CBSE Class 12-science Answered

• DNA-dependent RNA polymerase catalyzes the transcription of three types of RNAs in bacteria.
• Three types of RNA are mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosomal RNA).
• RNA polymerase binds to the promoter and initiates the transcription.
• RNA polymerase associates with the initiation factor and the termination factor for the initiation and termination of transcription.
• Polymerisation is carries out by using nucleoside triphosphates as substrate.
• Once polymerases reach the terminator site, RNA and RNa polymerase falls off which results in termination of transcription.

In Eukaryotes

• Three RNA polymerases I, II and III synthesize rRNA, mRNA, and tRNA, respectively.
• RNA polymerase I transcribe rRNAs, RNA polymerase II transcribes precursor of mRNA and heterogenous nuclear RNA (hnRNA) and RNA polymerase III transcribes tRNA, 5srRNA and small nuclear RNAs (snRNAs).
• The primary transcripts contain introns as well as exons. Hence, they are subjected to splicing in which introns are removed and exons are joined.
• hnRNA undergoes capping and tailing. In capping guanosine triphosphate is added to the 5′ end of hnRNA.
• In tailing, 200–300 adenylate residues are added at the 3′ end of hnRNA