Figure 3.

Analysis of GC content distribution in human introns with different isochoric location. Isochore definition is as described in [20]. (A) Scatter plot and loess fitting of intron size and GC content in light (blue) and heavy (red) isochores. (B) Analysis of GC₂₀₀ (see text). GC₂₀₀ significantly increases or decreases with residual size (percentile classes are shown) for introns located in heavy (red; breaks in bp = 681, 934, 1309, 1960, 3665) or light (blue; breaks in bp = 810, 1181, 1714, 2638, 5476) isochores, respectively (Kruskall Wallis Test, p = 1.3 × 10^-34 and 7.9 × 10^-7, respectively). The number of introns in each size class amounted to 2490 and 1567 for heavy and light isochores, respectively. (C) Distributions of within-gene correlation coefficients. For each gene having more than 15 introns (n = 500 and 1021 for light and heavy isochores, respectively) we calculated correlation coefficients between masked GC content and residual size. Hatched and dotted lines represent envelopes (1^st and 99^th percentiles, respectively) of correlation coefficient distributions obtained by randomization. (D) Scatter plot and loess fits of GC content over intron size (log₁₀ values) for introns (upper panel) and pseudointrons (lower panel). Spearman correlation coefficients (rho) are also shown (all p values were < 0.01). Introns and pseudointrons were divided on the basis of their isochoric location: blue for light isochores (501 introns-pseudointrons pairs), red for heavy ones (926 pairs).