Chromosomes are long molecules that contain millions of base pairs, making up a single chromosome. Most of these are special; they are known as genes. When a gene is expressed, a specific protein is produced. The first step is transcription. Enzymes use one of the DNA strands contained inside a gene as a template to create a messenger RNA(also known as an mRNA). Transcription is the basic process that occurs in both Prokaryotic and Eukaryotic Transcription.
Prokaryotic vs Eukaryotic Transcription
The main difference between prokaryotic transcription and eukaryotic transcription is that the prokaryotic transcription process occurs in the cytoplasm, while the eukaryotic transcription process occurs in the nucleus. The prokaryotic is the simple stage where there is DNA that is transcribed in RNA, which is fully functional which is translated into proteins, whereas in eukaryotes, the first RNA that is produced is called premature RNA, which doesn’t have the capability of making proteins then and there so, So there are modifications called splicing, 5 major end caps, and 3 major extensions.
In prokaryotic transcription and translation, both occur at the same time, so there is a small amount of mRNA processing. The RNA polymerase enzyme is required for prokaryotic transcription to be completed effectively. The enzyme is composed of five subunits, which bind to the σ factor and the promoter region before transcription is initiated by the holoenzyme.
In eukaryotic transcription, there is extensive mRNA processing, that is, removal of introns and addition of exons, the addition of 5 caps, and the addition of poly-a tails. A total of five different kinds of RNA polymerases exist in eukaryotes, each of which is specifically designed to meet a certain transcriptional need and comprises between 10 and 17 subunits.
Comparison Table Between Prokaryotic and Eukaryotic Transcription
| Parameters of Comparison | Prokaryotic Transcription | Eukaryotic Transcription |
| Site of Transcription | Transcription occurs in the cytoplasm. | Transcription occurs inside the nucleus. |
| Translation Association | Coupled transcription and translation. | Coupled transcription and translation are not possible. |
| RNA Polymerase | A single RNA polymerase synthesizes all types of RNA. | Three types of RNA polymerase. |
| Initiation | Generally, no proteins are required. | It requires proteins called transcription factors. |
| Transcriptional unit | Polycistronic | Monocistronic |
What is Prokaryotic Transcription?
Prokaryotic transcription often involves several genes, resulting in polycistronic mRNAs that specify multiple proteins in a single molecule. Multiple transcriptions and translation processes occurring at the same time on the same DNA template may quickly raise a bacterial protein’s intracellular level.
The prokaryotic transcription proceeds in three steps- Initiation, Elongation, Termination. And this process is driven by DNA dependent RNA polymerase enzyme, which transcribes the DNA. The RNA polymerase is a core enzyme which is having different subunits. It has α, α, β, and β subunits. Moreover, for this enzyme to work, there are sigma factors.
A promoter is a DNA sequence that the transcription machinery attaches to and uses to start transcription. There are two promoter consensus sequences located in the -10 and -35 regions upstream of the start site, which is the same in all promoters and bacterial species. TATA is the -10 consensus sequence, often known as the -10 region. The -35 sequence is recognized and bound by TTGACA.
Polymerase aborts when it is unable to synthesize. Once polymerase is synthesized, i.e. threshold 10+ nucleotides are synthesized, that’s called successful initiation. Elongation is the process by which mRNA is produced at a rate of 40 nucleotides per second in the 5′ to 3′ direction. The mRNA can be released through the termination of rho protein interaction or mRNA production.
What is Eukaryotic Transcription?
The transcription of eukaryotes is much more complicated than that of prokaryotes. In contrast to bacterial RNA polymerase, it can form a connection with the DNA template on its own. Eukaryotes need several additional proteins known as transcription factors to first bind to the promoter region and then aid in the selection of the proper polymerase.
The Eukaryotes have 3 RNA polymerase enzymes. RNA polymerase 2 is a major polymerase enzyme involved in the transcription of mRNA in eukaryotes. To reach the DNA template, each eukaryotic RNA polymerase needs a unique collection of transcription factors. The process of elongating a transcription takes a long time. When double-stranded DNA enters the enzyme from the front, it is unzipped to make room for the RNA template strand. One hybrid, RNA: DNA base pair, is generated extremely quickly for every DNA base pair separated by the advancing polymerase.
The DNA strands and the nascent RNA chain exit via separate channels; the two DNA strands reunite at the transcription bubble’s trailing end, while the single-strand RNA exits on its own at the transcription bubble’s leading end. When transcription is over, RNA polymerase is released from its template DNA, causing the entire transcript to dissociate from the template and be released. Each of the three RNA polymerases goes through a unique transformation process.
Main Differences Between Prokaryotic and Eukaryotic Transcription
- The prokaryotic process of transcription takes place in the cytoplasm. There is no nucleus in it, whereas in eukaryotic, the process takes place in the nucleus.
- The process between transcription and protein synthesis are coupled in prokaryotic, whereas in eukaryotic, transcription and protein synthesis are not coupled.
- In prokaryotic transcription, a single RNA polymerase synthesizes all types of RNA, whereas, in eukaryotic transcription, there are three types of RNA polymerase.
- In prokaryotic, no proteins are required, whereas eukaryotic transcription requires proteins called transcription factors.
- The prokaryotic transcriptional unit is polycistronic, whereas, in eukaryotic transcription, it is monocistronic.
Conclusion
The basic chemistry of the process is the same in both cases. RNA is the end product of transcription in both prokaryotes and eukaryotes, even though the processes of transcription differ. Prokaryotes and eukaryotes use RNA polymerase as a catalyst to stimulate the synthesis of RNA.
In prokaryotes, DNA replication and transcription and RNA translation occur in a single compartment (organisms lacking a nuclear membrane). All three processes have the capacity to occur once. In eukaryotes (organisms with a nuclear membrane), DNA replication and transcription occur in the nucleus, while protein synthesis occurs in the cytoplasm. RNA must first pass through the nuclear membrane before it can be translated. This means that transcription and translation are physically separated.
References
- https://www.nature.com/articles/nature14447
- https://www.sciencedirect.com/science/article/pii/S0969212601007031