Need for 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.
Mechanisms of Gene Regulation
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.
Mechanisms of Control of Gene Expression Contd.
c) Also available are a special set of molecules called activators whose job is to enhance the binding of RNA polymerase with DNA sequence, thereby increasing the mRNA copies of a gene.
d) There are certain proteins which can influence the conformation of the double helix called enhancers. These enhancers can make a promoter available for transcription by making it accessible by their conformational change as well as prevent them from being transcribed.
Transcriptional control of gene expression is probably the most employed and effective mode of gene regulation. It is one of the favoured modes of regulating genes during development because different sets of genes are expressed at different times of an organism’s life span. However, the biological system does come with a few more finetunable regulatory capabilities.
Post Transcription Control of Gene Expression
Post transcriptional regulation involves the control of gene expression after the mRNA or the pre-mRNA strand has been formed. There are three points of regulation here
1. Capping is a process that protects the newly synthesized mRNA from ribonucleases that destroy them. Hence if a protein is not needed the mRNA can be degraded at this point.
2. Poly A tailing involves the tagging of the mRNA strand with adenine residues, which prep the strand for further downstream processing.
3. Splicing is a very important phenomena where the RNA strand is edited or chopped into the require configuration for that specific protein (as the introns in the DNA sequence are also carried along in the mRNA or rather the pre-mRNA). At this point the splicing reaction actually controls what protein is formed since it is known that the pre-mRNA strand can be spliced is different combinations to create different proteins altogether.
Control of Gene Expression is Essential for Survival
Gene regulation can be described as a process that allows the organism or cell to adapt and quickly respond to its surroundings. It may react to external stimuli, metabolizing specific substances or as the most significant cellular differentiation. Although the DNA may carry the information for a majority of a cell’s or organism’s requirements, the various gene regulatory processes determine how the message is interpreted and acted upon.