mRNA

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.
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.
First, the triphosphate at the 5' end of the newly synthesized RNA is cleaved. The enzyme phosphohydrolase cleaves the gamma phosphodiester bonds while leaving the alpha and beta phosphates. Second, the enzyme guanylyltransferase transfers a guanine and its alpha phosphate onto the beta phosphate of the 5' end of the mRNA producing a 5'-5'-triphosphate linkage. Third, the nitrogen-7 (N-7) position of the newly added guanine is methylated (guaninemethylation) by the enzyme guanine-7-methyltransferase. Finally, 2'-O-methyltransferase methylates the 2' position of the ribose sugar. This methyl group provides extra stability to the RNA due to the protection from phosphoester cleavage by nucleophilic attack of the neighbor hydrogen. After the 5' end has been capped, it is released from the cap-synthesizing complex and is subsequently bound by a cap-binding complex associated with RNA polymerase.
Coding regions are composed of codons, which are decoded and translated into protein by the ribosome. Coding regions begin with the start codon and end with the one of three possible stop codons. In addition to protein-coding, portions of coding regions may also serve as regulatory sequences in the pre-mRNA as exonic splicing enhancers or exonic splicing silencers.
Untranslated regions (UTRs): are sections of the RNA 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 as part of the same transcript as the coding region. 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, and even inhibition of translation altogether, can be mediated by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by interfering with the ribosome's ability to bind to the mRNA. MicroRNAs also bind to the 3' UTR and may affect stability or efficiency.
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.
3' poly(A) tail: The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.
Anti-sense mRNA: During transcription, double stranded DNA produces mRNA from the sense strand; the other, complementary, strand of DNA is termed anti-sense. Anti-sense mRNA is an RNA complementary in sequence to one or more mRNAs. In some organisms, the presence of an anti-sense mRNA can inhibit gene expression by base-pairing with the specific mRNAs. In biochemical research, this effect has been used to study gene function, by simply shutting down the studied gene by adding its anti-sense mRNA transcript. Such studies have been done on the worm Caenorhabditis elegans and the bacterium Escherichia coli. This plays a part in RNA interference and RNA transcription.