Define a cistron. Giving examples,differentiate between monocistronic and polycistronic transcription units.

(N/A) cistron is defined as a segment of $DNA$ that contains the genetic information coding for a single polypeptide chain or a functional trait.
$1.$ Monocistronic transcription unit: These contain a single structural gene that codes for one polypeptide. This is characteristic of eukaryotes. Example: The structural gene for human insulin or hemoglobin.
$2.$ Polycistronic transcription unit: These contain multiple structural genes under the control of a single promoter and terminator,coding for several polypeptides. This is characteristic of prokaryotes (bacteria). Example: The $lac$ operon in $E. coli$.
In eukaryotes,monocistronic genes are often 'split' or 'interrupted' by non-coding sequences called introns. The coding sequences that appear in the mature $RNA$ are called exons.