Naylor S L, Sakaguchi A Y, Schmickel R D, Woodworth-Gutai M, Shows T B
J Mol Appl Genet. 1983;2(2):137-46.
Although the human rDNA gene family is organized in clusters of tandem repeats on five pairs of acrocentric chromosomes, all rDNA genes have undergone a "concerted" evolution resulting in a homogeneous population of genes. Two steps are necessary for a variant to spread to all rDNA genes: dissemination of the variant to all genes in a cluster; and exchange of rDNA genes between nonhomologous chromosomes. To study the organization of rDNA genes, a restriction fragment length polymorphism in the spacer region adjacent to the 28S gene was examined in somatic cell hybrids in which individual human acrocentric chromosomes could be isolated. Human DNA cut with BamHI and analyzed by Southern hybridization yields two to four major bands that hybridize to a 32P-labeled cloned fragment of the 28S gene. Hybrids containing single human acrocentric chromosomes do not recapitulate the parental patterns, but frequently have only one of the parental bands. The data suggest that the quantitative distribution of spacer length variants differs among the human acrocentric chromosomes in hybrids. The frequently observed homogeneity of the rDNA variants on individual acrocentric chromosomes in hybrid cells may reflect the individual rDNA clusters in the parental cell or may be a result of unequal crossing over in the hybrid cell.
尽管人类核糖体DNA（rDNA）基因家族以串联重复序列的形式聚集在五对近端着丝粒染色体上，但所有rDNA基因都经历了“协同”进化，从而产生了基因的同质化群体。一个变异体要传播到所有rDNA基因需要两个步骤：变异体在一个簇中的所有基因间传播；以及rDNA基因在非同源染色体之间的交换。为了研究rDNA基因的组织情况，在能够分离出单个人类近端着丝粒染色体的体细胞杂种中，检测了与28S基因相邻的间隔区的限制性片段长度多态性。用BamHI切割人类DNA并通过Southern杂交分析，可产生两到四条主要条带，这些条带与用32P标记的28S基因克隆片段杂交。含有单个人类近端着丝粒染色体的杂种不能重现亲本模式，而是常常只有一条亲本条带。数据表明，杂种中人类近端着丝粒染色体之间间隔区长度变异体的定量分布有所不同。在杂种细胞中，单个近端着丝粒染色体上rDNA变异体经常观察到的同质性，可能反映了亲本细胞中的单个rDNA簇，也可能是杂种细胞中不等交换的结果。
