● A `color{Violet}"transcription unit"` in DNA is defined primarily by the `color{Violet}"three regions"` in the DNA:
`star` `color{Violet}"A Promoter"`
`star` `color{Violet}"The Structural gene"`
`star` `color{Violet}"A Terminator"`
● There is a `color{Violet}"convention"` in `color{Violet}"defining the two strands"` of the DNA in the structural gene of a transcription unit.
● Since the two strands have `color{Violet}"opposite polarity"` and the DNA-dependent RNA polymerase also catalyse the polymerisation in only `color{Violet}"one direction"`, that is, 5'→3' , the strand that has the `color{Violet}"polarity (3'→5')"` acts as a template, and is also referred to as `color{Violet}"template strand"`.
● The other strand which has the `color{Violet}"polarity (5'→3')"` and the sequence same as `color{Violet}"RNA"` (except thymine at the place of uracil), is `color{Violet}"displaced"` during transcription.
● Strangely, this strand (which does not code for anything) is referred to as `color{Violet}"coding strand"`.
● All the `color{Violet}"reference point"` while defining a transcription unit is made with `color{Violet}"coding strand"`.
● To explain the point, a `color{Violet}"hypothetical sequence"` from a transcription unit is represented below:
`color{brown}"3'-ATGCAT"``color{brown}"GCATGCATG"``color{brown}"CATGCATGC-5'"` `color{violet}"Template Strand"`
`color{brown}"5'-TACGTAC"``color{brown}"GTACGTACG"``color{brown}"TACGTACG-3'"` `color{violet}"Coding Strand"`
● The `color{violet}"promoter"` and `color{violet}"terminator"` flank the `color{violet}"structural gene"` in a transcription unit.
● The `color{violet}"promoter"` is said to be located towards `color{violet}"5'-end"` (`color{violet}"upstream"`) of the structural gene (the reference is made with respect to the polarity of coding strand).
● It is a `color{violet}"DNA sequence"` that provides binding site for `color{violet}"RNA polymerase"`, and it is the presence of a `color{violet}"promoter"` in a transcription unit that also defines the template and coding strands.
● By `color{violet}"switching its position"` with `color{violet}"terminator"`, the definition of coding and template strands could be `color{violet}"reversed"`.
● The `color{violet}"terminator"` is located towards `color{violet}"3'-end"` (`color{violet}"downstream"`) of the coding strand and it usually defines the end of the process of transcription.
● There are additional `color{violet}"regulatory sequences"` that may be present `color{violet}"further upstream"` or `color{violet}"downstream"` to the promoter.
● A `color{Violet}"transcription unit"` in DNA is defined primarily by the `color{Violet}"three regions"` in the DNA:
`star` `color{Violet}"A Promoter"`
`star` `color{Violet}"The Structural gene"`
`star` `color{Violet}"A Terminator"`
● There is a `color{Violet}"convention"` in `color{Violet}"defining the two strands"` of the DNA in the structural gene of a transcription unit.
● Since the two strands have `color{Violet}"opposite polarity"` and the DNA-dependent RNA polymerase also catalyse the polymerisation in only `color{Violet}"one direction"`, that is, 5'→3' , the strand that has the `color{Violet}"polarity (3'→5')"` acts as a template, and is also referred to as `color{Violet}"template strand"`.
● The other strand which has the `color{Violet}"polarity (5'→3')"` and the sequence same as `color{Violet}"RNA"` (except thymine at the place of uracil), is `color{Violet}"displaced"` during transcription.
● Strangely, this strand (which does not code for anything) is referred to as `color{Violet}"coding strand"`.
● All the `color{Violet}"reference point"` while defining a transcription unit is made with `color{Violet}"coding strand"`.
● To explain the point, a `color{Violet}"hypothetical sequence"` from a transcription unit is represented below:
`color{brown}"3'-ATGCAT"``color{brown}"GCATGCATG"``color{brown}"CATGCATGC-5'"` `color{violet}"Template Strand"`
`color{brown}"5'-TACGTAC"``color{brown}"GTACGTACG"``color{brown}"TACGTACG-3'"` `color{violet}"Coding Strand"`
● The `color{violet}"promoter"` and `color{violet}"terminator"` flank the `color{violet}"structural gene"` in a transcription unit.
● The `color{violet}"promoter"` is said to be located towards `color{violet}"5'-end"` (`color{violet}"upstream"`) of the structural gene (the reference is made with respect to the polarity of coding strand).
● It is a `color{violet}"DNA sequence"` that provides binding site for `color{violet}"RNA polymerase"`, and it is the presence of a `color{violet}"promoter"` in a transcription unit that also defines the template and coding strands.
● By `color{violet}"switching its position"` with `color{violet}"terminator"`, the definition of coding and template strands could be `color{violet}"reversed"`.
● The `color{violet}"terminator"` is located towards `color{violet}"3'-end"` (`color{violet}"downstream"`) of the coding strand and it usually defines the end of the process of transcription.
● There are additional `color{violet}"regulatory sequences"` that may be present `color{violet}"further upstream"` or `color{violet}"downstream"` to the promoter.