(a) Describe the process of transcriptionin bacteria.
(b) Explain the processing the hRNA needs to undergo before becoming functional mRNA in
eukaryotes. (5 marks)
(a) Process of transcription in bacteria :
(i) Initiation DNA dependent RNA polymerase associates with the initiation factor or stigma
factor and binds to the promoter site of DNA. Thus initiates transcription.
(ii) Elongation One of the two strands of DNA functions as template. RNA polymerase helps in
opening of helix and continuous elongation. The two strands of DNA uncoil progressively from
the site of polymerase binding. RNA polymerase uses nucleotide triphosphate as a substrate
and polymerises nucleotide in a template in 5' to 3' direction, following the rule of complemen-
(ii) Termination When the RNA polymerase reaches the terminator region, m-RNA gets sepera-
ted from DNA. RNA polymerase associates with rho () and both the enzymes and the newly
formed/nascent RNA fall off from the DNA.
1. The structural genes are monocistronic in eukaryotes. The process of transcription is similar to
that in prokaryotes. It takes place in the nucleus.
2. The hnRNA undergoes two additional processes called capping and tailing.
3. In capping, methyl guanosine triphosphate is added to the 5' end of hnRNA.
4. In tailing, adenylate residues (about 200-300) are added at 3' end.
5. Now the hnRNA undergoes a process where the introns are removed and exons are joined to
form mRNA by the process called splicing.
In Eukaryotes, three types of RNA polymerases are found in the nucleus. These are :
(i) RNA polymerase I transcribes rRNAs (28S, 18S, 5.8S)
(ii) RNA polymerase II transcribes the precursor of mRNA called heterogenous nuclear RNA or
(iii) RNA polymerase III transcribes tRNA, 5srRNA and SnRNAs (small nuclear RNAs).
Messenger RNA (mRNA)