Cistron is a segment of a DNA coding for a polypeptide chain. The structural gene in a transcription unit can be said to be either monicistronic or polycistronic. Monicistronic structural gene is found in eukaryotes and polycistronic structure is mostly found in bacteria. In Exon, the coding sequences are mature or processed mRNA. In other words, Cistron is a part of DNA which codes for a protein and exon is the coding region of DNA. Exon can have  many cistrons within it.