High-level organization of isochores into gigantic superstructures in the human genome

Human DNA shows a complex structure with compositional features at many scales; the isochores-long DNA segments (similar to 10(5) bp) of relatively homogeneous guanine-cytosine (G + C) content-are the largest well-documented and well-analyzed compositional structures. However, we report here on the existence of a high-level compositional organization of isochores in the human genome. By using a segmentation algorithm incorporating the long-range correlations existing in human DNA, we find that every chromosome is composed of a few huge segments (similar to 10(7) bp) of relatively homogeneous G + C content, which become the largest compositional organization of the genome. Finally, we show evidence of the biological relevance of these superstructures, pointing to a large-scale functional organization of the human genome.