These nucleotides are not encoded in the DNA, but are inserted in a process called polyadenylation following transcription.The addition of 50 to 250 or more adenine nucleotides at the3′ end, forming a poly(A) tail, is a second kind of modification to eukaryotic mRNA.Additional methyl groups can rarely be attached to the bases of the second and third nucleotides of pre-mRNA.Next, in the second and third nucleotides, a methyl group may be attached to the 2′ position of the sugar.The first of these methyl groups is attached to the position 7 of the base of the terminal guanine nucleotide making the base 7-methyl guanine.To the 5′ end, one or more methyl groups are added.A special 5′-5′ bond connects this guanine nucleotide to the pre mRNA, which is somewhat different from the normal 5′-3′ phosphodiester bond that binds all the other RNA nucleotides.One of these phosphate groups is removed shortly after the start of transcription and a guanine nucleotide is added.
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