The process of transcription happens in both prokaryotic and eukaryotic cells. It involves copying a particular part of a DNA molecule. These parts, known as genes, are copied from the DNA so that the cell has the information it needs to make a variety of essential proteins. There are a few differences in the process of transcription between prokaryotic and eukaryotic cells.
In order for transcription to occur, the enzyme RNA polymerase is needed to bind to the DNA and copy the code for a particular gene. The coded information is stored in a strand of RNA that will then be used to make a particular protein.
Prokaryotic cells contain and use only one kind of RNA polymerase to make RNA strands. However, Eukaryotic cells have three variations of the RNA polymerase enzyme known as types I, II, and III. These different RNA polymerases each transcribe for a different class of gene: Messenger RNA (mRNA), Transfer RNA (tRNA) and Ribosomal RNA (rRNA). These differing classes of gene all have unique functions in protein synthesis.
The location within a cell where transcription happens is different between prokaryotic cells and eukaryotic cells. As prokaryotic cells lack a nucleus, the transcription and protein synthesis both occur within the cell body in the cytoplasm. However, eukaryotic cells do have a nucleus. This is the site of transcription in eukaryotes. Even though transcription happens in the nucleus, the coded RNA strand still needs to move from the nucleus to the cytoplasm to begin protein synthesis.
Promoters and Enhancers
Both eukaryotic and prokaryotic cells have promoters. As the name suggests, these are portions of a DNA molecule that encourage transcription by allowing RNA polymerase to attach to the DNA. Prokaryotic RNA polymerase can directly recognize a promoter site and attach to it. Eukaryotic RNA polymerase does not easily recognise promoter sites and instead uses other proteins to attach to the promoter to begin transcription.
Enhancers are another area of the DNA molecule that, believe it or not, exist to enhance the transcription process. These enhancers can only be found in eukaryotic cells and are not present in prokaryotic cells.
Genes are transcribed to make different proteins. At the end of the transcription process, the transcribed RNA strands then go on to create various proteins. In prokaryotic cells, the RNA strands do not need be processed and translation can begin even before the entire gene is transcribed. Eukaryotic cells adopt a more fine-tuned process and before translation can occur, the RNA strands must be carefully processed to create a finalised version of the RNA that is ready to create proteins.
Although similar, the process of transcription in prokaryotic cells and eukaryotic cells do have their differences:
• Eukaryotic cells contain three types of RNA polymerase. Prokaryotic cells have only one. • Eukaryotic transcription happens in the nucleus. Prokaryotic transcription happens in the cytoplasm. • RNA polymerase in eukaryotic cells uses other proteins to recognise promoter sites. In prokaryotic cells, the RNA polymerase directly recognises promoter sites. • Unlike prokaryotic DNA, eukaryotic DNA also contains enhancers to ‘enhance’ transcription. • In prokaryotic cells, translation can happen before transcription is finished. In eukaryotic cells, the RNA must be modified before it is ready for translation. • RNA polymerase has much easier access to DNA in prokaryotes than eukaryotes. Unlike prokaryotic DNA, eukaryotic DNA is shielded by structures called nucleosomes.