general absence of correspondence between C values and evolutionary rankings (C value paradox). A large fraction of the eukaryotic genome consists of noncoding DNA, in introns within genes and in spacers between genes, whose functions remain unclear for the most part. Split genes are made up of exons, which are coding segments that usually specify functional or structural domains in proteins, and introns, which are noncoding sequences between exons. It is possible that split gene organization is of great antiquity. If this is true, introns have been lost in prokaryotes and from many genes in simple eukaryotes, but have been retained during evolution leading to the most highly evolved plants and animals. Intronless DNA can be replicated rapidly, an advantage in prokaryotes and simple eukaryotes. Split genes take longer to be replicated, but they provide the potential for exon shuffling and thus for more rapid evolution of proteins than by mutation alone.
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