Untranslated region

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For other uses, see UTR.

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Compare non-coding RNA.
The flow of genetic information within a cell. DNA is initially transcribed into a messenger RNA molecule. The mRNA is then translated into protein.
mRNA structure, approximately to scale for a human mRNA

In molecular genetics, an untranslated region (or UTR) refers to either of two sections, one on each side of a coding sequence on a strand of mRNA. If it is found on the 5' side, it is called the 5' UTR (or leader sequence), or if it is found on the 3' side, it is called the 3' UTR (or trailer sequence). mRNA is RNA that carries information from DNA to the ribosome, the site of protein synthesis (translation) within a cell. The mRNA is initially transcribed from the corresponding DNA sequence and then translated into protein. However, several regions of the mRNA are not translated into protein, including the 5' and 3' UTRs.

The untranslated areas of mRNA are non-protein-coding, meaning that their sequence is not read as codons that are related to a specific amino acids.

The 5' UTR is upsteam from the coding sequence. Within the 5' UTR is a sequence that is recognized by the ribosome which allows the ribosome to bind and initiate translation. The mechanism of translation initiation differs in Prokaryotes and Eukaryotes. The 3' UTR is found immediately following the translation stop codon. The 3' UTR plays a critical role in translation termination as well as post transcriptional gene expression.[1]

These often long sequences were once thought to be useless or junk mRNA that has simply accumulated over evolutionary time. However, it is now known that the untranslated region of mRNA is involved in many regulatory aspects of gene expression in Eukaryotic organisms. The importance of these non-coding regions is supported by evolutionary reasoning, as there would be natural selection to eliminate unusable RNA.

It is important to distinguish the 5' and 3' UTRs from other non-protein-coding RNA. Within the coding sequence there can be found sections of RNA that will not be included in the protein product. These sections of RNA are called introns. The RNA that is coded into protein are called exons. The reason why introns are not considered untranslated regions is that the introns are spliced out in the process of RNA splicing. The introns are not included in the mature mRNA molecule.

History

The untranslated regions of mRNA became a subject of study at early as the late 1970s, after the first mRNA molecule was fully sequenced. In 1978, the 5' UTR of the human gamma-globin mRNA was fully sequenced.[2] In 1980, a study was conducted on the 3' UTR of the duplicated human alpha-globin genes.[3] In the decades following, scientists have continued to study the untranslated regions of mRNA.

Evolution

The untranslated region is seen across in Prokaryotes and Eukaryotes, although the length and composition may vary. In Prokaryotes, the 5’ UTR is typically between 3 and 10 nucleotides long. In Eukaryotes, the 5’ UTR can be hundreds to thousands of nucleotides long. This is consistent with the higher complexity of the genomes of Eukaryotes compared to Prokaryotes. The 3’ UTR has variation in length as well. The poly-A tail is essential for keeping the mRNA from being degraded. Although there is variation in lengths of both the 5' and 3' UTR, it has been seen that the 5' UTR length is more highly conserved than the 3' UTR length.[4]

Prokaryotes

The 5’ UTR of Prokaryotes consists of the Shine-Dalgarno sequence (AGGAGGU).[5] This sequence is found 3-10 base pairs upstream from the initiation codon. The initiation codon is the start site of translation into protein.

Eukaryotes

The 5’ UTR of Eukaryotes is more complex than prokaryotes. It contains a kozak consensus sequence (ACCAUGG)(put internal link).[6] This sequence contains the initiation codon. The initiation codon is the start site of translation into protein.

Links to Disease

The importance of these untranslated regions of mRNA is just beginning to be understood. Various medical studies are being conducted that have found connections between mutations in untranslated regions and increased risk for developing a particular disease, such as cancer. For example, associations between polymorphisms in the HLA-G 3′UTR region and development of colorectal cancer have been discovered.[7] Single Nucleotide Polymorphisms 3' UTR has also been associated with susceptibility to preterm birth.[8] Mutations in the 3' UTR are also related to development of cerebral amyloid angiopathy.[9]

Further Study

Through the recent study of untranslated regions, general information has been gathered about the nature and function of these elements. However, there is still much that is unknown about these regions of mRNA. Since the regulation of gene expression is critical in the proper function of cells, this is an area of study that needs to be investigated further. It is important to consider that mutations in 3' untranslated regions have the potential to alter the expression of several genes that may appear unrelated.[10] We are only beginning to understand the links between proper untranslated region function, and disease states of cells.

See also

References

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External links

Transcription: http://highered.mheducation.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html

Translation: http://highered.mheducation.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

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