Abstract
Once a speciation process has initiated, other factors may replace (G+C)% as a barrier to reproduction (preventing intergenomic recombination between species). This leaves (G+C)% free to assume other roles, such as preventing intragenomic (e.g. intergenic) recombination within a species. Many organisms have “macroisochores,” defined as long segments of relatively uniform (G+C)% that are coinherited with specific sequences of bases. Isochores may facilitate gene duplication. Indeed, each gene has a “homostabilizing propensity” to maintain itself as a “microisochore” of relatively uniform (G+C)%. The protection, afforded by differences in (G+C)%, against inadvertent recombination, facilitates the duplication both of genes and of genomes (speciation). George Williams’ definition of a gene as a unit of recombination, rather than of function, is now seen to have a chemical basis. For organisms living in extreme environments (e.g. thermophiles), natural selection seems to influence values for (G+C)% less than for (A+G)%.
Genetic information is ‘written’ by a variation in sequence on the one hand, and the physical stability of the double-stranded structure is determined by the base composition on the other hand. … DNA is found to consist of a number of homostability regions which come from homogenous base sequences consisting of 500 base pairs or more. … Biologically, it is hard … to believe that such regional homostability originates in a fundamental characteristic of the genetic code itself. It is quite plausible … that the homostability region plays an important part somewhere in the biological process within which the DNA is closely related. If so, then the evolutionary selective force can be considered to have fixed such regions of DNA. From the size of the homostability region, recombination might be one possible process which is aided by it. In any case, the wobble bases [in codon third position s] must give the necessary redundancy to make a homostability region without spoiling the biological meaning of the genetic code.
Akiyoshi Wada et al. (1975) [1]
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Forsdyke, D.R. (2016). Homostability. In: Evolutionary Bioinformatics. Springer, Cham. https://doi.org/10.1007/978-3-319-28755-3_11
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