A 5 cap (also termed an RNA cap, an RNA 7-methylguanosine cap, or an RNA mG 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 that 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.