Och3

14.6 Most eukaryotic mRNAs have a 5' cap.

The cap consists of a nucleotide with 7-methyl guanine attached to the pre-mRNA by a unique 5'-5' bond (shown in detail in the bottom box). The cap is added shortly after the initiation of transcription. A methyl group is added to position 7 of the guanine base of the newly added (now the terminal) nucleotide and to the 2' position of each sugar of the next two nucleotides.

from the first ribonucleoside triphosphate in the transcription reaction. The 5' end of pre-mRNA can be represented as 5 '-pppNpNpN..., in which the letter N represents a ribonucleotide and p represents a phosphate. Shortly after the initiation of transcription, one of these phosphates is removed and a guanine nucleotide is added (see Figure 14.6). This guanine nucleotide is attached to the pre-mRNA by a unique 5' -5' bond, which is quite different from the usual 5'-3' phosphodiester bond that joins all the other nucleotides in RNA. One or more methyl groups are then added to the 5' end; the first of these methyl groups is added to position 7 of the base of the terminal guanine nucleotide, making the base 7-methylguanine. Next, a methyl group may be added to the 2' position of the sugar in the second and third nu-cleotides, as shown in Figure 14.6. Rarely, additional methyl groups may be attached to the bases of the second and third nucleotides of the pre-mRNA.

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