The new G residue is in the reverse orientation with respect to all other nucleotides and undergoes methylation (using a methyl transferase at its 7th position. The cap with a single methyl group at this terminal guanine residue is found in unicellular eukaryotes and described as cap 0, but in most eukaryotes, methyl group may also be present on the penultimate base at 2' position of sugar moiety, so that with methyl groups at two terminal nucleotides, it is now described as cap 1. In still other cases (10-15% of all caps), methyl group may be present at the third base also at the 2' position of sugar and this cap is rtow described as cap 2 (Fig. 33.1). It is. estimated that, at a particular moment, only ~20% of hnRNA is capped following transcription.
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