POSSIBLE ROLE OF NATURAL-SELECTION IN THE FORMATION OF TANDEM-REPETITIVE NONCODING DNA

A simulation model of sequence-dependent amplification, unequal crossi ng over and mutation is analyzed. This model predicts the spontaneous formation of tandem-repetitive patterns of noncoding DNA from arbitrar y sequences for a wide range of parameter values. Natural selection is found to play an essential role in this self-organizing process. Natu ral selection which is modeled as a mechanism for controlling the leng th of a nucleotide string but not the sequence itself favors the forma tion of tandem-repetitive structures. Two measures of sequence heterog eneity, inter-repeat variability and repeat length, are analyzed in de tail. For fixed mutation rate, both inter-repeat variability and repea t length are found to increase with decreasing rates of (unequal) cros sing over. The results are compared with data on micro-, mini- and sat ellite DNAs. The properties of minisatellites and satellite DNAs resem ble the simulated structures very closely. This suggests that unequal crossing over is a dominant long-range ordering force which keeps thes e arrays homogeneous even in regions of very low recombination rates, such as at satellite DNA loci. Our analysis also indicates that in reg ions of low rates of (unequal) crossing over, inter-repeat variability is maintained at a low level at the expense of much larger repeat uni ts (multimeric repeats), which are characteristic of satellite DNA. In contrast, the microsatellite data do not fit the proposed model well, suggesting that unequal crossing over does not act on these very shor t tandem arrays.