Write an explanatory note on gene expression.

(N/A) Gene expression is the process by which the information encoded in a gene is used to synthesize a functional gene product,typically a polypeptide.
Gene regulation is the mechanism of switching 'off' and 'on' genes depending upon the requirements 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: Formation of primary transcript.
$(ii)$ Processing level: Regulation of splicing.
$(iii)$ Transport of $mRNA$: From the nucleus to the cytoplasm.
$(iv)$ Translational level: Protein synthesis.
In prokaryotes,gene expression is primarily regulated by controlling the rate of initiation of transcription.
Genes in a cell are expressed to perform a particular function or a set of functions.
The metabolic,physiological,or environmental conditions regulate the expression of genes.
The development and differentiation of an embryo into an adult organism are also a result of the coordinated regulation of the expression of several sets of genes.
In a transcription unit,the activity of $RNA$ polymerase at a given promoter is regulated by the interaction with accessory proteins,which affect its ability to recognize start sites.
The accessibility of promoter regions of prokaryotic $DNA$ is often 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; for example,the $lac$ operator is present only in the $lac$ operon and interacts only with the $lac$ repressor.