Evolution of chromosome bands: Molecular ecology of noncoding DNA |
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Authors: | Gerald P Holmquist |
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Institution: | (1) Department of Biology, Beckman Research Institute of the City of Hope, 1450 E. Duarte Rd., 91010 Duarte, California, USA |
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Abstract: | Summary Giemsa dark bands, G-bands, are a derived chromatin character that evolved along the chromosomes of early chordates. They
are facultative heterochromatin reflecting acquisition of a late replication mechanism to repress tissue-specific genes. Subsequently,
R-bands, the primitive chromatin state, became directionally GC rich as evidenced by Q-banding of mammalian and avian chromosomes.
Contrary to predictions from the neutral mutation theory, noncoding DNA is positionally constrained along the banding pattern
with short interspersed repeats in R-bands and long interspersed repeats in G-bands. Chromosomes seem dynamically stable:
the banding pattern and gene arrangement along several human and murine autosomes has remained constant for 100 million years,
whereas much of the noncoding DNA, especially retroposons, has changed. Several coding sequence attributes and probably mutation
rates are determined more by where a gene lives than by what it does. R-band exons in homeotherms but not G-band exons have
directionally acquired GC-rich wobble bases and the corresponding codon usage: CpG islands in mammals are specific to R-band
exons, exons not facultatively heterochromatinized, and are independent of the tissue expression pattern of the gene. The
dynamic organization of noncoding DNA suggests a feedback loop that could influence codon usage and stabilize the chromosome’s
chromatin pattern: DNA sequences determine affinities of → proteins that together form → a chromatin that modulates → rate
constants for DNA modification that determine → DNA sequences. Theories of hierarchical selection and molecular ecology show
how selection can act on Darwinian units of noncoding DNA at the genome level thus creating positionally constrained DNA and
contributing minimal genetic load at the individual level.
Presented in part at the FEBS Symposium on Genome Organization and Evolution, held in Crete, Greece, September 1–5, 1986 |
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Keywords: | Hierarchical selection Chromosome bands Base composition isochores Natural selection Replication time Retroposons Neutral mutation theory |
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