Mullinger A M, Johnson R T
J Cell Sci. 1980 Dec;46:61-86. doi: 10.1242/jcs.46.1.61.
Electron microscopy of HeLa metaphase nucleoids (i.e. whole metaphase cells exposed to 2 M salt and non-ionic detergent) spread by the Kleinschmidt technique, reveals a variety of protein-depleted structures (spreads) derived from chromosomes. Spreads vary in size and shape. At one extreme are oval structures with one or more cores surrounded by a network of supercoiled fibres. These fibres are probably arranged as loops and we estimate that 600-1000 may emerge from a single, large core region. At the other extreme are chromosome-shaped spreads with an elongated core which takes the form of a multifibred axis. At intervals groups of lateral fibres appear to emerge from each axis to produce the network. Spreads intermediate between these extremes occur in which axial fibres can be resolved in only part of the elongated core. Similar structures are observed in chromosomes deproteinized and spread after isolation by a procedure which preserves high molecular weight DNA. The appearance of chromosomes isolated by the Wray-Stubblefield hexylene glycol procedure agrees in general with previous findings of others, except that in some more extended spreads axial fibres are visible. We believe our observations are consistent with the idea that the chromonema of each metaphase chromatid contains regions of multistranded DNA. We do not propose, however, that the chromatid is functionally multineme, but rather that axial fibre folding either within or between chromomere regions contributes to packing of DNA in the metaphase chromosome. These regions of constraint re also postulated as the locations of emergence of lateral loops. In spread preparations the axial fibres are seen clearly only when chromatids have been elongated beyond the contracted metaphase length. Elongation would be produced both by relaxation of chromosomal coils (gyres) and by extension of the chromonema upon deproteinization and spreading. Whereas in deproteinized nucleoids the long axis is liable to elongate, fragment or collapse, the chromomena of Wray-Stubblefield chromosomes is locked at the metaphase (gyred) length and axial fibres are generally not visible. We propose that the assembly of the complex DNA axis of the metaphase chromosome from its extended interphase counterpart plays a major part in increasing the DNA packing ratio in the mitotic cell.
用克莱因施密特技术铺展的海拉中期类核（即暴露于2M盐和非离子去污剂的整个中期细胞）的电子显微镜观察显示，有多种源自染色体的蛋白质缺失结构（铺展物）。铺展物的大小和形状各不相同。一端是椭圆形结构，有一个或多个核心，被超螺旋纤维网络包围。这些纤维可能排列成环，我们估计600 - 1000个可能从单个大的核心区域发出。另一端是染色体形状的铺展物，有一个细长的核心，呈多纤维轴的形式。每隔一段距离，横向纤维组似乎从每个轴上发出形成网络。在这些极端之间出现中间型铺展物，其中轴向纤维仅在细长核心的一部分中可分辨。在用保留高分子量DNA的方法分离后去蛋白并铺展的染色体中观察到类似结构。用雷 - 斯塔布菲尔德己二醇法分离的染色体的外观总体上与其他人先前的发现一致，只是在一些更伸展的铺展物中可见轴向纤维。我们相信我们的观察结果与每个中期染色单体的染色线包含多链DNA区域的观点一致。然而，我们并不认为染色单体在功能上是多线的，而是认为染色粒区域内或之间的轴向纤维折叠有助于中期染色体中DNA的包装。这些约束区域也被假定为侧环出现的位置。在铺展制剂中，只有当染色单体伸长超过收缩的中期长度时，轴向纤维才能清晰可见。伸长将通过染色体螺旋（回旋）的松弛以及去蛋白和铺展时染色线的延伸而产生。虽然在去蛋白的类核中长轴容易伸长、断裂或塌陷，但雷 - 斯塔布菲尔德染色体的染色线锁定在中期（回旋）长度，轴向纤维通常不可见。我们提出，从中期染色体的延伸间期对应物组装复杂的DNA轴在增加有丝分裂细胞中的DNA包装比方面起主要作用。
