5' cap
A 5' cap (also called 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" (5' end) of the messenger RNA after transcription. The RNA cap is critical for recognition and proper attachment of the messenger RNA to the ribosome. The 5' cap consists of a terminal 7-methylguanosine residue which is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. The cap is added during transcription of DNA into mRNA, and its synthesis proceeds in a five-step reaction. First, the 5' end of the mRNA that is being synthesized is bound by a cap-synthesizing complex, which is associated with RNA polymerase. This complex catalyzes the chemical changes that are required for the cap. First, a guanine nucleotide (derived from GTP) is added to the 5' end of the chain. The guanosine residue is then methylated to yield 7-methylguanosine by action of guanine-7-methyltransferase. The methyl group which is required for this process is derived from S-adenosylmethionine. In most mRNAs, the nucleotide immediately next to the added guanosine is methylated as well. After the 5' end has been capped, it is released from the cap-synthesizing complex and is subsequently bound by a cap-binding complex that is linked to an RNA polymerase subdomain. 5' capping of mRNA only occurs in eukaryotes and is necessary for proper exit of the nucleus. Furthermore, the 5' cap provides protection from (5') ribonucleases, enzymes that degrade RNA from one end of the mRNA chain. Capping of the 5' end is part of a three-step process referred to as RNA processing. In RNA processing, the primary mRNA transcript is modified to create mature mRNA. RNA processing also includes RNA splicing and synthesis of a 3' polyadenylate tail (poly(A) tail).
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