Therman E, Kuhn E M
Cancer Genet Cytogenet. 1985 Feb 15;15(3-4):293-301. doi: 10.1016/0165-4608(85)90173-6.
The incidence and origin of dicentric chromosomes has been analyzed in Bloom's syndrome (BS) lymphocytes. In diploid cells, the dicentrics are predominantly asymmetric, consisting of two nonhomologous chromosomes. In contrast, in tetraploid cells, the majority of dicentrics are formed by two homologs, which, judging from Q-banded metaphases, have broken at identical points. Our earlier assumption that most, possibly all, chromosome abnormalities in BS originate in S-G2 is further supported by the present observation that only 26/79 cells with dicentrics also had a fragment. In other words, the dicentrics have not arisen in the previous G1 but in S-G2 of a preceding cell cycle. The symmetric dicentrics would arise in diploid cells from an adjacent counterpart to a mitotic chiasma between two homologs, and in tetraploid cells, from an adjacent exchange between two sister chromosomes in a diplochromosome. The asymmetric dicentrics can be assumed to result primarily from segregation of an adjacent QR between two nonhomologous chromosomes. Based on the frequency of symmetric dicentrics in tetraploid cells (25/61 cells), and assuming that the ratio of adjacent exchanges to mitotic chiasmata is around 1/50, we predict that the chiasma frequency in diplochromosomes of BS lymphocytes will turn out to be as high as 20/cell.
在布卢姆综合征（BS）淋巴细胞中分析了双着丝粒染色体的发生率和起源。在二倍体细胞中，双着丝粒主要是不对称的，由两条非同源染色体组成。相比之下，在四倍体细胞中，大多数双着丝粒是由两条同源染色体形成的，从Q带中期判断，它们在相同的点断裂。我们早期的假设，即BS中大多数（可能是所有）染色体异常起源于S-G2期，进一步得到了目前观察结果的支持，即只有26/79个有双着丝粒的细胞也有一个片段。换句话说，双着丝粒不是在前一个G1期出现的，而是在前一个细胞周期的S-G2期出现的。对称双着丝粒将在二倍体细胞中由两条同源染色体之间有丝分裂交叉的相邻对应物产生，而在四倍体细胞中，由双染色体中两条姐妹染色体之间的相邻交换产生。不对称双着丝粒可以假定主要是由两条非同源染色体之间相邻的QR分离产生的。根据四倍体细胞中对称双着丝粒的频率（25/61个细胞），并假设相邻交换与有丝分裂交叉的比率约为1/50，我们预测BS淋巴细胞双染色体中的交叉频率将高达20/细胞。
