Compositional overview of human chromosomes and their GC levels. The color-coded map shows 100-kb moving window plots using the program draw_chromosome_gc.pl (Pačes et al. 2004) (http://genomat.img.cas.cz). The color code spans the spectrum of GC levels in five steps, indicated by broken horizontal lines, from ultramarine blue (GC-poorest L1 isochores) to scarlet red (GC-richest H3 isochores). Gray vertical lines correspond to the few gaps still present in the sequences, gray vertical regions to the centromeres. Figure 1 differs from a previous, similar figure (Pavliček et al. 2002) in that it is based on the May 2004 UCSC release (Kent et al. 2002) of the finished sequence (International Human Genome Sequencing Consortium 2004) (http://genome.ucsc.edu) and lacks the 5000 large gaps of the original draft sequence (Lander et al. 2001) that would not have allowed the present work. The isochore family borders as defined here (see color bar and main text) are slightly different from the previous ones in the case of the L2/H1 and the H2/H3 borders. Several characteristic features of isochores are already visible in this plot, such as the larger extent of the GC-poor compared to GC-rich regions, and the higher compositional heterogeneity of the GC-rich compared to the GC-poor regions.
