Margaret R. McDonald

Organizational patterns within chromosomes.

Publisher Summary This chapter discusses the structure of chromosome. The condensed chromosomes of somatic and meiotic mitoses are characterized by helically disposed chromonemata. When seen in end view-both in living material and in fixed preparations, the chromosome appears as a ring with its constituent chromonemata surrounding a central core. In fixed and stained preparations, the chromonemata reveal a high degree of basophilia, except in the zones of lesser stainability that mark the positions of primary and secondary constrictions. The primary constriction indicates the location of the centromere. Secondary constrictions are often detectable, such as those that separate terminal satellites from the body of the chromosome. Chromosomes of higher plants and animals—and probably the chromosomes of microorganisms—are multistranded. In the terminology of Darlington, they are polynemic. A chromosome in a given type of cell generally maintains a constant number of strands, apart from the variability imposed by their recurrent doubling and halving in chromosomal replication and distribution during the mitotic cycles. Further, polynemic and polytenic chromosomes are assumedly composed of genetically equivalent subsidiary strands. Within the limits of its stability, the chromosome is subject to changes in the patterns of association of its constituent materials during somatic and meiotic mitoses and in the course of differentiation. A consideration of these changes recommends a measure of caution in attempting to designate any one component as the essential structure or functional material. Deoxyribonucleic acid has often been selected to fill this role, largely on the basis of studies of cells that function in the transfer of “genetic information” from one individual to another.

Organization of the Chromosome

The observational and experimental results presented here constitute another chapter in a continuing research program that is directed toward an understanding of patterns of organization of chromosomal materials. Evidence has been sought over the years at various levels of analysis, beginning with descriptive cytology, progressing into the areas of experimental and analytical cell research, and extending in recent months to the realm of fine detail afforded by the resolving powers of the electron microscope. Attention has been focussed throughout on the chromosomes of higher plants and animals, because their large size favored microscopic examination and because they offered the prospect of correlating chromosome activity with developmental processes. This progress report includes a brief examination of the background information accumulated in the earlier studies and then attempts to fit the more recent findings into that frame of reference.

Localization of cellular proteins by enzymatic hydrolysis

Identification of types of cellular proteins and determination of their patterns of distribution and association with nucleic acids may be regarded as essential to an understanding of the processes involved in duplication of the living cell. A more specific evaluation of these processes with respect to replication of the gene complex requires precise information concerning the organization of the chromosome and the arrangement of its constituent materials during the various phases of mitosis. Inferences concerning the chemical composition of chromosomes derived from the earlier observations of descriptive cytologists have been supplemented in recent years by information provided by a variety of experimental procedures. Descriptions of patterns of linear organization in terms of achromatic and chromatic materials have accordingly been replaced by descriptions in terms of proteins and nucleic acids.

The degradation by ribonuclease of substrates other than ribonucleic acid.

These experiments support the premise which is the basis for cytochemical procedures that employ ribonuclease for the identification of ribonucleic acid, namely, that the substrate specifically degraded by ribonuclease is ribonucleic acid. The reduction in Feulgen stainability of chromatin effected by ribonuclease has been shown to be due to an intracellular deoxyribonuclease, which hydrolyzes intracellular deoxyribonucleic acid only after ribonucleic acid has been removed. The degradation of apurinic acid by ribonuclease can be duplicated by other proteins.

Production of mitotic abnormalities by ethylene-diaminetetraacetic acid

Treatment of growing roots of onion with solutions of EDTA between 0.0004 and 0.002 M was found to modify the form and distribution of the chromosomes of the meristematic cells. Similar results were obtained with 0.002 M CaCl2 or MgCl2. The effects of EDTA are attributed to alterations in the general metabolism of the cell, rather than to direct breakage of chromosomes through chelation of divalent cations serving as bridges between macromolecular complexes of protein and nucleic acids.