(i) The DNA dependent RNA polymerase helps in DNA replication by catalysing the polymerisation in only one direction, i.e., 5′→3′. In bacteria, the RNA polymerase has co-factors β, β′, α, α′, ω and σ which catalyse the process. 

Transcription in Prokaryotes 

▪ In prokaryotes, the structural gene is polycistronic and continuous. 

▪ In bacteria, the transcription of all the three types of RNA (mRNA, tRNA and rRNA) is catalysed by single DNA-dependent enzyme, called the RNA polymerase. 

▪ In E. coli bacterium, the RNA polymerase has co-factors β, β′, α, α′ and ω along with σ (sigma) factor, to catalyse the process. 

▪ The transcription is completed in three steps: initiation, elongation and termination. 

▪ Initiation: σ (sigma) factor recognises the start signal and promotor region on DNA which then along with RNA polymerase binds to the promoter to initiate transcription. 

▪ Elongation: The RNA polymerase after initiation of RNA transcription loses the σ factor but continues the polymerisation of ribonucleotides to form RNA. 

▪ Termination: Once the RNA polymerase reaches the termination region of DNA, the RNA polymerase is separated from DNA–RNA hybrid, as a result nascent RNA separates. This process is called termination which is facilitated by a termination factor ρ (rho). 

▪ In prokaryotes, mRNA does not require any processing, so both transcription and translation occur in the cytosol. It can be said that transcription and translation are coupled together.

(ii) Transcription in Eukaryotes 

▪ The structural genes are monocistronic in eukaryotes. 

▪ The process of transcription is similar to that in prokaryotes. 

▪ It takes place in the nucleus. 

▪ Coding gene sequences called exons form the part of mRNA and non-coding sequence called introns are removed during RNA splicing. 

▪ In eukaryotes, three types of RNA polymerases are found in the nucleus: 

i. RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S). 

ii. RNA polymerase II transcribes the precursor of mRNA (called heterogeneous nuclear RNA or hnRNA). 

iii. RNA polymerase III transcribes tRNA, 5S rRNA and snRNAs (small nuclear RNAs).