Alternative Processing Pathways

Another finding that complicates the view of a gene as a sequence of nucleotides that specifies the amino acid sequence of a protein is the existence of alternative processing pathways, in which a single pre-mRNA is processed in different ways to produce alternative types of mRNA, resulting in the production of different proteins from the same DNA sequence.

One type of alternative processing is alternative splicing, in which the same pre-mRNA can be spliced in more than one way to yield multiple mRNAs that are translated into proteins with different amino acid sequences (IFigure 14.16a). Another type of alternative processing requires the use of multiple 3' cleavage sites (IFigure 14.16b); two or more potential sites for cleavage and polyadenylation are present in the pre-mRNA. In our example, cleavage at the first site produces a relatively short mRNA, compared with the mRNAs produced through cleavage at other sites.

Both alternative splicing and multiple 3' cleavage sites can exist in the same pre-mRNA transcript; an example is seen in the mammalian calcitonin gene, which contains six

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