In genetics, an isochore is a large region of genomic DNA (greater than 300 kilobases) with a high degree of uniformity in GC content; that is, guanine (G) and cytosine (C) bases.
Bernardi and colleagues first noticed the compositional non-uniformity of vertebrate genomes using thermal melting and density gradient centrifugation.
[1] [2] [3] The DNA fragments extracted by the gradient centrifugation were later termed "isochores",[4] 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".
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.
The first view was that isochores reflect variable mutation processes among genomic regions consistent with the neutral model.
[41] Isochores are detected manually by visual inspection of GC content curves ,[42] however because this approach lacks scientific merit and is difficult to replicate by independent groups, the findings remain disputed.
[35] The theory defines "compositional domains" as genomic regions with distinct GC-contents as determined by a computational segmentation algorithm.
[35] Since its inception the theory received wide attention and was extensively used to explain findings emerging from over dozen new genome sequencing studies.