Origin of Split Gene Structure Figure 6. Frequency distributions of the lengths of ORFs, exons, and coding-sequence of genes from different genomes. The frequency distributions of the lengths of exons and the complete coding sequence of genes and the ORFs from the genomes of H. sapiens (A), A. thaliana (B), C. elegans (C), D. melanogaster (D), P. falciparum (E), and combined fungal genomes of S. pombe and S. cerevisia (F), and the lengths of the coding sequence of genes and ORFs from the combined prokaryotic genomes of E. coli K12 and S. pneumoniae R6 (G) were plotted. Frequencies of every 100 consecutive ORF lengths were binned. The frequencies of the TCSs are shown on the left Y-axis and the frequencies of the ORFs and exons are shown on the right Y-axis. In order to magnify the frequencies within the region of non-randomness, the frequencies above certain threshold values (250 and 500 respectively for left and right Y-axes) are not shown, as the frequencies of only ORFs/exons of very short lengths exhibit such large frequencies (Figure 2). doi:10.1371/journal.pone.0003456.g006 random nucleotide sequence, and the possibility that split coding genes may occur intrinsically within a random genetic sequence, this approach may explain the major characteristics underlying the split-structure of eukaryotic genes. The current study with complete genome sequences strongly corroborates this fundamen- tal theme, and brings out novel insights concerning the randomness of the genome sequences, and the inter-relations between the intron-rich and intron-poor genomes. It shows that the vast majority of genomic sequences (.99%) in all the genomes examined here exhibit random characteristics, and that there exists only a tiny fraction of the ORFs that do not conform to the random model. It is striking that this tiny fraction of ORFs in a genome corresponds and correlates with the fraction of all the long, non-conforming exons in each of the genomes. Although the current results indicate that the probability of long ORFs occurring in random DNA is small, we found that the smaller genomes of the simpler organisms had a higher frequency of non-conforming ORFs and exons than the larger genomes of the more complex organisms. The study indicates that, because the ORFs and exons are highly restricted in length in a random PLoS ONE | www.plosone.org 8 October 2008 | Volume 3 | Issue 10 | e3456