An electron microscopic observation was made on the DNA's extracted from purified HeLa cell nuclei, mitochondria, and the whole cell, and fractionated by ethidium bromide-cesium chloride density gradient method
or sucrose density gradient method. Nuclear DNA presents mainly long linear DNA derived from fragmented chromosomal DNA. In addition to this, the existence of small circular DNA molecules measuring 0.32 -1.78 μ, was confirmed. Mitochondrial DNA was mainly circular DNA, which measured 4.87 μ in the mean value of the contour lengths in the
highest frequency group, and small circular DNA molecules, measuring 0.3-1.01 μ in contour length, were also found in an extremely low frequency.</p
