Information transfer in biological systems: targeting of proteins to specific organelles or to the extracellular...

Orderliness is the salient characteristic of living systems. Cells are intolerant of disorder. They express this by rapidly eliminating or degrading out-of-place molecules. When cells are broken apart and their constituent organelles separated and analysed, the same types of macromolecules are always associated with the same subcellular structures. One finds, for example, the same proteins in mitochondria time after time, and these differ from the sets of proteins found in nuclei, secretory granules, or plasma membranes. The information necessary to target each protein to its appropriate intracellular destination is determined primarily by the gene for that protein. Encoded within the DNA structure of genes are signals that specify where each protein molecule belongs. Thus, it is the transfer of information from one macromolecule to another that maintains the integrity and orderliness of living cells.