Transcription in Prokaryotes and Eukaryotes

Transcription in Prokaryotes and Eukaryotes

Proteins holds significant important functions for every living organism, these functions include enzymes in the body, carrier proteins for cellular transport, hormones and many other uses. The first step of making proteins in prokaryotes and eukaryotes is the process of transcription. Transcription is the process of making messenger ribonucleic acid (mRNA) by using the DNA genetic information code. The process of transcription starts when the hydrogen bonds in the DNA double helix unwind and each DNA base links with a matching base to create the mRNA molecule. Guanine pairs with cytosine and cytosine pairs with Guanine, thymine pairs with adenine and adenine pairs with thymine. However, adenine that would normally pairs to thymine in DNA template, pairs with Uracil during the mechanism of transcription. Transcription in pro and eukaryotes is different due to the location of the genetic information in the cell. Therefore, this essay will focus on the different processes of making mRNA in both of prokaryotes and eukaryotes.

The fundamental features of RNA synthesis are shared between prokaryotes and eukaryotes; however, the process of transcription in eukaryotes differs and it is much more complex as it takes place inside the nucleus. First, rather than having a single RNA polymerase, eukaryotes have three different RNA polymerases, each of which transcribes a different set of genes. RNA polymerase I transcribes three types of rRNA, RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNA and the smallest rRNA. There are different stages before mRNA can be synthesized in eukaryotic transcription. Theses stages are carried out simultaneously in order to produce a faithful copy of the genetic information from the DNA template to the mRNA. The initiation stage requires RNA polymerase (RNAP). The RNA polymerase is such an important protein complex which reads the DNA genetic code and produces a complimentary RNA from that code. Prokaryotes and eukaryotic polymerases, besides having similar function, carry much of the same structural motifs and similar binding domains. Bacteria only have a single polymerase to take care of each duty that RNA polymerases have. Where he RNA polymerase would require transcription factors (TF) to regulate its function.

Unlike the bacterial RNA polymerase, eukaryotic RNA polymerases cannot initiate transcription by themselves but need the help of a set of proteins called the basic transcription factors. The basic transcription factors perform a number of functions, including binding to gene promoter regions and attracting the appropriate RNA polymerase to the initiation site, as well as unwinding the DNA double helix to allow access of the incoming ribonucleotides of the growing RNA chain.

In contrast to prokaryotes, transcription termination by RNA polymerase II does not occur simply by release of the RNA molecule. Rather, transcription continues well beyond the termination point, and the transcript is later cleaved to the appropriate length. Following cleavage an enzyme called poly-A polymerase adds approximately 250 adenine residues to the tail end of the transcript.

After the initial pre-mRNA transcript has been created the first major modification is the addition of a 5' methyl guanosine cap. The addition of the 5' 7-MG cap is important for two reasons: the 5' caps are recognized by protein factors that initiate translation, and it also helps protect the transcript from nucleases. Nucleases are very common in the cell and because of this unprotected RNA has a very short half-life inside the cell. Nucleases are actually so common that working with RNA in the laboratory can be quite difficult because the samples have a tendency to disintegrate into useless bits.

Eukaryotic genes contain two types of transcribed regions: introns and exons. Exons are the regions of the genome that contain actual coding information. Introns are non-coding, meaning that intronic sequences are never translated to protein. Introns are never included in the final processed mRNA transcript. Splicing is the process of removing introns from the pre-mRNA transcript to produce an exon-only mRNA molecule.

Polymer is read in the 3' to 5' direction and the new, complementary fragments are generated in the 5' to 3' direction.

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