Portion of $DNA$ having information for an entire polypeptide or trait is called cistron. However by defining a cistron as a segment of $DNA$ coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes). In eukaryotes, the monocistronic structural genes have interrupted coding sequences-the genes in eukaryotes are split.

The coding sequences or expressed sequences are defined as exons. Exons are said to be those sequence that appear in mature or processed $RNA$. The exons are interrupted by introns. Introns or intervening sequences do not appear in mature or processed $RNA.$