Write an explanatory note on the transcription unit and the gene.

(N/A) gene is defined as the functional unit of inheritance. Although there is no ambiguity that genes are located on the $DNA$,it is difficult to literally define a gene in terms of a $DNA$ sequence.
The $DNA$ sequence coding for a $tRNA$ or $rRNA$ molecule also defines a gene. However,by defining a $cistron$ as a segment of $DNA$ coding for a polypeptide,the structural gene in a transcription unit can be described as $monocistronic$ (mostly in eukaryotes) or $polycistronic$ (mostly in bacteria or prokaryotes).
In eukaryotes,$monocistronic$ structural genes have interrupted coding sequences; the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as $exons$. $Exons$ are sequences that appear in mature or processed $RNA$. The $exons$ are interrupted by $introns$. $Introns$ or intervening sequences do not appear in mature or processed $RNA$. The split-gene arrangement further complicates the definition of a gene in terms of a $DNA$ segment.
Inheritance of a character is also affected by promoter and regulatory sequences of a structural gene. Hence,sometimes the regulatory sequences are loosely defined as regulatory genes,even though these sequences do not code for any $RNA$ or protein.