Electron microscopic identification of the interphase chromosomes of Amoeba proteus and Amoeba discoides using autoradiography; with some notes on helices and other nuclear components

Abstract

Data obtained from light and electron microscope autoradiographs of cells of Amoeba proteus and Amoeba discoides previously incubated in medium supplemented with H3 thymidine, indicate that fibrous material, the basic unit of which is about 150 A in diameter, represents the interphase chromosomes of these amoebae. The helices of interphase nuclei do not appear to incorporate H3 thymidine, which is in opposition to the hypothesis of Taylor (1963) that they are G2 chromosomes, and makes it unlikely that they represent any form of the DNA-containing component of the amoeba's interphase nucleus. Stereo-electron microscopy reveals that the direction of spiralization of helices may be either left or right handed and that the direction of spiralization of a single helix can reverse. The specific location of helices and of 850 A–1150 A electron dense bodies suggests that they are either primary chromosome products which subdivide before entering the cytoplasm, or units for the intranuclear transportation of primary chromosome products. In each nuclear membrane pore complex one central and eight peripheral regions of dense material are found. At each of the nine points, the dense material appears to traverse the nuclear membrane.