Levinson G, Gutman G A
Department of Microbiology and Molecular Genetics, University of California, Irvine 92717.
Mol Biol Evol. 1987 May;4(3):203-21. doi: 10.1093/oxfordjournals.molbev.a040442.
Simple repetitive DNA sequences are a widespread and abundant feature of genomic DNA. The following several features characterize such sequences: (1) they typically consist of a variety of repeated motifs of 1-10 bases--but may include much larger repeats as well; (2) larger repeat units often include shorter ones within them; (3) long polypyrimidine and poly-CA tracts are often found; and (4) tandem arrangements of closely related motifs are often found. We propose that slipped-strand mispairing events, in concert with unequal crossing-over, can readily account for all of these features. The frequent occurrence of long tandem repeats of particular motifs (polypyrimidine and poly-CA tracts) appears to result from nonrandom patterns of nucleotide substitution. We argue that the intrahelical process of slipped-strand mispairing is much more likely to be the major factor in the initial expansion of short repeated motifs and that, after initial expansion, simple tandem repeats may be predisposed to further expansion by unequal crossing-over or other interhelical events because of their propensity to mispair. Evidence is presented that single-base repeats (the shortest possible motifs) are represented by longer runs in mammalian introns than would be expected on a random basis, supporting the idea that SSM may be a ubiquitous force in the evolution of the eukaryotic genome. Simple repetitive sequences may therefore represent a natural ground state of DNA unselected for coding functions.
简单重复DNA序列是基因组DNA广泛且丰富的特征。以下几个特征可表征此类序列：（1）它们通常由1至10个碱基的各种重复基序组成，但也可能包括更大的重复序列；（2）较大的重复单元通常包含较短的重复单元；（3）经常发现长的聚嘧啶和聚CA序列；（4）经常发现密切相关基序的串联排列。我们提出，滑链错配事件与不等交换协同作用，能够很容易地解释所有这些特征。特定基序（聚嘧啶和聚CA序列）长串联重复的频繁出现似乎源于核苷酸取代的非随机模式。我们认为，滑链错配的螺旋内过程更有可能是短重复基序初始扩展的主要因素，并且在初始扩展之后，由于其错配倾向，简单串联重复序列可能因不等交换或其他螺旋间事件而易于进一步扩展。有证据表明，单碱基重复序列（可能最短的基序）在哺乳动物内含子中的连续长度比随机情况下预期的更长，这支持了滑链错配可能是真核基因组进化中普遍存在的力量这一观点。因此，简单重复序列可能代表了未被选择用于编码功能的DNA的自然基态。
