Maintenance and evolution of repeated genes in eukaryotes

Abstract

The genes for 5S RNA and ribosomal RNA are highly repetitive and occur as tandemly repeated clusters within eukaryotic genomes. These genes are separated by so-called spacer sequences which are not transcribed. Two main theories have been proposed to explain the mechanism by which tandemly arranged sequences are kept relatively homogeneous and how the genes and their spacers have evolved together. At the one extreme, are the sudden correction mechanisms such as Callan's "Master-Slave" hypothesis in which many genes are simultaneously corrected against a master template. At the other extreme, gradual correction mechanisms have been proposed which involve unequal crossing-over between homologous chromatids in such a way that variants could be spread or eliminated from tandem genes. The spacers of the genes for 5S DNA and rDNA in Xenopus laevis have now been subjected to elegant molecular analysis and this has cast some light on these problems.

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