CHROMOSOMES AS INTERDEPENDENT ACCOUNTING UNITS: THE ASSIGNED ORIENTATION OFC. ELEGANSCHROMOSOMES MINIMIZES THE TOTAL W-BASE CHARGAFF DIFFERENCE

Abstract

DNAs of individual chromosomes violate, albeit perhaps by only one in a thousand bases, Chargaff's second parity rule, which is that Chargaff's first parity rule for duplex DNA (A = T, G = C) applies, to a close approximation, to single stranded DNA. If the "top" strand of one chromosome has A > T and the "top" strand of another has T > A, can they complement to approach even parity (A = T)? Assignment of orientation to the six chromosomes of Caenorhabditis elegans is said to have been arbitrary and, of 26 (= 64) possible combinations of top (T) and bottom (B) strands, the GenBank orientation (designated "TTTTTT") is but one. Yet, for the W bases (A and T) the chromosomes in the GenBank orientation complement to reduce the Chargaff difference (A–T) to only 200 bases (i.e. only one in 323,658 bases does not have a potential Watson-Crick pairing partner). This suggests that the assignment was not arbitrary. However, the GenBank orientation for the S bases (G and C) allows an approach to even parity less well than many other orientations, the best of which is BBBBTT (indicating a disparity between the GenBank orientations of the first four autosomes and those of chromosomes V and X). Although only the euchromatic regions of Drosophila melanogaster chromosomes have been sequenced, there are orientations that allow an approach to even parity. We conclude that, with respect to their Chargaff differences, the chromosomes of C. elegans have the potential to engage in interdependent base accounting. Since this might also apply to D. melanogaster, even when heterochromatin-associated DNA rich in tandem repeats (microsatellite DNA) is excluded, then heterochromatic DNA might not normally participate in the hypothetical accounting process.

The common origin of symmetry and structure in genetic sequences

“…4, show that not only the hierarchy is present, but also that the scales L D , L S , and L M are comparable across chromosomes. This remarkable similarity (see also [37][38][39] ) suggests that some of the mechanisms shaping simultaneously structure and symmetry work similarly in every chromosomes and/or act across them (e.g.chromosome rearrangements mediated by transposable elements). This, and the scales of L D , L S , and L M , provide a hint on the origin of our observations, which we explore below through the proposal of a minimal model.…”