REGULATION OF GENE EXPRESSION

In prokaryotes, the synthesis of some proteins is regulated so that they are made only when required. Proteins that are made only in the presence of a particular compound are called inducible proteins and the compound is known as inducer. Proteins that are made at a constant rate regardless of conditions are constitutive proteins. A principal means of genetic regulation in prokaryotes is the operon system. An operon contains not only a linear sequence of genes coding for a group of functionally related protein but also has adjacent DNA sequences known as promoter and operator. Transcription from the operon is controlled by the promoter the binding site for RNA polymerase,and the operator, the binding site for  the repressor. The lac operon is an example of an inducible operon. The regulatory gene codes for and the operator regulatory protein, such as repressor.   When the repressor is attached to the DNA molecule at the operator, RNA polymerase cannot initiate the transcription of RNA. When the repressor is inactivated, RNA polymerase can attach to the DNA, permitting transcription and protein synthesis to take place. The tryptophan operon is a repressible operon, in which the presence of a co-repressor causes the repressor to bind to the operator and stop transcription.

 Eukaryotic gene expression can be regulated before or during pre-mRNA processing and during or after translation. The regulator of DNA sequences include regulator sequences, which bind regulator proteins and activate transcription, enhancer sequences, which stimulate transcription, and silencer sequences, which bind repressor proteins and turn off transcription.

 One of the most important ways of post-transcriptional regulation is alternative RNA splicing, which allows more than one protein to be made from a gene. The stability of mRNA in the cytoplasm can also be regulated. Micro RNAs and RNA editing are two recently discovered mechanism of regulation.

Homeotic genes –these are master control genes; they coordinate the action of number of genes, which, in turn, determine the development of a large region or body segment. It was first discovered in fruitflies in 1983, these genes are conserved (are the same) in organisms that are very different from one another, such as yeast, mice and human beings. Scientists have found the genes that are functionally equivalent to the Drosophila eyeless gene in mice.