THE STATE OF THE CHROMOSOMES IN THE INTERPHASE NUCLEUS

In the living interphase nucleus no chromosomal structures are visible. Yet in the injured cell and after treatment with most histological fixatives chromatin structures become apparent. Under certain conditions this appearance of structure in the living interphase nucleus is reversible. We have found that this change in the interphase nucleus is the result of a change in the state of the chromosomes. In the living nucleus the chromosomes are in a greatly extended state, filling the entire nucleus. Upon injury the chromosomes condense and therefore become visible. At the same time the nuclear volume decreases. This behavior of the chromosomes is connected with their content of desoxyribonucleic acid (DNA). This view is based on the following observations: (a) Distribution of DNA in the Nucleus.—(1) The living interphase nucleus of uninjured cells absorbs diffusely at 2537 Å. No chromosomal structures are visible in ultraviolet photographs unless they are also distinct in ordinary light. If the chromosomes are made to condense they become visible and the absorption at 2537 Å is now localized in these structures. (2) After fixation with formalin and osmic acid interphase nuclei stain diffusely with Feulgen. These fixatives preserve the extended state of the chromosomes. (3) If nuclei are teased out in non-electrolytes (sucrose, glycerin) the chromosomes are extended. Such nuclei stain homogeneously with methyl green. On adding salts the chromosomes condense and the methyl green is now restricted to the visible structures. (b) Extension and Condensation of Isolated Chromosomes.—When chromosomes isolated from interphase nuclei of calf thymus are suspended in sucrose, their volume is four to five times larger than in saline, but they retain their characteristic shapes. Chromosomes from which DNA and histone have been removed do not show this reversible extension and condensation, neither do lampbrush chromosomes of frog oocytes which contain very little DNA. During mitosis a partial condensation of the DNA occurs in prophase, so that the mitotic chromosomes now occupy a much smaller volume of the nucleus. At telophase the chromosomes swell again to fill the entire nucleus.

Some new evidence on the physical nature of plant nuclei from intra-specific polyploids

The object of this paper is to make clear certain facts regarding the resting nuclei and mitotic phenomena of plants. These facts are of material importance to both experimental and morphological cytologists, but so far do not seem to have been clearly appreciated by either. Indeed, the extreme diversity of outlook between the two branches of what should be one science may easily be shown by a simple quotation from current text-books of each. Thus Gray (1931), on p. 122 of his “ Text-Book of Experimental Cytology,” remarks: “ Since all nuclei exhibit a visible granular or fibrillar structure after coagulative fixation, it is generally supposed that the structures seen as preserved preparations or in moribund nuclei are to be regarded as purely artificial products of coagulation, which cannot be correlated with the fundamental structure of a living nucleus. This view, developed many years ago by Hardy, is now accepted by the majority of animal cytologists.” Sharp, on the other hand, in the new edition of the “ Introduction to Cytology ” (1934), on p. 54, remarks: “ These results [from plants], together with the analogous findings of Chambers on the prophases in animal spermatocytes, indicate that the reticulum appearing in a well-fixed nucleus fairly represents a delicate thready structure actually present during life and rendered more distinctly visible by fixation.” While admitting, as will be shown, the inadequacy of the “ reticulum ” conception as a complete description of any nucleus, it must be confessed that the entirely negative view of “ the majority of animal cytologists ” comes as nothing less than a shock to anyone familiar with the degree of precision and certainty of which plant cytology is becoming increasingly capable. This precision is generally recognized where it concerns the morphology and behaviour of chromosomes in the definitive state. To deny all validity to the morphological approach to the resting nucleus appears, in contrast, both unjustified and undesirable. It is undesirable, owing to the very obscurity of most nuclei in the living state; a state so difficult to observe and interpret that every possible source of information should be exploited. That such excess of caution is also unjustified may perhaps become more apparent from the new evidence to be presented. This admittedly deals with dead cells, but the type of reasoning is such that it does not depend for its validity on the detailed interpretation of fixation alone. It may therefore help to bridge the gap between the morphological and experimental fields of observation.