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).Post-transcriptional modifications
▪ The primary transcripts are non-functional, containing both the coding region, exon, and region, intron, in RNA and are called heterogenous RNA or hnRNA.
▪ The hnRNA undergoes two additional processes called capping and tailing.
Posttranscriptional modifications in eukaryotes
▪ In capping, an unusual nucleotide, methyl guanosine triphosphate, is added to the 5′-end of hnRNA.
▪ In tailing, adenylate residues (about 200–300) are added at 3′-end in a template independent manner.
▪ Now the hnRNA undergoes a process where the introns are removed and exons are joined to form mRNA by the process called splicing.
