There are three types of RNAs involved in the process of protein synthesis—messenger RNA or mRNA, transfer RNA or tRNA and ribosomal RNA or rRNA. The mRNA helps to combine amino acids to make proteins. The tRNA carries each amino acid to the ribosomes according to the coded message in the mRNA, while the rRNA provides a mechanism to decode mRNA into amino acids. The DNA containing the genetic information is transferred to mRNA through transcription.
In prokaryotes, both transcription and translation, involved in protein synthesis take place inside the prokaryotic cell at the same time. Only one DNA-dependent RNA polymerase enzyme catalyses the transcription of all types of RNA. Transcription occurs in three stages–initiation, chain elongation and termination. In the initiation stage, the RNA polymerase along with the initiation factor (denoted by sigma) binds to the promoter sequence in DNA. This helps in the opening the DNA helix and separates the two DNA strands. In the elongation stage, the RNA polymerase builds a strand of RNA using one of the DNA strands as a template. This RNA chain growth takes place in the five prime to three prime direction. Once the chain elongation commences, the sigma factor dissociates from the RNA polymerase which can be reused. In the termination stage, the polymerase along with the termination-factor, represented by rho reaches the terminator region and the newly created mRNA falls off along with the enzyme.
The stages of transcription in eukaryotes are similar to that of prokaryotic transcription with few differences. In eukaryotes, transcription occurs in the nucleus and involves three RNA polymerases. The RNA polymerase one transcribes various rRNAs, the RNA polymerase two transcribes the precursor of mRNA, known as heterogeneous nuclear RNA or hnRNA, while the RNA polymerase three transcribes the tRNA, five sub unit ribosomal RNA or 5srRNA and the small nuclear RNA or snRNA. The processing of hnRNA marks major difference between the prokaryotic and eukaryotic transcription.
The mRNA in prokaryotes does not require any processing, but in eukaryotes, the hnRNA is inactive since it has both exons and introns. So, to remove the non-coding sequence called introns, it has to undergo processes such as capping, splicing and tailing. Capping involves the addition of methyl guanosine triphosphate to the five prime end of the hnRNA, which helps in binding small ribosomal subunits during protein synthesis. Splicing involves the removal of introns and joining of exons in a specific order. Tailing involves the addition of about two to three hundred adenylate residues to the three prime end of the hnRNA in a template-independent manner, giving rise to a poly A tail. This process is also known as polyadenylation. After these processes, the hnRNA becomes mRNA and the mRNA is transported out of the nucleus and further used for the process of protein synthesis during translation.