Transcription in Prokaryotes and Eukaryotes
Transcription is the process of making ribonucleic acid (RNA) by using the DNA genetic information code. The process of transcription starts when the hydrogen bonds in the DNA double helix unwind and each DNA base links with a complementary base to create the RNA molecule. If the gene that is transcribed codes for a protein, the result of transcription is messenger RNA (mRNA). Guanine pairs with cytosine and cytosine pairs with guanine, thymine pairs with adenine and adenine pairs with thymine. However, adenine that would normally pairs to thymine in DNA template, pairs with uracil during the mechanism of transcription. Transcription in pro and eukaryotes is different due to the location of the genetic information in the cell. Therefore, this essay will focus on the processes of transcription in both of prokaryotes and eukaryotes.
The fundamental features in the synthesis of RNA are shared amongst prokaryotes and eukaryotes; however, the process of transcription in eukaryotes differs and it is much more complex as it takes place inside the nucleus. There are three stages in transcription which occur simultaneously, named initiation, elongation and termination. In prokaryotic transcription, the initiation step begins when the RNA polymerase (RNAP) binds to the DNA helix, e.g. RNAP enzyme in E.coli is a holoenzyme that contains core enzymes and sigma (σ) factors. The role of the sigma factor is to allow the initiation of transcription by making the RNA polymerase holoenzyme to bind to a promoter and causes a region of the DNA to unwind. The promoter covers a 60 nucleotides region within which it reads for the -35 and -10 (figure 1). Then the polymerase complex extends from the -10 region to form a bubble that is usually about 17 base pairs. In the elongation stage, the RNAP reads the DNA sense strand in the 5' to 3' direction instead of 3' to 5' direction, and adds nucleotides to the 3' end of the growing RNA chain. There are two types of prokaryotic transcription termination stages, rho-independent terminator is an inverted repeats that transcribe sequences which folds and creates hairpin loops, then causing RNAP to stop and resulting in release of the transcript. On the other hand, rho-dependent terminators use rho factor, which unwinds the DNA-RNA hybrid formed during transcription, thus it releases the newly made RNA.
Eukaryotic transcription involves three different RNA polymerases; each RNAP transcribes a different set of genes. RNA polymerase I transcribes ribosomal RNA (rRNA), RNA polymerase II transcribes messenger (mRNA) and RNA polymerase III transcribes transfer RNA (tRNA) as well as the smallest rRNA. Unlike the prokaryotic RNA polymerase, eukaryotic RNA polymerases cannot begin the process of transcription without the aid of basic transcription factors (BTF). The BTFs help in the binding of the RNA polymerase to the promoter region in the initiation site also unwind the DNA double helix to permit the access of the ribonucleotides of the growing RNA chain.
After the pre-RNA initiation, eukaryotic transcription goes through further modifications of the 5' end. This begins by the addition of the 7-methyl guanosine Cap (G-Cap) which is done by an unusual 5' to 5' triphosphodiester bond. Eukaryotic transcripts genes contain exons and interons. The exons contain the genetic code whereas the introns are non-coding regions. Thus, the introns are removed by the process of splicing. The splicing reaction is done with the help of splicosomes. It starts with a cut at the region of the 5-exon and 3. A second cut is made at the region of the intron with the 3-exon this is called the transesterification reaction. The exon at the upstream attacks the downstream 5-phosphate exon-introns boundary and the circle structure containing the intron gets released and hydrolysed. Eventually, the introns are removed and discarded. The mRNA also undergoes addition of poly Adenylation tail; first, the 3' end of the mRNA transcript gets cleaved to free a 3' hydroxyl. Then a chain of adenine (A) bases is added between 100 and 250 residues long by an enzyme called poly-A polymerase to the RNA. The poly-A tail is such an important process as it stabilizes the RNA molecule and prevents its degradation.
Even there are some differences that distinguish between prokaryotic and eukaryotic transcription but each of which produces RNA molecule.