McCready S J, Godwin J, Mason D W, Brazell I A, Cook P R
J Cell Sci. 1980 Dec;46:365-86. doi: 10.1242/jcs.46.1.365.
Structures resembling nuclei are released when HeLa cells are lysed in a detergent and 2 M salt. These nucleoids, which lack any organized membrane structure, contain all the nuclear DNA packaged within a cage of RNA and protein. Their DNA is supercoiled so that the linear DNA must remain unbroken and looped during lysis. Following digestion with the restriction endonuclease, EcoRI, cages and associated DNA were filtered free of unattached DNA. Pulse-labelled (i.e. newly synthesized) DNA remains preferentially associated with the cages. This association has been confirmed by autoradiography. When nucleoids are prepared for electron microscopy by the Kleinschmidt procedure the DNA spills out to form a skirt around the flattened cage. Labelling, which is restricted to the region of the cage after short pulses, extends out into the skirt as the labelling time increases. A model, based on the premise that replication takes place at the nuclear cage, is presented in the Appendix. The results of the biochemical experiments and electron microscopy both indicate that the average size of the unit of replication is approximately 2 micrometer. This is about one-quarter the size of the average structural unit - the loop. Therefore sequences in the loop must become attached to the nuclear cage prior to the initiation of DNA synthesis.
当海拉细胞在去污剂和2M盐中裂解时，会释放出类似细胞核的结构。这些核小体缺乏任何有组织的膜结构，包含包装在RNA和蛋白质笼子内的所有核DNA。它们的DNA是超螺旋的，因此线性DNA在裂解过程中必须保持完整且成环。用限制性内切酶EcoRI消化后，将笼子和相关DNA过滤，去除未附着的DNA。脉冲标记（即新合成）的DNA优先与笼子结合。这种结合已通过放射自显影得到证实。当通过克莱因施密特方法制备用于电子显微镜观察的核小体时，DNA会溢出，在扁平的笼子周围形成一条裙边。短脉冲后标记仅限于笼子区域，随着标记时间增加，标记会延伸到裙边。附录中提出了一个基于复制发生在核笼子这一前提的模型。生化实验和电子显微镜观察结果均表明，复制单位的平均大小约为2微米。这大约是平均结构单位——环大小的四分之一。因此，环中的序列在DNA合成起始之前必须附着到核笼子上。
