exocytosis The process in which a membrane-enclosed vesicle Þrst fuses with the plasma membrane and then opens and releases its contents to the outside. exon The DNA bases that code for an amino acid sequence . Exons are separated by introns that code for no amino acid sequences exoskeleton A hard, jointed, external covering that encloses the muscles and organs of an organism; typical of many arthropods including insects. exothermic A reaction where the product is at a higher energy level than the reactants. exponential rate An extremely rapid increase, e.g., in the rate of population growth. expression In relation to genes, the phenotypic manifestation of a trait. Expression may be age-dependent (e.g., Huntington disease) or affected by environmental factors (e.g., dark fur on Siamese cats).
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