STUDIES ON THE FIXATION OF ARTIFICIAL AND BACTERIAL DNA PLASMS FOR THE ELECTRON MICROSCOPY OF THIN SECTIONS

The process of fixation of DNA-containing plasms is investigated by macroscopical and electron microscopical observations on solutions of DNA, nucleohistones, as well as on bacterial nuclei. The following treatments were found to produce a gelation of a solution of DNA or nucleohistones: (a) OsO₄ fixation at pH 6 in the presence of amino acids (tryptone) and Ca⁺⁺. (b) Exposure to aqueous solutions of uranyl acetate. (c) Exposure to aqueous solutions of indium chloride. Observed in the electron microscope, these gels show a fine fibrillar material. From experiments in which solutions of DNA or nucleohistones are mixed with bacteria and treated together, it is concluded that the behavior of the bacterial nucleoplasm is similar to that of the DNA solutions. The appearance of birefringence indicates that uranyl acetate and indium chloride produce an orientation of the molecules of a DNA solution during gelation. Bacterial chromosomes fixed by these agents also show a certain order, while those fixed by the OsO₄-amino acid-Ca⁺⁺ formula do not. Whether or not the order can be considered to be artificial is discussed, and a tentative conclusion is presented: (a) Uranyl acetate may induce artificial order. (b) Fixatives which do not gel DNA probably result in the grossest artifacts. (c) OsO₄ fixation at pH 6 in the presence of amino acids (tryptone) and Ca⁺⁺ may give the most accurate preservation of the in vivo disposition of DNA (RK⁺ fixation).