(prokaryotic) RNA polymerase consists of the core enzyme and the sigma subunit. The core enzyme ( α, β, β and ω ) is responsible for RNA synthesis whereas a sigma subunit is responsible for recognition of the promoter. Promoter sequences vary in different organisms. RNA polymerase opens Fig.
. Schematic structure of a transcription unit XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics In Eukaryotes, there are at least three RNA polymerases in the nucleus (in addition to RNA polymerase found in the organelles). There is a clear division of labour. The RNA polymerase I transcribes rRNAs (28S, 18S and .8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5S rRNA and snRNA.
The RNA polymerase II transcribes precursor of mRNA, the hnRNA (heterogenous nuclear RNA). In eukaryotes, the monocistronic structural genes have interrupted coding sequences known as exons (expressed sequences) and non- coding sequences called introns (intervening sequences). The introns are removed by a process called splicing . hnRNA undergoes additional processing called capping and tailing .
In capping an unusual nucleotide, methyl guanosine triphosphate is added at the ' end, whereas adenylate residues ( - ) (Poly A) are added at the ' end in tailing ( Fig. . ). Thereafter, this processed hnRNA, now called mRNA is transported out of the nucleus for translation.
The split gene feature of eukaryotic genes is almost entirely absent in prokaryotes. Originally each exon may have coded for a single polypeptide chain with a specific function. Since exon arrangement and intron removal are flexible, the exon coding for these polypeptide subunits act as domains combining in various ways to form new genes. Single genes can produce different functional elongation and termination?
The RNA polymerase is only capable of catalyzing the process of elongation. The RNA polymerase associates transiently with initiation factor sigma ( σ ) and termination factor rho ( r ) to initiate and terminate the transcription, respectively. Association of RNA with these factors instructs the RNA polymerase either to initiate or terminate the