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In both single celled and multicellular organisms, communication is vital to development, and survival. Cellular pathways convert signals from the exterior of the cell (for example the presence of a metabolite or hormone) or from inside the cell (for example the completion of DNA replication) into a response, such as enzyme activation or nuclear transcription. Communication between cells can also be achieved by direct contact between molecules exhibited on the cell membrane. Both mechanisms require that molecules specifically recognise their proper partners at the right time and in the right place (see Section 1.3).