Prokaryotic vs Eukaryotic Transcription
In a cell, DNA carries information from generation to generation controlling the activities of a cell. DNA is responsible for synthesizing proteins, which have a functional role or structural role in a cell. By synthesizing such proteins, DNA controls the activities of a cell.
In protein synthesis, two main steps can be distinguished; they are transcription and translation. In the first step, which is transcription, DNA expression is carried out, making complementary RNA strand from the DNA sequence (gene). In the second step, the mRNA is converted into a chain of polypeptides.
Transcription in Prokaryotes
Transcription in prokaryotic cell has four stages: binding, initiation, elongation and termination. The synthesis of RNA strand is catalyzed by the enzyme called RNA polymerase. Binding of RNA polymerase to the promoter sequence is the first step in transcription. In a bacterial cell, only one kind of RNA polymerase exists which synthesis all classes of RNA: mRNA, tRNA and rRNA. RNA polymerase found in Escherichia coli (E-coli) consists of two α subunits and two β subunits and sigma factor.
When this sigma factor binds to the DNA promoter sequence resulting in the unwinding of DNA double helix, initiation takes place. Using one of the DNA strand as a template, RNA polymerase synthesizes the RNA strand moving along the DNA strand unwinding the helix bit by bit. This RNA strand grows from 5′ to 3′ forming short hybrid with the DNA strand, and that is called elongation. Elongation is ceased with the transcription of special sequence called termination signal. In prokaryotes, there are two types of termination, factor dependent termination and intrinsic termination. Factor dependent termination needs Rho factor, and intrinsic termination happens when template contains a short GC rich sequence near the 3′ end after several Uracil bases.
Transcription in Eukaryotes
Transcription in eukaryote cell also has the same four stages as in prokaryotes; i.e. binding, initiation, elongation and termination. However, transcriptional process is more complicated in eukaryotic cells. In an eukaryotic cell, three different kinds of RNA polymerases occur to catalyze the synthesis of RNA strand from DNA template. These RNA polymerases are denoted as I, II, III, and differ from their location and types of RNA they synthesize. This polymerase then binding the DNA promoter with the help of transcriptional factors. Unwinding the DNA helix into single strands, RNA polymerase synthesizes the RNA strand.
After RNA polymerase has bounded to the DNA promoter sequence resulting in the unwinding of the DNA double helix, initiation takes place. RNA polymerase synthesizes the RNA strand moving along the DNA strand unwinding the helix. This RNA strand grows from 5′ to 3′ forming short hybrid with the DNA strand and that is called elongation. Elongation is ceased with the transcription of special sequence called termination signal. Termination is controlled by a variety of signals which vary with the enzyme involved.
What is the difference between the transcriptions in Eukaryotes and Prokaryotes ?
• The transcription in a eukaryotic cell is much more complicated than in a prokaryotic cell.
• In prokaryotic transcription, single kind of RNA polymerase is involved, whereas a eukaryotic cell has three different kinds of RNA polymerases.
• Eukaryotes transcription needs additional set of proteins called transcriptional factors to bind RNA polymerase to the promoter, and they are not a part of RNA polymerase, whereas prokaryotic transcription needs sigma factor to bind to the promoter.
• Eukaryotic promoters have more variation than prokaryote promoters.
• Termination of transcription needs a Rho factor in prokaryotes, whereas eukaryotes do not need that.
• In prokaryotes, two types of transcription can occur; they are factor dependent termination and intrinsic termination, whereas eukaryotic transcription is controlled by different signals, which vary with the enzyme involved.