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The orderly patterns of metabolic and developmental reactions giving rise to the unique characteristics of the individual and of its species. Biological specificity is most pronounced and best understood at the cellular and molecular levels of organization, where the shapes of individual molecules allow them to selectively recognize and bind to one another. The main principle which guides this recognition is termed complementarity. Just as a hand fits perfectly into a glove, molecules which are complementary have mirror-image shapes that allow them to selectively bind to each other.
This ability of complementary molecules to specifically bind to one another plays many essential roles in living systems. For example, the transmission of specific hereditary traits from parent to offspring depends upon the ability of the individual strands of a deoxyribonucleic acid (DNA) molecule to specifically generate two new strands with complementary sequences. Similarly, metabolism, which provides organisms with both the energy and chemical building blocks needed for survival, is made possible by the ability of enzymes to specifically bind to the substrates whose interconversions they catalyze. During embryonic development, individual cells associate with each other in precise patterns to form tissues, organs, and organ systems. These ordered interactions are ultimately dependent upon the ability of individual cells to recognize and specifically bind to other cells of a similar type. See Deoxyribonucleic acid (DNA), Enzyme, Metabolism
In addition to binding to one another, cells can interact by releasing hormones into the bloodstream. Though all of an organism's cells are exposed to hormones circulating in the bloodstream, only a small number of target cells respond to any particular hormone. This selectivity occurs because the specific receptor molecules to which hormones bind are restricted to certain cell types. Thus each hormone exerts its effects on a few selected cell types because only those cells contain the proper receptor. Specific receptors are also involved in interactions between neurotransmitters and the cells they stimulate or inhibit, between certain types of drugs and the cells they affect, and between viruses and the cells they infect. This last phenomenon has an important influence on the susceptibility of individuals to virally transmitted diseases. See Endocrine mechanisms, Hormone
Although most examples of biological specificity are based upon interactions occurring at the molecular level, such phenomena affect many properties manifested at the level of the whole organism. The ability of individuals to defend against infectious diseases, for example, requires the production of antibody molecules which specifically bind to bacteria and viruses. The fertilization of an egg by a sperm is facilitated by specific recognition between molecules present on the surfaces of the sperm and egg cells. Even communication between organisms can be mediated by specific chemical signals, called pheromones. Such chemical signals are utilized in trail marking by ants and bees, in territory marking by certain mammals, and as sexual attractants. Specific molecular interactions thus exert influences ranging from the replication of genes to the behavior of organisms. See Immunology, Molecular biology, Pheromone