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A structural gene is a gene that codes for any RNA or protein product other than a regulatory factor (i.e. regulatory protein). It may code for a structural protein, an enzyme, or an RNA molecule not involved in regulation. Structural genes represent an enormous variety of protein structures and functions, including structural proteins, enzymes with catalytic activities and so on. These genes are needed for the morphological or functional traits of the cell. In eukaryotes, these occur in spilt-form, segmented into introns and exons. But in prokaryotes, these are continuous. The structural genes are concerned with the synthesis of a polypeptide chain or a number of polypeptide chains and are also concerned with the synthesis of different types of RNA required during protein synthesis. Structural genes are different from regulatory or promoter genes, as they serve the significant purpose of actually transcribing for proteins.
In the Lac-Operon concept, these genes are associated with the synthesis of those enzymes that are needed for the catabolism of Lactose. In lac-operon, there are three structural genes:
a. Lac Z gene for beta-galactosidase enzyme.
b. Lac Y gene for permease enzyme.
c. Lac A gene for acetylase enzyme.
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