Isochore (genetics) - Principles of The Isochore Theory

Principles of The Isochore Theory

The isochore theory was the first to identify the nonuniformity of nucleotide composition within vertebrate genomes and predict that the genome of "warm-blooded" vertebrates such as mammals and birds are mosaic of isochores (Bernardi et al. 1985). The human genome, for example, was described as a mosaic of alternating low and high GC content isochores belonging to five compositional families, L1, L2, H1, H2, and H3, whose corresponding ranges of GC contents were said to be <38%, 38%-42%, 42%-47%, 47%-52%, and >52%, respectively.

The main predictions of the isochore theory are that:

• GC content of the third codon position (GC3) of protein coding genes is correlated

with the GC content of the isochores embedding the corresponding genes.

• The genome organization of warm-blooded vertebrates is a mosaic of mostly GC-rich isochores.
• Genome organization of cold-

blooded vertebrates is characterized by low GC content levels and lower compositional heterogeneity than warm-blooded vertebrates. Homogeneous domains do not reach the high GC levels attained by the genomes of warm-blooded vertebrates.