Origin of new gene copies through gene duplication. (A) DNA-based duplication. A common type of segmental duplication—tandem duplication—is shown. It may occur via unequal crossing-over that is mediated by transposable elements (light green). There are different fates of the resulting duplicate genes. For example, one of the duplicates may acquire new functions by evolving new expression patterns and/or novel biochemical protein or RNA functions (see main text for details). (Gold and blue boxes) Exons, (black connecting lines) exon splicing, (red right-angled arrows) transcriptional start sites (TSSs), (gray tubes) nonexonic chromatin. (B) RNA-based duplication (termed retroposition or retroduplication). New retroposed gene copies may arise through the reverse transcription of messenger RNAs (mRNAs) from parental source genes. Functional retrogenes with new functional properties may evolve from these copies after acquisition or evolution of promoters in their 5′ flanking regions that may drive their transcription. (Pink right-angled arrow) TSS, (transparent pink box) additionally transcribed flanking sequence at the insertion site.
