Electron microscopy of human interphase nuclei. Determination of total dry mass and DNA-packing ratio.

Abstract

Quantitative electron microscopy was used to analyze surface-spread, critical-point-dried human interphase nuclei and chromatin. The following information is presented: (1) Unstimulated interphase nuclei of lymphocytes from peripheral blood have a mean dry mass of 50.30×10−12 g. The mean dry mass of stimulated nuclei of lymphocytes was determined to be 59.34×10−12 g, a significant statistical difference from the unstimulated ones. (2) Mean diameter of chromatin fibers and mean fiber mass per micron were 199A±15% coefficient of variation (C.V.) and 5.95×10−16g×29% C.V., respectively. (3) A line of regression of fiber mass on fiber diameter for 83 fibers indicated that a 200-A fiber has a mass of 5.86×10−16g/μ, or almost the same as the mean fiber mass of 5.95× 10−16g/μ. (4) With the value 7×10−12g for the DNA content of an unstimulated lymphocyte nucleus, a total length of 215 cm is calculated for the DNA double helix. When this length is compared to the mean length of chromatin fiber per nucleus (7.59 cm), a ratio of 28.3 to 1 results, which is called the DNA-packing ratio. (5) This DNA-packing ratio of 28.3 is reasonably close to the packing ratio of 26.9 suggested from model calculations for the second DNA supercoil in a 200-A chromatin fiber.