Eukaryotic Transcription Process: DNA to RNA
According to the Central Dogma, DNA is transcribed into RNA and then translated to protein which marks the pathway in molecular biology. This process of the Central Dogma can be explained by four major stages: replication, transcription, processing and translation. We will be focusing on transcription, the process of creating messenger RNA that is complementary to the DNA template. Transcription requires four important events: preinitiation, initiation, elongation and termination that is mainly responsible for gene expression. There are many processes that require multiple jobs of an RNA polymerase, which is why there are three types. The first RNA polymerase is responsible for transcribing rRNA genes. Next, RNA polymerase II has a role in transcribing all protein encoding genes of mRNA. Finally, RNA polymerase III is used for transcribing small RNA genes such as snRNAs and 5S rRNA.
When RNA polymerase II travels through the DNA segment, it unwinds the double helix of the DNA then when transcription is complete, it unwinds back. First, RNA polymerase II breaks the hydrogen bonds between the base pairs in DNA as it moves along the transcription bubble. During preinitiation, the TATA-binding protein (TBP) binds to the TATA box which attracts the six general transcription factors and RNA polymerase II to the promoter. RNA polymerase II binds to the promoter located at -30, -75 and -90 base pairs. Before the start of elongation, the carboxyl tail domain (CTD), will be phosphorylated by a GTF, allowing the interaction between the polymerase and proteins to be weakened. Next, RNA polymerase II scans and reads the DNA sequence thus producing an antiparallel complementary RNA transcript. Because the DNA template is read from 3’ to 5’ in transcription, RNA polymerase adds nucleotides to the 3’ end of the 5’ to 3’ direction of the RNA complementary transcript. In this RNA sequence, however, the base pair Thymine is replaced with Uracil that complements Adenine. In addition, Guanine is always paired with Cytosine. With a couple of RNA polymerases processing RNA copies on the DNA molecule, the result will consist of many mRNAs for the same gene.
Finally, during the processing event, RNA is destined to undergo three events before it can be translated. The first step is the addition of a cap at the 5’ end of the mRNA transcript once the nascent RNA leaves RNA polymerase II. The purpose of this cap is to protect the RNA while it travels down to the site of translation. The next step is to splice the unwanted introns of the mRNA. There are two parts to the transcript, exons and introns. Exons are the segments of RNA that codes for the translation of proteins while introns are segments that do not code for a purpose. As a result, only exons are remained on the mRNA which is the exact coding sequence for the protein that will be produced. After all these complicated yet crucial activities take place, the fully processed coding mRNA is prepared to be translated into a protein.