an organic molecule that acts within a cell to initiate the response to a signal carried by a chemical messenger (e.g. a hormone) that does not itself enter the cell. Examples of second messengers are *inositol triphosphate and cyclic *AMP.
a type of RNA that carries the information of the *genetic code of the DNA from the cell nucleus to the ribosomes, where the code is translated into protein. See transcription; translation.... messenger rna
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