Medial Constriction Research Articles

Spindle and kinetochore morphology of Dictyostelium discoideum.

The metaphase spindle of haploid Dictyostelium discoideum (n = 7) is 2 mum long. It consists of some 20 microtubules which seem continuous between the spindle pole bodies and there are about 20 chromosomal microtubules at each end of the spindle. During anaphase the central spindle elongates and the chromosomal microtubules shorten. The spindle length and structure at this stage suggests that lengthening is caused by elongation as well as parallel sliding of the nonchromosomal microtubules. The nuclear envelope remains mostly intact during mitosis, and nuclear separation through medial constriction takes place when the spindle is 6 mum long. Cytokinesis occurs when the spindle is 10 mum long. At that time the kinetochores double in size. During interphase, the spindle pole body separates from the nucleus to a distance of 0.7 mum, and it returns at the onset of the next prophase when it becomes functionally double, thereby starting the formation of a central spindle. When comparing mitosis in the cellular slime molds Polysphondylium violaceum and D. discoideum, several similarities and some differences are apparent.

Modification of sporulation in yeast strains with two-spored asci (Saccharomyces, Ascomycetes).

ABSTRACT Electron-microscope observations indicate that the single nuclear division of sporulating yeasts which form 2-spored asci resembles, in several aspects, the nuclear behaviour during sporulation of the more common 4-spored yeasts. Specifically the parental nucleolus remains behind in the ascus cytoplasm, a characteristic nucleolus-associatcd spherical structure is formed, and the nucleus divides by budding rather than by elongation and medial constriction.

NUCLEAR MORPHOGENESIS DURING THE DEVELOPMENTAL CYCLES OF SOME MEMBERS OF THE GENUS NOCARDIA.

SUMMARY: Observations of nuclear morphogenesis were carried out using a modification of the acid hydrolysis-Giemsa technique. Throughout the development cycles studied, nuclear division was found to occur by means of elongation of the chromatin bodies with subsequent medial constriction and separation into daughter structures. Nuclear division greatly exceeded septum deposition during the early phases of the developmental cycles and, as a result, coenocytic filaments were formed. Fragmentation of these filaments resulted in the production of single cells, short chains and short coenocytes like those found in the inoculum. No evidence for complex nuclear fusions or reductional divisions was obtained during any period of nuclear morphogenesis and chromatin body divisions were considered to be amitotic. A simple model explaining previously reported genetic recombination in the genus Nocardia is presented.

Electron microscopical studies of frozen-dried yeast: II. The nature of basophile particles and vesicular nuclei in Saccharomyces

Haploid and diploid vegetative nuclei of Saccharomyces were stained selectively for basophilia after post-fixation in vacuo with anhydrous, vapor reagents specifically effective for proteins and polypeptides and studied in conjunction with enzymatic and other controls. The nuclei cleave simply by elongation and medial constriction of hour-glass forms with no indications of a spindle apparatus or resolvable chromosomes with classical morphological attributes, or a breakdown of the nuclear boundary at any time until final cleavage. Instead, the nuclear contents are particulate, measuring about 50–80 Å and seemingly dispersed at random in most sectioning planes, but frequently detectable as short rows streaming into the developing daughter nucleus. Association of these particles with faintly stainable submicroscopic cords of anastomotic form in sections is detectable; one or two peripheral clusters of similar particles, of hitherto undetermined function, occur in the dividing nucleus regardless of haploidy or diploidy. No organization of obviously light microscopically resolvable dimension occurs in the nucleus. Integrity of the particulate matter in the nucleus is destroyed or very greatly reduced by DNA-ase, but preserved after RNA-ase. The latter abolishes basophily of the dense deposits of ribosomes in the cytoplasm with the exception of a small resistant fraction mostly in the vicinity of the nucleus. These resistant particles are not seen in specimens subjected successively to both the nucleic acid depolymerases. The cytogenetical implications of these findings are discussed very briefly in view of Mendelian heredity established in yeast, and in terms of an evolution of cytogenetical mechanisms.