzyme is not present at all times; if it were, my reserve of this enzyme will become depleted.

This points to the need for control of gene expression or gene regulation. Gene regulation involves several degrees of control. Let us start by answering the question: why do organisms require gene regulation? In the case of eukaryotes, gene regulation plays the crucial role in helping differentiation of the cells. Eukaryotes are multicellular organisms and therefore require specialized cells in the different parts of the organism. In the case of prokaryotes, gene regulation allows the organism to change quickly to its surrounding conditions i.e presence or absence of a nutrient or a harmful agent.

Here are three mechanisms of gene regulation:

1. Epigenetic Regulation

2. Transcriptional Regulation

3. Post transcriptional regulation

Epigenetic regulation involves chromatin remodeling. This is a process in which a stretch of DNA sequence is made unavailable for the protein synthesis machinery to act upon, and therefore the corresponding gene is not expressed. This mode of gene regulation has been shown to be prevalent in cancer, where a loss of function of a gene due to epigenetic regulation may trigger cancer development. Histones are responsible for chromatin modifications and they are controlled in turn by DNA methylation.

Transcriptional regulation is very potent method of controlling gene expression by influencing the availability of RNA for protein synthesis. Transcription of DNA by RNA polymerase can be regulated in these ways:

a) Since transcription is a complex process it involves several co-factors called transcription factors whose availability can be regulated, which in turn controls the transcription reaction.

b) There are special repressor molecules available that bind the promoter region of a gene, and therefore make it impossible for RNA polymerase to bind. A classical model of this would be the lac operon.