F ractionation experiments in the early 1960s suggested that certain organelles may contain DNA (see Gibor and Granick, 1964, for review), but varying levels of nuclear contamination marred those results. Meanwhile, oddities in genetic transmission chipped away at the nucleus-only theory of genetic inheritance. But it took direct visualization to convince most skeptics that organelles could harbor genetic material.
First under the EM was the chloroplast of the alga Chlamydomonas. Ris and Plaut (1962) visualized chloroplast DNA both via direct staining by dyes and as DNase-sensitive fibers under the EM.
The fibers were evident only after the use of a staining procedure designed for bacterial cells, and this parallel gave the authors an idea. “With the demonstration of ultrastructural similarity of a cell organelle and free living organisms,” stated Ris and Plaut, “endosymbiosis must again be considered as a possible evolutionary step in the...
