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.
- One of the terminal phosphate groups is removed (by RNA terminal phosphatase), leaving two terminal phosphates.
- 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′–5′ triphosphate linkage.
- The 7-nitrogen of guanine is methylated (by a methyl transferase).
- If the second base from the terminal is adenine, it can be methylated; and the third base from the terminal is generally methylated 10–15% of the time.
Read more about this topic: Five Prime Cap
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