Write a short note on the transcription unit.

(N/A) transcription unit in $DNA$ is defined primarily by three regions in the $DNA$: $(i)$ $A$ promoter,$(ii)$ The structural gene,and $(iii)$ $A$ terminator.
There is a convention in defining the two strands of the $DNA$ in the structural gene of a transcription unit.
Since the two strands have opposite polarity and the $DNA$-dependent $RNA$ polymerase also catalyzes the polymerization in only one direction,that is $5' \rightarrow 3'$,the strand that has the polarity $3' \rightarrow 5'$ acts as a template and is referred to as the template strand.
The other strand,which has the polarity $5' \rightarrow 3'$ and the same sequence as $RNA$ (except thymine in place of uracil),is displaced during transcription. Strangely,this strand (which does not code for anything) is referred to as the coding strand.
All reference points while defining a transcription unit are made with respect to the coding strand.
$(i)$ $A$ promoter is a $DNA$ sequence that provides a binding site for $RNA$ polymerase. It is located at the $5'$ end (upstream) of the structural gene. The presence of the promoter in a transcription unit defines the template and coding strands. By switching its position with the terminator,the definition of coding and template strands could be reversed.
$(ii)$ The structural gene in a transcription unit is the segment of $DNA$ that is flanked by the promoter and the terminator.
$(iii)$ $A$ terminator is located towards the $3'$ end (downstream) of the coding strand. It usually defines the end of the process of transcription. There are additional regulatory sequences that may be present further upstream or downstream to the promoter.