All cells express a certain set of genes, often called ‘housekeeping’ genes. These genes encode proteins that are essential for every type of cell. For instance, proteins that make up the structural cytoskeleton (the cell’s ‘skeleton’) and proteins that are involved in replicating, or copying, DNA before a cell divides are expressed in every cell of an organism.
Each specialized type of cell also expresses a tissue-specific set of genes, which are unique to that particular tissue or organ. Kidney cells express a set of kidney-specific genes, while bone cells express a set of bone-specific genes. There may be some overlap—some of the kidney-specific genes may be the same as some of the bone-specific genes—but the combination of genes will be specific for each tissue. The cells become specialized for a particular function during the development of the organism—plant, fungus, or human. A totipotent embryonic stem cell, which has the potential to become any type of specialized cell, will begin to differentiate, or specialize, on the basis of cues it receives from its environment—cells right next to it, cells nearby, or cells far away. These cues, together with when the different signals are received (the timing), instruct the different cells of the embryo to become the different types of specialized cells. The differentiation is made permanent when the tissue-specific genes are turned on and the cell has committed to a lineage.