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Entities within operons whose expression is controlled by the operon regulatory sequences.
Structural genes contrast specifically with regulatory genes. That is, structural genes give rise to enzymes as well as various structural components of cells (e.g., cytoskeleton members), but not included among structural genes are those genes that give rise specifically to factors, RNA or protein, that solely serve to regulate the expression of other genes.
The distinction between structural and regulatory genes, though often presented as profound, really is based historically on the original development of the operon model. In particular, there were genes that were expressed when the operon was expressed (structural genes) and genes that controlled the expression of these structural genes (regulatory genes). Since the primary goal was one of determining how operons function, great weight was put on determining the role of the regulatory genes, hence their being placed in a separate category from the structural genes. In either case, though, genes give rise to gene products and gene products have functions within cells, whether we describe those genes as structural or instead as regulatory.
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