Transcription

Transcription is initiated by adjoining of RNA polymerase II to the beginning of the gene – to the promotor. The presence of enzymes causes unwinding of the double helix. As polymerase moves along the chosen chain it catalyzes adding succeeding nucleotides that are complementary to the DNA matrix. DNA letters A, T, C, G are translated into RNA A, U, C, G. Special sequence of bases in DNA causes terminating translation. The new mRNA molecule undergo further treatment that consist of three stages.

On the 5’ ending a cap is joined. It facilitates adjoining of the ribosome and enables translation. On the 3’ ending of mRNA a poli-A sequence which is from 50 to 200 adenines is added. Both cap and the poli-A sequence probably protect from enzymes and makes mRNA molecule more stabile.

The third modification is splicing the mRNA. Introns are cut out and exons are spliced together to form functional molecule that go to the cytoplasm. Some of the exons are removed by special enzymes called snRNP (small nuclear ribonucleoprotein) that consist of proteins and snRNA. SnRNP catalyze splicing mRNA. Some of the introns can cut out by themselves – exon splicing is then spontaneous.