Biology TRANSCRIPTION

### KEY TOPICS

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

### REPLICATION v/s TRANSCRIPTION

● 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.

### WHY BOTH THE STRANDS ARE NOT COPIED DURING TRANSCRIPTION?

● 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.

### TRANSCRIPTION UNIT

● 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.