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In genetics, an isochore is a large region of DNA (greater than 300 kb) with a high degree uniformity in guanine (G) and cytosine (C): G- C and C-G (collectively GC content). Bernardi and colleagues first uncovered the compositional non-uniformity within vertebrate genomes using thermal melting and density gradient centrifugation. The DNA fragments extracted by the gradient centrifugation were later termed "isochores", which was subsequently defined as "very long (much greater than 200 KB) DNA segments" that "are fairly homogeneous in base composition and belong to a small number of major classes distinguished by differences in guanine-cytosine (GC) content".〔 Subsequently, the isochores "grew" and were claimed to be ">300 kb in size." The theory proposed that isochore’s composition varied markedly between "warm-blooded" (homeotherm) vertebrates and "cold-blooded" (poikilotherm) vertebrates〔 and later became known as the isochore theory. == The thermodynamic stability hypothesis == The isochore theory purported that the genome of "warm-blooded" vertebrates (mammals and birds) are mosaics of long isochoric regions of alternating GC-poor and GC-rich composition, as opposed to the genome of "cold-blooded" vertebrates (fishes and amphibians) that were supposed to lack GC-rich isochores.〔 These findings were explained by the thermodynamic stability hypothesis, attributing genomic structure to body temperature. GC-rich isochores were purported to be a form of adaptation to environmental pressures, as an increase in genomic GC-content could protect DNA, RNA, and proteins from degradation by heat. 〔〔 Despite its attractive simplicity, the thermodynamic stability hypothesis has been repeatedly shown to be in error . Many authors showed the absence of a relationship between temperature and GC-content in vertebrates,〔〔 while others showed the existence of GC-rich domains in "cold-blooded" vertebrates such as crocodiles, amphibians, and fish.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isochore (genetics)」の詳細全文を読む スポンサード リンク
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