Another way to create new genes is by using pre-made modules. Segments from two or more genes may be fused together by DNA rearrangements so generating a novel gene consisting of regions derived from several sources (Fig. 20.14A). An example of the formation of a new gene from several diverse components is the LDL receptor (Fig. 20.14B). LDL is low-density lipoprotein that carries cholesterol around in the blood. The LDL receptor is found on the surface of cells that take up LDL. The gene for the LDL receptor consists of several regions, two of which are derived from other genes. Towards the front are seven repeats of a sequence also appearing in the C9 factor of complement, an immune system protein. Farther along is a segment related to part of epidermal growth factor (a hormone). When such a gene mosaic is transcribed and translated, we get a patchwork protein consisting of several different domains.
The sequences of different genes and proteins evolve at different rates. Less critical sequences are free to evolve faster.
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