Minimal repeats are ubiquitous sites of crossover and recombination across the human genome.

backgroundCrossover and recombination create genetic diversity that reflects differences in the DNA sequences of different organisms. We previously reported that trinucleotide 2-repeat units (T2Us) are sites of crossover and consequent colonization, which are massively spread and shared across the genomes of human and several other primates. These sites underscore the preference for AT- over CG-rich sequences as recombination sites.

methodsWe extended our study to simpler repeat cores, consisting of AT/TA and CG/GC dinucleotides. An algorithm was designed to extract the genomic regions with a higher probability of recombination. To this end, we hypothesized that dinucleotide 3-repeat units (D3Us) are, at least in part, the basic overlapping units resulting from unequal crossover between dinucleotide 2-repeat units (D2Us). We mapped TATATA, ATATAT, CGCGCG, and GCGCGC across the human genome and analyzed their colonization (the distance between consecutive D3Us < 500 bp). We also studied several randomly selected colonies of diverse sizes in up to 100 vertebrate species using the UCSC and Ensembl Genome Browsers.

resultsWe found approximately four million AT/TA D3Us and one hundred thousand CG/GC D3Us across the human genome. The majority of these D3Us resided in colonies and spread ubiquitously along all chromosomes. AT/TA colonies were significantly larger and more intricate than CG/GC colonies. D2Us and D3Us were the primary sites of unequal crossover in these colonies, resulting in the emergence of primary recombinants (overlapping recombinants of D2Us/D3Us) and a vast repertoire of secondary recombinants (non-overlapping recombinants of D2Us/D3Us) and eventually, colonies of enormous intricacy and significance based on Poisson distribution. Intricacy was consistently detected across diverse colony sizes, from the smallest to the largest. The randomly selected colonies that were studied in other species were specific to or of their largest size in human.

conclusionWe report ubiquitous and intricate colonies, in which D2Us and D3Us were the primary sites of crossover and recombination. It is plausible that minimal repeats such as D2Us, D3Us, and T2Us mark recombination as a ubiquitous rule across the human genome. This phenomenon is likely to transform our understanding of the magnitude, biological, and evolutionary outcomes of crossover and recombination.