Definition of gene expression
Gene expression is the process by which the information encoded in the genotype is used to create a functional gene product (i.e. a protein or RNA) from genes encoded within DNA.
Control of gene expression , or gene regulation is essential for all living organisms. Even single-celled bacteria need to control when and where their genes are expressed.
In multicellular animals and plants, gene expression is even more complicated.
Plants and animals need to be able to control gene expression in a single cell. They also need to be able to control gene expression across a whole host of cell types.