· Transcription is also governed by the complementarity of bases as in DNA. (Except that uracil in place of thymine is complementary to adenine).
· Only one of the strands of the DNA acts as the template for RNA synthesis for the following reasons:
(I) If both the strands code for RNA, two different (complementary) RNA molecules and two different proteins would be formed; hence the genetic information-transfer machinery would become complicated.
(II) Since the two RNA molecules produced would be complementary to each other, they would wind together to form a double-stranded RNA without carrying out translation; that means the process of transcription would become futile.
· A transcription unit in DNA has three regions:
(I) A Promoter
(II) Structural and
(III) A terminator
Schematic structure of transcriptional unit
· The process is catalysed by DNA-dependent RNA-polymerase, which catalyses the polymerisation of nucleotides only in 5’-3’ direction.
· The DNA strand with 3’-5’ polarity is called ‘template strand’, while the other strand with 5’-3’ polarity is called ‘coding strand’.
· The coding strand is displaced and does not code for RNA, but reference points regarding transcription are made in relation to it.
· The promoter refers to a particular sequence of DNA located towards the 5’ end (upstream) of the coding strand, where the RNA polymerase becomes bound for transcription.
· The terminator is a sequence of DNA located towards the 3’ end (downstream) of the coding strand, where the process of transcription would stop.
· There are additional regulatory sequences that may be present upstream or downstream to the promoter.
(A) Transcription in prokaryotes
¾ In prokaryotes, the structural genes are polycistronic and continuous.
¾ In prokaryotes, there is a single DNA-dependent RNA polymerase, that catalyses the transcription of all the three types of RNA (mRNA, tRNA, rRNA).
¾ RNA polymerase binds to the promoter and initiates the process along with certain initiation factors sigma.
¾ It uses ribonucleoside triphosphates (also called ribonucleotides) for polymerisation on a DNA template following complementarity of bases.
¾ The enzyme facilitates the opening of the DNA-helix and elongation continues.
¾ Once the RNA polymerase reaches the terminator, the nascent RNA falls off and the RNA polymerase also separates; it is called termination of transcription and is facilitated by certain termination factors Rho.
¾ In prokaryotes, the mRNA synthesised does not required any processing to become active and both transcription and translation occur in the same cytosol; translation can start much before the mRNA is fully transcribed, i.e., transcription and translation can be coupled.
Process of transcription in Bacteria
(B) Transcription in Eukaryotes
¾ In eukaryotes, the structural genes are monocistronic and ‘split’.
¾ They have coding sequences called exons that form part of mRNA and non-coding sequences, called introns, that do not form part of the mRNA and are removed during splicing.
¾ In eukaryotes, there are at least three different RNA polymerases in the nucleus, apart from the RNA polymerase in the organelles, which function as follows:
Process of transcription in Eukaryotes
Þ RNA polymerase I transcribes rRNAs (26S, 18S and 5.8S),
Þ RNA polymerase II transcribes the precursor of mRNA (called as heterogenous nuclear RNA (hnRNA) and
Þ RNA-polymerase III catalyses transcription of tRNA
¾ The primary transcript contains both exons and introns and it is subjected to a process, called splicing, where the introns are removed and the exons are joined in a definite order to form mRNA
¾ The hnRNA undergoes two additional processes called ‘capping’ and ‘tailing’.
¾ In capping, methyl guanosine triphosphate is added to the 5’ end of hnRNA.
¾ In tailing, adenylate residues (about 200-300) are added at the 3’- end of hnRNA
¾ The fully processed hnRNA is called mRNA and is released from the nucleus into the cytoplasm