explain the process of transcription in eukaryotes with diagram.
Transcription in eukaryotes is a very complex multi-step procedure which involves a number of transcription factors and enzymes. There are three different RNA polymerases found in eukaryotes. These are:
1) RNAP I which synthesizes all ribosomal RNAs (5.8S, 18S, 28S) except for 5S.
2) RNAP II which synthesizes all nuclear genes encoding proteins (mRNAs).
3) RNAP III which synthesizes all tRNAs, the 5S rRNA, and all snRNAs.
The process of eukaryotic transcription is divided into 3 stages:
During this stage the transcription process is initiated with the help six different transcription factors which guide the binding of RNA polymerase to the promoter region of the gene to be transcribed. This results in unwinding of the DNA to facilitate the working of RNA polymerase. The complex of DNA, RNA pol, and transcription factors is formed which is known as the pre-initiation complex.
During this stage, RNA polymerase synthesizes the complete RNA from the DNA by addition of nucleotide triphosphates (NTP's) to the growing chain. Most of the transcription factors get dissociated from RNA pol during this phase.
The final step of transcription is the termination of synthesis. As the enzyme, RNA polymerase encounters the transcription end site on the gene, it stops transcription and dissociates from DNA. RNA formed is released from enzyme and further processed to form a complete mRNA.
There are three stages in these process:
Eukaryotic transcription is more complex than prokaryotic transcription. For instance, in eukaryotes the genetic material (DNA), and therefore transcription, is primarily localized to the nucleus, where it is separated from the cytoplasm (in which translation occurs) by the entaamoeboic nuclear membrane. DNA is also present in mitochondria in the cytoplasm and mitochondria utilize a specialized RNA polymerase for transcription. This allows for the temporal regulation of gene expression through the sequestration of the RNA in the nucleus, and allows for selective transport of RNAs to the cytoplasm, where the ribosomes reside.
The basal eukaryotic transcription complex includes the RNA polymerase and additional proteins that are necessary for correct initiation and elongation.