5' cap
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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, it also helps to protect mRNA from ribonucleases.
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. The first step in formation of the 5' cap involves the triphosphate at the 5' end of the newly synthesized RNA. The enzyme phosphohydrolase cleaves the gamma phosphodiester bonds leaving the alpha and beta phosphate behind. Next the enzyme guanylyltransferase transferes Guanine and its alpha phosphate onto the beta phosphate of the 5' end of RNA. This process gives the 5',5'-Triphosphate linkage. The next step involves the methylation of the N-7 position of Guanine, which is catalyzed by guanine-7-methyltrasnferase. 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 attact 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 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).