Transcription and RNA Processing

ID #2159

Why is the 5' end of a primary transcript occupied by a triphosphate group?

When RNA polymerase puts in the first RNA base at the 5' end, it is simple hybridization, and no energy is required, which means the NTP keeps all three phosphate groups. The first base is just held in place by those hydrogen bonds, and the 5' end is a free end and is not involved in a phosphodiester linkage. For all of the other NTPs, a pyrophosphate is cleaved off leaving a monophosphate. This is where the energy comes from to form the phosphodiester linkage.

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