Regulation of gene expression refers to a very broad term that may occur at various levels. Considering that gene expression results in the formation of a polypeptide, it can be regulated at several levels.
In eukaryotes, the regulation of gene expression could be exerted at four levels.
i) Transcriptional level: Formation of the primary transcript.
ii) Processing level: Formation of primary transcript
iii) Transport of mRNA from nucleus to the cytoplasm.
iv) Translational level.
The genes in a cell are expressed to perform a particular function or a set of functions. In eukaryotes, functionally related genes do not represent an operon but are present on different sites, chromosomes. Here structural gene is called split gene which is a mosaic of exons and introns, i.e., the base triplet – amino acid matching is not continous. The entire split gene is transcribed to form a continuous strip of mRNA. The removal of non – coding intronic part and fusion of exonic coding parts of RNA is called RNA splicing. About 50-90% of primary transcribed RNA is discarded during processing. The development and differentiation of embryo into adult organisms are also a result of the coordinated regulation of expression of several sets of genes.