Fig. 4.

Computational analysis of intron size and alternative splicing in the Drosophila and human genomes. (A and B) Every internal exon within the human or the Drosophila genomes is displayed as a function of the nucleotide length of its upstream (x axis) and downstream (y axis) introns. Each point within the scatter plots represents a unique exon. The x and y axes are shown in log scale. (A) Exon profile of the human genome. The majority of introns are long; however, ≈25% of exons are flanked by at least one short intron. The vertical and horizontal lines demarcate the experimentally determined 200- to 250-nt transition from cross-intron to cross-exon recognition. (B) Exon profile of the Drosophila genome. The majority (>85%) of exons are flanked by at least one short intron. (C–F) Color diagrams displaying the probability of an exon's undergoing alternative splicing as a function of the length of its flanking introns. The upstream and downstream intron length increases exponentially along the x and y axes. The color scale (Right) represents the fold increase in the probability of an exon's being alternatively spliced relative to the probability calculated for exons that are flanked by introns <225 nt. (C) Probability diagram for exon skipping within the Drosophila genome. (D) Drosophila alternative 5′ or 3′ splice-site usage. (E) Human exon skipping. (F) Human alternative 5′ or 3′ splice-site usage.