Write an explanatory note of gene expression.
 

Gene expression results in the formation of a polypeptide.

Gene regulation is the mechanism of switching 'off' and switching 'on' of the genes depending upon the requirement of the cells and the stage of development.

The regulation of gene expression may occur at various levels.

In eukaryotes, it takes place at the following levels :

$(i)$ Transcriptional level : A primary transcript is formed.

$(ii)$ Processing level : Regulation of splicing.

$(iii)$ Transport of $mRNA$ : From nucleus to the cytoplasm.

$(iv)$ Translational level : In prokaryotes, gene expression is regulated by controlling the rate of inition of transcription.

Genes in a cell are expressed to perform a particular function or a set of functions.

The metabolic physiological or environmental conditions regulates the expression of genes.

The development and differentiation of embryo into adult organisms are also a result of the coordinated regulation of expression of several sets of genes.

In prokaryotes, control of the transcriptional rate of initiation is the predominant site for control of gene expression.

In a transcription unit the activity of $RNA$ polymerase at a given promoter is in turn regulated by the interaction with accessory proteins which affect its ability to recognise start sites.

The accessibility of promoter regions of prokaryotic $DNA$ in many cases is regulated by the interaction of proteins with sequences termed as operators.

The operator region is adjacent to the promoter elements in most operons and in most cases the sequences of the operator bind a repressor protein.

Each operon has its specific operator and specific repressor eg. lac operator is present only in the lac operon and it interacts with lac repressor only.