Kato H
Chromosoma. 1977 Feb 3;59(3):179-91. doi: 10.1007/BF00292776.
By taking advantage of the fact that fluorescent light (FL) induces strand breaks only in bromodeoxyuridine(BrdUy-substituted DNA, and that those breaks eventually lead to the formation of sister chromatid exchanges (SCEs), the response of SCEs to FL was studied carefully in Chinese hamster chromosomes in which, out of four DNA strands, BrdU-substitution had occurred either in one or three strands. The FL-induced SCE frequency did not differ greatly between these two types of chromosomes. However, when they were submitted to caffeine treatment, a drastic increase in the frequency was detected in the trifilarly-substituted chromosomes while a significant decrease occurred in the unifilarly-substituted chromosomes. Based on these results, a working hypothesis was developed that the SCE can arise by at least two different mechanisms, one operating at replicating points probably utilizing the machinery of DNA replication, and the other acting only in the post-replicational DNA portion, probably in a similar fashion as assumed in a general model of crossing over in the eukaryote. These dual mechansims may account for the discrepancy encountered in the explanations of the induction of SCEs by various exogenous agents as well as spontaneous SCEs. The present study also showed that some, but clearly not all, of chromatid deletions are the outcome of the failure to complete SCEs arising through these mechanisms.
利用荧光（FL）仅在溴脱氧尿苷（BrdU）取代的DNA中诱导链断裂这一事实，并且这些断裂最终导致姐妹染色单体交换（SCE）的形成，对中国仓鼠染色体中SCE对FL的反应进行了仔细研究。在四条DNA链中，BrdU取代发生在一条或三条链上。这两种类型的染色体之间，FL诱导的SCE频率没有很大差异。然而，当它们接受咖啡因处理时，三线取代染色体中的频率急剧增加，而单线取代染色体中的频率则显著下降。基于这些结果，提出了一个工作假设，即SCE可能通过至少两种不同的机制产生，一种在复制点起作用，可能利用DNA复制机制，另一种仅在复制后DNA部分起作用，可能与真核生物中一般交叉模型所假设的方式类似。这些双重机制可能解释了在各种外源性因素以及自发SCE诱导解释中遇到的差异。本研究还表明，一些但显然不是所有的染色单体缺失是这些机制产生的SCE未能完成的结果。
