Definition of Gene Expression
Gene expression is a process through which the information that a gene is carrying is used for the synthesis of a gene product that is functional. The information in the gene is first transcribed and mRNA is formed and then this mRNA is used to form the proteins which affect the phenotypes (the traits which are observed) of the organisms.
This is known as central dogma. Gene expression is a highly regulated process as it turns on and off the machinery in order to control the amount of protein made. All organisms i.e. prokaryotes and eukaryotes use this process of gene expression.