How is gene expression regulated in eukaryotes?

(N/A) Gene expression results in the formation of a polypeptide.
In eukaryotes,regulation can be exerted at various 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.
Genes in a cell are expressed to perform a specific function or a set of functions.
For example,the enzyme $\beta$-galactosidase in $E. coli$ hydrolyzes lactose into galactose and glucose. However,if lactose is absent in the medium,the synthesis of $\beta$-galactosidase is not required.
Thus,metabolic,physiological,or environmental conditions regulate gene expression.