After transcription, a cap forms at the 5′ end of the mRNA in eukaryotes. The cap forms by the attachment of 7-methyl guanosine (m7G). 7-methyl guanosine is a modified guanosine nucleotide where a methyl (-CH3) group is attached at the 7′ position of guanosine nucleotide.
During the capping, the 5′ end of the 7-methyl guanosine link to the 5′ end of the terminal nucleotide of the mRNA. The m7G capping protects the eukaryotic mRNA from the 5′ to 3′ exonucleolytic activity of exonucleases. The cap also helps in the nuclear export of the mRNA and recognition by translational machinery.