Concept 35 DNA responds to signals from outside the cell.

Growth and development require that cells communicate with each other and react to signals that come from other parts of the body. Notably, hormones released by various glands travel throughout the body to stimulate the growth of certain cell types. Cells capable of being stimulated by a particular hormone possess a specific receptor anchored in the cell membrane. The binding of a hormone to its receptor initiates a series of molecular transformations, called signal transduction, that relay the growth signal through the cell.

First, the receptor transduces the signal through the cell membrane to the internal membrane surface, where it activates protein "messengers." These messengers are part of and initiate a cascade of chemical reactions, often involving the addition of phosphate groups. This is the signal that passes through the cytoplasm and into the nucleus. In the final step of signal transduction, DNA binding proteins attach to regulatory sequences and start DNA replication or transcription.