Fletcher H L, Rafferty J A
Division of Genetic Engineering, School of Biology and Biochemistry, Queens University of Belfast.
J Theor Biol. 1993 Oct 21;164(4):507-14. doi: 10.1006/jtbi.1993.1169.
Models of homogenization of repeated sequences by unequal sister chromatid exchange (USCE) assume that no significant changes in array length occur. The effects of successive USCEs on the lengths of arrays of repeated sequences has been examined mathematically and by simulation, assuming misalignment to be greater in longer arrays, a condition necessary for homogenization. The series of duplications and deletions gives a wide, asymmetric variation in copy number. Frequencies follow a binomial distribution, but if misalignment increases with array length, lengths become logarithmically distributed, most arrays being shorter than the original, counterbalanced by a few very long arrays. The median length, which is also the modal length, decreases exponentially or asymptotically. Given a proportional misalignment of a, the median (mode) decreases by a2/2 per unequal crossover. Since drift is a random transmission of available alleles in the population, it follows that fixation of common short arrays is much more probable than fixation of rare long arrays. This process will continue inexorably until each array is too short to undergo further unequal crossing over, and no more variation is generated. The number of unequal crossovers required by some published models of homogenization would almost certainly cause dramatic reduction in number, or complete loss, of repeats. Frequent unequal sister chromatid exchanges are not compatible with survival of arrays unless counteracted by an independent amplification mechanism or selection, so are unlikely to be important as a long-term homogenization mechanism in non-essential repeated sequences.
通过不等姐妹染色单体交换（USCE）实现重复序列同质化的模型假定阵列长度不会发生显著变化。已通过数学方法和模拟研究了连续的USCE对重复序列阵列长度的影响，假定在较长的阵列中错配更大，这是实现同质化的必要条件。一系列的重复和缺失会导致拷贝数出现广泛的、不对称的变化。频率遵循二项分布，但如果错配随阵列长度增加，长度会呈对数分布，大多数阵列比原始阵列短，由少数非常长的阵列来平衡。中位数长度也是众数长度，呈指数或渐近减少。给定比例错配为a，每次不等交换时中位数（众数）减少a2/2。由于漂变是群体中可用等位基因的随机传递，因此常见短阵列的固定比罕见长阵列的固定更有可能。这个过程将无情地持续下去，直到每个阵列短到无法再进行进一步的不等交换，不再产生更多变异。一些已发表的同质化模型所需的不等交换次数几乎肯定会导致重复序列数量急剧减少或完全丧失。频繁的不等姐妹染色单体交换与阵列的存活不兼容，除非有独立的扩增机制或选择来抵消，因此作为非必需重复序列的长期同质化机制不太可能重要。
