Investing in distant rewards Press, William H
Science (American Association for the Advancement of Science),
2013-Feb-08, 2013-02-08, 20130208, Letnik:
339, Številka:
6120
Journal Article
Recenzirano
Odprti dostop
Would you spend money today to make the world a substantially better place for your children and grandchildren? Most of us would. But what if the benefit would accrue only to your ...great-great-great-great-grandchildren, not born until the 22nd century? That's an awfully distant time horizon for most people. Many would probably spend today's resources on more immediate concerns.
The genomes of mammals and birds can be partitioned into megabase-long regions, termed isochores, with consistently high, or low, average C + G content. Isochores with high CG contain a mixture of ...CG-rich and AT-rich genes, while high-AT isochores contain predominantly AT-rich genes. The two gene populations in the high-CG isochores are functionally distinguishable by statistical analysis of their gene ontology categories. However, the aggregate of the two populations in CG isochores is not statistically distinct from AT-rich genes in AT isochores. Genes tend to be located at local extrema of composition within the isochores, indicating that the CG-enriching mechanism acted differently when near to genes. On the other hand, maximum-likelihood reconstruction of molecular phylogenetic trees shows that branch lengths (evolutionary distances) for third codon positions in CG-rich genes are not substantially larger than those for AT-rich genes. In the context of neutral mutation theory this argues against any strong positive selection. Disparate features of isochores might be explained by a model in which about half of all genes functionally require AT richness, while, in warm-blooded organisms, about half the genome (in large coherent blocks) acquired a strong bias for mutations to CG. Using mutations in CG-rich genes as convenient indicators, we show that approximately 20% of amino acids in proteins are broadly substitutable, without regard to chemical similarity.
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