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20-September-2008 09:29:09 - Messenger RNA The life cycle of an mRNA in a eukaryotic cell. RNA is transcribed in the nucleus; once completely processed, it is transported to the cytoplasm and translated by the ribosome. At the end of its life, the mRNA is degraded. The life cycle of an mRNA in a eukaryotic cell. RNA is transcribed in the nucleus; once completely processed, it is transported to the cytoplasm and translated by the ribosome. At the end of its life, the mRNA is degraded. Messenger ribonucleic acid mRNA is a molecule of RNA encoding a chemical blueprint for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein. In mRNA as in DNA, genetic information is encoded in the sequence of four nucleotides arranged into codons of three bases each. Each codon encodes for a specific amino acid, except the stop codons that terminate protein synthesis. This process requires two other types of RNA: transfer RNA tRNA mediates recognition of the codon and provides the corresponding amino acid, while ribosomal RNA rRNA is the central component of the ribosome's protein manufacturing machinery. Contents 1 Processing and function 1.1 Transcription 1.2 Eukaryotic pre-mRNA processing 1.2.1 5' cap addition 1.2.2 Splicing 1.2.3 ing 1.2.4 Polyadenylation 1.3 Transport 1.4 Translation 1.5 Degradation 2 Structure 2.1 5' cap 2.2 Coding regions 2.3 Untranslated regions 2.4 PolyA tail 2.5 Monocistronic versus polycistronic mRNA 3 References 4 External links Processing and function The brief existence of an mRNA molecule begins with transcription and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, ed, and transported prior to translation. Eukaryotic mRNA molecules often require extensive processing and transport, while prokaryotic molecules do not. Transcription Main article: Transcription genetics During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One notable difference, however, is that eukaryotic RNA polymerase associates with mRNA processing enzymes during transcription so that processing can proceed quickly after the start of transcription. The short-lived, unprocessed or partially processed, product is termed pre-mRNA; once completely processed, it is termed mature mRNA. Eukaryotic pre-mRNA processing Main article: Post-transcriptional modification Processing of mRNA differs greatly among eukaryotes, bacteria and archea. Non-eukaryotic mRNA is essentially mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing. 5' cap addition Main article: 5' cap A 5' cap also termed an RNA cap, an RNA 7-methylguanosine cap or an RNA m7G cap is a modified guanine nucleotide that has been added to the front or 5' end of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue which is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the ribosome and protection from RNases. Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with RNA polymerase. This enzymatic complex catalyzes the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemical reaction. Splicing Main article: Splicing genetics Splicing is the process by which pre-mRNA is modified to remove certain stretches of non-coding sequences called introns; the stretches that remain include protein-coding sequences and are called exons. Sometimes pre-mRNA messages may be spliced in several different ways, allowing a single gene to encode multiple proteins. This process is called alternative splicing. Splicing is usually performed by an RNA-protein complex called the spliceosome, but some RNA molecules are also capable of catalyzing their own splicing see ribozymes. ing In some instances, an mRNA will be ed, changing the nucleotide composition of that mRNA. An example in humans is the apolipoprotein B mRNA, which is ed in some tissues, but not others. The ing creates an early stop codon, which upon translation, produces a shorter protein. Polyadenylation Main article: Polyadenylation Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, most messenger RNA mRNA molecules are polyadenylated at the 3' end. The polyA tail and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where polyA tails act to facilitate, rather than impede, exonucleolytic degradation. Polyadenylation occurs during and immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. After the mRNA has been cleaved, around 250 adenosine residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase. Just as in alternative splicing, there can be more than one polyadenylation variant of a mRNA. Transport Another difference between eukaryotes and prokaryotes is mRNA transport. Because eukaryotic transcription and translation is compartmentally separated, eukaryotic mRNAs must be exported from the nucleus to the cytoplasm. Mature mRNAs are recognized by their processed modifications and then exported through the nuclear pore. Translation Main article: Translation genetics Because prokaryotic mRNA does not need to be processed or transported, translation by the ribosome can begin immediately after the end of transcription. Therefore, it can be said that prokaryotic translation is coupled to transcription and occurs co-transcriptionally. Eukaryotic mRNA that has been processed and transported to the cytoplasm i.e. mature mRNA can then be translated by the ribosome. Translation may occur at ribosomes free-floating in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle. Therefore, unlike prokaryotes, eukaryotic translation is not directly coupled to transcription. Degradation After a certain amount of time, the message is degraded by RNases. The limited lifetime of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs. Different mRNAs within the same cell have distinct lifetimes