Strand Symmetry: Characteristics and Origins

In order to reveal the genetic information contained in the primordial genome (defined as the most primitive nucleic acid genome for Earth's life), one of the possible means is to search for primitive traits or relics remained in modern genomes. As the shorter is a sequence, the less probable it would be modified during genome evolution. For that reason, some characteristics of very short nucleotide sequences would have considerable chances to persist during billions of years of evolution. Consequently, conservation of certain genomic features of mononucleotides, dinucleotides, and higher-order oligonucleotides across various genomes may exist; some, if not all, of these features would be relics of the primordial genome. Based on this assumption, we analyzed the pattern of frequencies of mononucleotides, dinucleotides, and higher-order oligonucleotides of the whole-genome sequences from 458 species (including archaea, bacteria, and eukaryotes) to study the phenomenon of strand symmetry. The results show that strand symmetry is a ubiquitous and explicit phenomenon. Considering and evaluating various views on the origins of the phenomenon of strand symmetry, we propose a new hypothesis about its origin as well as about the constitution of early genomes. We conclude that the phenomenon of strand symmetry would be the original feature and relic of the primordial genome.