title: Evolutionary implications of inversions that have caused intra-strand parity in DNA.
abstract: BACKGROUND
  Chargaff's rule of DNA base composition, stating that DNA comprises equal amounts of adenine and thymine (%A = %T) and of guanine and cytosine (%C = %G), is well known because it was fundamental to the conception of the Watson-Crick model of DNA structure. His second parity rule stating that the base proportions of double-stranded DNA are also reflected in single-stranded DNA (%A = %T, %C = %G) is more obscure, likely because its biological basis and significance are still unresolved. Within each strand, the symmetry of single nucleotide composition extends even further, being demonstrated in the balance of di-, tri-, and multi-nucleotides with their respective complementary oligonucleotides.
  
  
  RESULTS
  Here, we propose that inversions are sufficient to account for the symmetry within each single-stranded DNA. Human mitochondrial DNA does not demonstrate such intra-strand parity, and we consider how its different functional drivers may relate to our theory. This concept is supported by the recent observation that inversions occur frequently.
  
  
  CONCLUSIONS
  Along with chromosomal duplications, inversions must have been shaping the architecture of genomes since the origin of life.
authors.affiliation: The Centre for Applied Genomics, Program in Genetics and Genome Biology, The Hospital for Sick Children, MaRS Centre, Toronto, Ontario, Canada. kohji@genet.sickkids.on.ca <kohji@genet.sickkids.on.ca>
authors.forename: Kohji
authors.initials: K
authors.lastname: Okamura
authors.forename: John
authors.initials: J
authors.lastname: Wei
authors.forename: Stephen W
authors.initials: SW
authors.lastname: Scherer
mesh_terms: D001482:Base Composition; D005075:Biological Evolution; D007446:Chromosome Inversion; D004247:DNA / Q000737:chemistry* / Q000235:genetics; D004272:DNA, Mitochondrial / Q000737:chemistry / Q000235:genetics; D020440:Gene Duplication; D023281:Genomics
chemical_list: D004272:DNA, Mitochondrial; D004247:DNA