Individual nucleotides in the interior of pre-mRNA may be changed, added, or deleted by RNA editing. The amino acid sequence produced by the edited mRNA is not the same as that encoded by DNA. More information on RNA editing and a database of guide RNA sequences

Connecting Concepts)

Eukaryotic Gene Structure and Pre-mRNA Processing

Chapters 13 and 14 have introduced a number of different components of genes and RNA molecules, including promoters, 5' untranslated regions, coding sequences, introns, 3' untranslated regions, poly(A) tails, and caps. Let's see how some of these components are combined to create a typical eukaryotic gene and how a mature mRNA is produced from them.

The promoter, which typically encompasses about 100 nucleotides upstream of the transcription start site, is necessary for transcription to take place but is itself not usually transcribed when protein-encoding genes are transcribed by RNA polymerase II (I Figure 14.19a). Farther upstream or downstream of the start site, there may be enhancers that also regulate transcription.

In transcription, all the nucleotides between the transcription start site and the stop site are transcribed into pre-mRNA, including exons, introns, and a long 3' end that is later cleaved from the transcript (I Figure 14.19b). Notice that the 5' end of the first exon contains the sequence that codes for the 5' untranslated region, and the 3' end of the last exon contains the sequence that codes for the 3' untranslated region.

The pre-mRNA is then processed to yield a mature mRNA. The first step in this processing is the addition of a cap to the 5' end of the pre-mRNA (IFigure 14.19c). Next, the 3' end is cleaved at a site downstream of the AAUAAA consensus sequence in the last exon (I Figure 14.19d). Immediately after cleavage, a poly(A) tail is added to the 3' end (I Figure 14.19e). Finally, the introns are removed to yield the mature mRNA (I Figure 14.19f). The mRNA now contains 5' and 3' untranslated regions, which are not translated into amino acids, and the nu-cleotides that carry the protein-coding sequences. The nu-

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