They control it with regulatory sites found on each gene, with specific regulatory proteins, and by determining when individual genes are to be transcribed.
A variety of genes are involved in the control of cell growth and division. The cell cycle is the cell's way of replicating itself in an organized, step-by-step fashion. Tight regulation of this...
No cell must express all genes all the time. Out of all the genes in the entire genome, only a specific subset is required for a specific cell type. To make sure that a type of cell effectively...
... so that the cell is regulated. If all the genes were swithced on at one time the cell would go haywire and produce excess amounts of proteins not necessarily needed, and possible causing problems...