- There are three different types of RNA polymerase, one for each type of RNA that is synthesized.
- RNA polymerase I. It is involved in the transcription of ribosomal RNA (minus 5 S).
- RNA polymerase II. It is involved in the transcription of messenger RNA.
- RNA polymerase III. Role in the transcription of the RNA transfer of the ribosomal RNA 5 S and performs the transcription of genes that contain information for the histones.
- All RNAs formed (primary transcribed RNA) require a maturation process before being functional. During maturation, introns (nonsense sequences) are removed and exons (sense sequences) are spliced. As an exception, genes that contain information about histones do not have introns.
In the case of mRNA synthesis, the following stages are distinguished:
As in prokaryotes, DNA has a promoter region in which RNA polymerase II binds, but in this case the consensus sequences are CAAT and TATA, at different distances before the starting point.
In eukaryotes, DNA has to bind to proteins called transcription initiation factors in order for RNA polymerase II to bind and initiate transcription.
RNA synthesis always occurs in the 5'→3' direction. After having joined the first 30 transcribed nucleotides, a cap consisting of a methylguanosine triphosphate is added at the 5' end , which will serve as a starting signal in the transcription process.
The mRNA is synthesized until RNA polymerase II encounters the TTATTT sequence, which is the cut signal that indicates that RNA synthesis has ended. The RNA is separated, and an enzyme adds to the 3 'end a sequence of about 200 adenine ribonucleotides, the so-called poly-A tail.
Transcription in eukaryotes is distinguished from that of prokaryotes by the presence of Gppp and polyA tails.
The maturation of the RNA takes place in the nucleus. As already mentioned, the precursors of mRNA, rRNA, and tRNA, are not functional and have to undergo a maturation process. When this process has been completed, the RNA can leave the nucleus through the nuclear pores and reach the cytoplasm, where it will perform its function.
The introns are transcribed but not translated, and the exons are the ones that remain in the mature RNA.
The introns are removed in a process of splicing (splicing) wherein the exons are joined to form the final sequence of the RNA.