In genetics, an isochore is a large region of DNA (greater than 300 KB) with a high degree uniformity in G-C and C-G (collectively GC) which tends to have more genes, higher local melting or denaturation temperatures, and different flexibility. Overall, isochores are largely homogeneous in GC content in contrast to the heterogeneity of the entire genome.
There are five families of isochores ranging in overall GC content:
In vertebrates, gene expression is tissue-dependent and the genome is heterogeneous in terms of GC content. Bernardi et.al (1985,1988) have shown that vertebrate genomes are a mosaic composed of GC-rich regions and GC-poor regions, and some GC-rich regions have a GC content of about 60% and some GC-poor regions about 30%. Each of these GC-rich or GC-poor regions may be as long as 300kb and contain many functional genes. These GC-rich or GC-poor regions are called isochores.
There are four major groups of isochores (two GC-rich and two GC-poor isochores) in warm-blooded vertebrates such as mammals and birds, but in cold-blooded vertebrates GC-rich isochores are rare or nearly absent. The boundary between the GC-rich and GC-poor isochores is known to be quite narrow (Ikemura and Aota 1988).
The origin of isochores in vertebrates has been a subject of controversy, and no consensus has been reached at the present time. The reader who is interested in this subject may refer to Wolfe et al. (1989), Holmquist and Filipski (1994) and Bernardi (1995). However, it is important to note that the genes located in different isochores are expected to have different patterns of codon usage biases and since codon usage bias affects the rate of nucleotide substitution (Shields et al. 1988; Sharp et al. 1989), they may evolve at different rates.
Isochores have been described using CsCl ultracentrifugation.[1]