`star` Replication v/s Transcription
`star` Why both the strands are not copied during Transcription
`star` Transcription Unit


● The process of `color{Violet}"copying genetic information"` from one strand of the `color{Violet}"DNA into RNA"` is termed as `color{Violet}"transcription"`.

● Here also, the `color{Violet}"principle of complementarity"` governs the process of transcription, except the `color{Violet}"adenosine"` now forms base pair with `color{Violet}"uracil"` instead of thymine.

● However, unlike in the process of replication, which once set in, the `color{Violet}"total DNA"` of an organism gets duplicated, in transcription `color{Violet}"only a segment"` of DNA and `color{Violet}"only one of"` `color{Violet}"the strands"` is copied into RNA.

● This necessitates `color{Violet}"defining the boundaries"` that would `color{Violet}"demarcate"` the region and the strand of DNA that would be transcribed.


● First, if `color{Violet}"both strands"` `color{Violet}"act a template"`, they would code for `color{Violet}"RNA"` molecule with `color{Violet}"different sequences"` (Since complementarity does not mean identical).

● Then in turn, if they code for `color{Violet}"proteins"`, the `color{Violet}"sequence of amino acids"` in the proteins would be `color{Violet}"differen"`t.

● Hence, one segment of the DNA would be coding for `color{Violet}"two different proteins"`, and this would `color{Violet}"complicate"` the genetic information transfer machinery.

● Second, the `color{Violet}"two RNA"` molecules if produced `color{Violet}"simultaneously"` would be `color{Violet}"complementary"` to each other, hence would form a `color{Violet}"double stranded RNA"`.

● This would `color{Violet}"prevent RNA"` from being `color{Violet}"translated into protein"` and the exercise of transcription would become a futile one.


● 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.