Almost all eukaryotic pre-mRNAs are modified at their 5' ends by the addition of a structure called a 5' cap. This capping consists of the addition of an extra nucleotide at the 5' end of the mRNA and methylation by the addition of a methyl group (CH3) to the base in the newly added neucleotide and to the 2' -OH group of the sugar of one or more nucleotides at the 5' end (I Figure 14.6). Capping takes place rapidly after the initiation of transcription and, as will be discussed in more depth in Chapter 15, the 5' cap functions in the initiation of translation. Cap-binding proteins recognize the cap and attach to it; a ribo-some then binds to these proteins and moves downstream along the mRNA until the start codon is reached and translation begins. The presence of a 5' cap also increases the stability of mRNA and influences the removal of introns.
In the discussion of transcription in Chapter 13, it was noted that three phosphates are present at the 5' end of all RNA molecules, because phosphates are not cleaved
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