stabilities. In bacterial cells, individual mRNAs can survive from seconds to more than an hour; in mammalian cells, mRNA lifetimes range from several minutes to days. The greater the stability of an mRNA, the more protein may be produced from that mRNA. The presence of AU-rich elements in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these motifs. Rapid mRNA degradation via AU-rich elements is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor TNF and granulocyte-macrophage colony stimulating factor GM-CSF.1 Base pairing with a small interfering RNA siRNA or microRNA miRNA can also accelerate mRNA degradation. Structure The structure of a mature eukaryotic mRNA. A fully processed mRNA includes a 5' cap, 5' UTR, coding region, 3' UTR, and polyA tail. The structure of a mature eukaryotic mRNA. A fully processed mRNA includes a 5' cap, 5' UTR, coding region, 3' UTR, and polyA tail. 5' cap Main article: 5' cap The 5' cap is a modified guanine nucleotide added to the front 5' end of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport. Coding regions Main article: Coding region Coding regions are composed of codons, which are decoded and translated into one mostly eukaryotes or several mostly prokaryotes proteins by the ribosome. Coding regions begin with the start codon and end with the a stop codons. Generally, the start codon is an AUG triplet and the stop codon is UAA, UAG, or UGA. The coding regions tend to be stabilised by internal base pairs, this impedes degradation.23 In addition to being protein-coding, portions of coding regions may serve as regulatory sequences in the pre-mRNA as exonic splicing enhancers or exonic splicing silencers. Untranslated regions Main articles: 5' UTR and 3' UTR Untranslated regions UTRs are sections of the mRNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region 5' UTR and three prime untranslated region 3' UTR, respectively. These regions are transcribed with the coding region and thus are exonic as they are present in the mature mRNA. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency. The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs. The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called ribonucleases and for ancillary proteins that can promote or inhibit RNA degradation. Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA. MicroRNAs bound to the 3' UTR also may affect translational efficiency or mRNA stability. Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can actually be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation. Some of the elements contained in untranslated regions form a characteristic secondary structure when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the SECIS element, are targets for proteins to bind. One class of mRNA element, the riboswitches, directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself. PolyA tail Main article: Polyadenylation The 3' polyA tail is a long sequence of adenine nucleotides often several hundred at the 3' end of the pre-mRNA. This tail promotes export from the nucleus and translation, and protects the mRNA from degradation. Monocistronic versus polycistronic mRNA An mRNA molecule is said to be monocistronic when it contains the genetic information to translate only a single protein. This is the case for most of the eukaryotic mRNAs4. On the other hand, polycistronic mRNA carries the information of several proteins, which are translated into several proteins. Most of the mRNA found in bacteria and archea are polycistronic4. Dicistronic is the term used to describe a mRNA that encodes only two proteins. References ^ Shaw G, Kamen R August 1986. A conserved AU sequence from the 3' untranslated region of GM-CSF mRNA mediates selective mRNA degradation. Cell 46 5: 659-67. doi:10.1016/0092-86748690341-7. PMID 3488815. ^ Shabalina SA, Ogurtsov AY, Spiridonov NA 2006. A periodic pattern of mRNA secondary structure created by the genetic code. Nucleic Acids Res. 34 8: 2428-37. doi:10.1093/nar/gkl287. PMID 16682450. ^ Katz L, Burge CB September 2003. Widespread selection for local RNA secondary structure in coding regions of bacterial genes. Genome Res. 13 9: 2042-51. doi:10.1101/gr.1257503. PMID 12952875. ^ a b Kozak, M. March 1983. Comparison of initiation of protein synthesis in procaryotes, eucaryotes, and organelles PDF. Microbiological Reviews 47 1: 1-45. PMID 6343825. Retrieved on 2006-08-12. External links Life of mRNA Flash animation v d e Types of nucleic acids Constituents Nucleobases | Nucleosides | Nucleotides | Deoxynucleotides Ribonucleic acids RNA | mRNA pre-mRNA/hnRNA | tRNA | rRNA | aRNA | gRNA | miRNA | ncRNA | piRNA | shRNA | siRNA | snRNA | snoRNA | stRNA | ta-siRNA | tmRNA Deoxyribonucleic acids DNA | cDNA | gDNA | msDNA | mtDNA Nucleic acid analogues GNA | LNA | PNA | TNA | morpholino Cloning vectors phagemid | plasmid | lambda phage | cosmid | P1 phage | fosmid | BAC | YAC | HAC Major families of biochemicals Saccharides | Carbohydrates | Glycosides | | Amino acids | Peptides | Proteins | Glycoproteins | | Lipids | Terpenes | Steroids | Carotenoids Alkaloids | Nucleobases | Nucleic acids | | Enzyme cofactors | Flavonoids | Polyketides | Tetrapyrroles Retrieved from http://en..org/wiki/Messenger_RNA Categories: RNA | Gene expression | Protein biosynthesis | Genetics | Molecular genetics Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية БългарÑ?ки Català Česky Dansk Deutsch Eesti Español Français Bahasa Indonesia Italiano עברית Latina Lietuvių Nederlands 日本語 Occitan Polski Português Română РуÑ?Ñ?кий SlovenÄ?ina Suomi Svenska Tiếng Việt Türkçe УкраїнÑ?ька اردو ä¸æ–‡ This page was last modified on 16 August 2008, at 10:5
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