Explain how gene expression is more complicated than it looks.

A cell typically expresses only a fraction of its genes, and the different types of cells in multicellular organisms arise because different sets of genes are expressed. Moreover, cells can change the pattern of genes they express in response to changes in their environment, such as signals from other cells. Genes can't control an organism on their own; rather, they must interact with and respond to the organism's environment. Some genes are constitutive, or always "on," regardless of environmental conditions. Such genes are among the most important elements of a cell's genome, and they control the ability of DNA to replicate, express itself, and repair itself. These genes also control protein synthesis and much of an organism's central metabolism.