Answer: Transcription is the synthesis of RNA on a DNA template. In eukaryotes it occurs in the nucleus and is carried out by three RNA polymerases: RNA polymerase I (rRNAs 28S, 18S, 5.8S), RNA polymerase II (hnRNA, precursor of mRNA) and RNA polymerase III (tRNA, 5S rRNA and snRNAs).
Steps of transcription:
- Initiation: with the help of transcription factors, RNA polymerase II binds to the promoter of the transcription unit, the DNA unwinds locally and the template strand (3’ → 5’) is exposed.
- Elongation: RNA polymerase adds ribonucleotides (ATP, GTP, CTP, UTP) complementary to the template in the 5’ → 3’ direction; uracil pairs with adenine.
- Termination: at the terminator sequence the polymerase and the newly formed RNA are released. The primary transcript is hnRNA (heterogeneous nuclear RNA), which contains both exons (coding) and introns (non-coding).
Processing of hnRNA to functional mRNA:
- Capping: an unusual nucleotide, 7-methyl guanosine triphosphate (methyl GTP cap), is added to the 5’ end. It protects the mRNA and helps it bind to the ribosome.
- Tailing (polyadenylation): about 200 to 300 adenylate residues (poly-A tail) are added to the 3’ end, without a template. It increases the stability of mRNA.
- Splicing: introns are removed and the exons are joined in a definite order by spliceosomes (snRNPs).
The processed, fully functional mRNA is then transported from the nucleus to the cytoplasm for translation.
