Transcription: Central Dogma | Additional Study Material for NEET PDF Download

Transcription is the process of copying genetic information from one strand of the i DNJS. into RNA. The principle of complementarity governs the process of transcription, except the adenosine now forms base pair with uracil instead of l thymine.

(i) In transcription, only a segment of DNA is duplicated and on Iv one of the strands is . copied into RNA. Both the strands are not copied because

• If both the strands code for RNA, two different RNA molecules and two different proteins would be formed, hence complicating the genetic information transfer machinery.
• Since two RNA produced would be complementary to each other, they would form a double-stranded RNA without translation, making the process of transcription futile.

(ii) A transcription unit in DNA is defined by three regions in the DNA which are as follows:
(a) A promoter (b) The structural gene (c) A terminator

(iii) The two strands of DNA have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerization in only one direction that is 5′ -» 3′.

(iv) The strand that has the polarity 3′-» 5′ acts as a template and is referred to as template strand. The other strand which has the polarity (5′ -> 3′) and the sequence same as RNA (T at the place of U) is displaced during transcription. This strand is called as coding strand.

(v) The promoter and terminator flank the structural gene in a transcription unit.

(vi) The promoter is located towards 5′ end (upstream) of the structural gene.

(vii) It is the DNA sequence that provides binding site for RNA polymerase and the presence of promoter defines the template and coding strands. By switching its position with terminator, the definition of coding and template strands could be reversed.

(viii) The terminator is located towards 3f-end (downstream) of the coding strand and it usually defines the end of the process of transcription.

(ix) There are additional regulatory sequences that may be present further upstream or downstream to the promoter.

Transcription Unit and the Gene
(i) A gene can be defined as the functional unit of inheritance.
(ii) A cistron is a segment of DNA coding for a polypeptide.
(iii) The structural gene in a transcription unit could be said as monodstronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes).
(iv) The coding sequences or expressed sequences are defined as exons. Exons appear in mature or processed RNA. The exons are interrupted by introns.

(v) Introns or intervening sequences do not appear in mature or processed RNA.
(vi) Sometimes, the regulatory sequences are loosely defined as regulate 5: even though these sequences do not code for any RNA or protein.

Transcription in prokaryotes occur in the following steps:
(i) A single DNA-dependent RNA polymerase catalyses the transcription of all types of RNA in bacteria.
(ii) RNA polymerase binds to promoter and initiates transcription (initiation).
(iii) It uses nucleoside triphosphates as substrate and polymerises in a template depended fashion following the rule of complementarity. It also facilitates opening of the helix and continues elongation.
(v) Once the polymerase reaches the terminator region, the nascent RNA falls off, so also the RNA polymerase. This results in termination of transcription.
(vi) RNA polymerase is only capable of catalysing the process of elongation. It associates transiently with initiation-factor (a) and terminator factor (b), to initiate and terminate the transcription, respectively. Thus, catalysing all the three steps.
(vii) Since, the mRNA does not require any processing to become active and also since transcription and translation take place in the same compartment, many times the translation can begin much before the mRNA is fully transcribed. As a result, transcription and translation can be coupled in bacteria.

Transcription in eukaryotes have additional complexities than prokaryotes.
(i) There are at least three RNA polymerases in the nucleus other than the RNA polymerase in organelles. The RNA polymerase I transcribes rRNAs (28S, 18S and 5.8S). RNA polymerase III is responsible for transcription of fRNA, 5srRNA and SnRNAs (small nuclear RNAs). RNA polymerase II transcribes precursor of mRNA, the heterogenous nuclear RNA (/mRNA).

(ii) Another complexity is that, the primary transcripts contain both the exons and the introns and are non-functional. Hence, subject to a process called splicing. In this process, introns are removed and exons are joined in a definite order.

(iii) mRNA undergoes additional processing called as capping and tailing. In capping, an unusual nucleotide is added to the 5′-end of /mRNA. In tailing, adenylate residues
(200-300) are added at 3′-end in a template. It is the fully processed /mRNA, now called mRNA, that is transported out of the nucleus for translation process.

Significance of these complexities are:
(i) The split gene arrangements represent an ancient feature of genome.
(ii) The presence of introns is reminescent of antiquity.
(iii) The process of splicing represents the dominance of RNA world.