MITOCHONDRIAL DNA - ADVANCES PROBLEMS AND GOALS
MITOCHONDRIAL DNA - ADVANCES PROBLEMS AND GOALS
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DOI：
10.1126/science.165.3888.25
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发表时间：
1969-01-01
期刊：
影响因子：
56.9
通讯作者：
NASS, MMK
中科院分区：
文献类型：
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作者：
NASS, MMK
Fig. 1.从小鼠成纤维细胞（L细胞）线粒体中分离的环状DNA分子的电子显微照片。一个高度扭曲或超螺旋的单体位于一个开放的单体（左下）;三个松散扭曲的单体（右）;一个开放的二聚体（左上）。这些分子被分散在蛋白质单层的表面上，随后通过从各个方向投射阴影与蒸发的铂铱形成对比。比例尺为0.5 ja。腹水肿瘤细胞和人肝脏的线粒体DNA的差异也得到了类似的证实（15）。另一组DNA分子的大小是5 y的两倍或几倍，在来自几种哺乳动物细胞类型（2，12-15）和海胆卵（18）的线粒体中也有描述。这些分子是开环二聚体（图1）或由互锁单元组成的二聚体和低聚体（图2）。通过电子显微镜的聚焦，以及通过连续循环的旋转和无方向的蒸发金属阴影投射来增强结构的三维图像，通常可以将分子的互锁与仅仅的重叠（很少发生）区分开来。在氯化铯-溴化乙锭梯度中，在含有共价连接的单体和二聚体的下部区域与含有切口环和线性DNA的上部区域之间的中间密度带中发现互锁环。有人认为，互锁的二聚体位于中间带，因为一个成员是有切口的，另一个是共价连接的（13）。二聚体具有稳定的共价键的事实证明它们是在体内形成的，而不是作为人工产物出现的
Fig. 1. Electron micrographs of circular DNA molecules isolated from mitochondria of mouse fibroblasts (L cells). A highly twisted or supercoiled monomer is lying inside an open monomer (lower left); three loosely twisted monomers (right); one open dimer (upper left). The molecules have been spread on the surface of a monolayer of protein and subsequently contrasted by shadow casting from all directions with vaporized platinum-iridium. Scale is 0.5 ja. differences was similarly confirmed for mitochondrial DNA of ascites tumor cells and human liver (15). Another group of DNA molecules that are twice or several times the size of 5 y has beendescribed in mitochondria derived from several mammalian cell types (2, 12-15) and from sea urchin eggs (18). These molecules are open circular dimers (Fig. 1) or dimers and oligomers consisting of interlocked units (Fig. 2). Interlocking of molecules can usually be distinguished from mere overlapping (which occurs rarely) by focusing of the electron microscope and by enhancing the three-dimensional image of the structures by successive cycles of rotary and undirectional shadow casting with vaporized metal. In cesium chloride-ethidium bromide gradients, interlocked circles are found in a band of intermediate density between the lower region containing cova-lently linked monomers and dimers and the upper band containing nicked circles and linear DNA. It has been suggested that the interlocked dimers are foundin the middle band because one member is nicked and the other is covalently linked (13). The fact that the dimers have a stable covalent bond is evidence that they are formed in vivo rather than that they arise as artifacts