Define a cistron. Giving examples differentiate between monocistronic and polycistronic transcription unit.
A segment of DNA coding for a polypeptide is called cistron. A cistron is basically a gene. If a stretch of replicating DNA contains a single cistron (or gene), it is called monocistronic. e.g. eukaryotes. If a stretch of replicating DNA contains more than one cistron, it is called polycistronic, e.g. bacteria and prokaryotes.