RNA 5' Cap:

The 5' cap is a specially altered nucleotide end to the 5' end of precursor messenger RNA and some other primary RNA transcripts as found in eukaryotes and, as a special exception, caliciviruses such as norovirus. The process of 5' capping is vital to creating mature messenger RNA which is then able to undergo translation. Capping ensures the messenger RNA's stability while it undergoes translation in the process of protein synthesis, and is a highly regulated process which occurs in the cell nucleus. 5' cap structure Ribose structure showing the positions of the 2', 3' and 5' carbonsThe 5' cap is found on the 5' end of an mRNA molecule and consists of a guanine nucleotide connected to the mRNA via an unusual 5' to 5' triphosphate linkage. This guanosine is methylated on the 7 position directly after capping in vitro by a methyl transferase. It is referred to as a 7-methylguanosine cap, abbreviated m7G/ Further modifications include the possible methylation of the 2' hydroxy-groups of the first 3 ribose sugars of the 5' end of the mRNA. The methylation of both 2' hydroxy-groups is shown on the diagram

Functionally the 5' cap looks like the 3' end of an RNA molecule (the 5' carbon of the cap ribose is bonded, and the 3' unbonded). This provides significant resistance to 5' exonucleases… Capping process The starting point is the unaltered 5' end of an RNA molecule. This features a final nucleotide followed by three phosphate groups attached to the 5' carbon. 1. One of the terminal phosphate groups is removed (by a phosphatase), leaving two terminal phosphates. 2. GTP is added to the terminal phosphates (by a guanylyl transferase), losing two phosphate groups (from the GTP) in the process. This results in the 5' to 5' triphosphate linkage. 3. The 7-Nitrogen of guanine is methylated (by a methyl transferase). 4. Other methyltransferases are optionally used to carry out methylation of 5' proximal nucleotides.