Relatively semi-conservative replication and a folded slippage model for simple sequence repeats

Simple sequence repeats (SSRs) are found ubiquitously in almost all genome, and their formation mechanism is ambiguous yet. Here, the SSRs were analyzed in 55 randomly selected segments of genomes from a fairly wide range of species, with introducing more open standard for extensively mining repeats. A high percentage of repeats were discovered in these segments, which is inconsistent with the current theory suggested that repeats tend to disappear over long-term evolution. Therefore, a mechanism is most probably responsible for continually producing repeats during replication to balance continuous repeat disappearance, which may makes the replicating process relatively semi-conservative. To improve the current straight-line slippage model, we proposed a folded slippage model involving the geometric space of nucleotides and hydrogen bond stability to explain the high-percent SSR occurrence, which can describe SSR expansion and contraction more reasonably. And analysis of external forces in the folding template strands suggested that the microsatellites tend to expand than contract. Our research may provide implements for contributions of microsatellites to genome evolution and complement semi-conservative replication.