CHROMOSOME MICROMANIPULATION : III. Spindle Fiber Tension and the Reorientation of Mal-Oriented Chromosomes

Kinetochore reorientation is the critical process ensuring normal chromosome distribution. Reorientation has been studied in living grasshopper spermatocytes, in which bivalents with both chromosomes oriented to the same pole (unipolar orientation) occur but are unstable: sooner or later one chromosome reorients, the stable, bipolar orientation results, and normal anaphase segregation to opposite poles follows. One possible source of stability in bipolar orientations is the normal spindle forces toward opposite poles, which slightly stretch the bivalent. This tension is lacking in unipolar orientations because all the chromosomal spindle fibers and spindle forces are directed toward one pole. The possible role of tension has been tested directly by micromanipulation of bivalents in unipolar orientation to artificially create the missing tension. Without exception, such bivalents never reorient before the tension is released; a total time "under tension" of over 5 hr has been accumulated in experiments on eight bivalents in eight cells. In control experiments these same bivalents reoriented from a unipolar orientation within 16 min, on the average, in the absence of tension. Controlled reorientation and chromosome segregation can be explained from the results of these and related experiments.     

Fine structure of polysaccharide microcrystals

Summary Negative staining showed the presence of microcrystals in various polysaccharides. Cellulose microcrystals from Valoniopsis, Vaucheria, and an unidentified tunicate had widths of 20, 27, and 30 Å, respectively. Mannan microcrystals from Acetabularia were 10x25 Å and were oriented in linear arrays with their long axis perpendicular to the array axis. dichotomosiphon and Caulerpa xylans had respective microcrystal widths of 22 and 24 Å. All microcrystals appeared as component part of microfibrils.     

Unique Protein Moieties for 30S and 50S Ribosomes of Escherichia coli

Each major component of the proteins of 30S ribosomes from Escherichia coli was compared with the proteins of 50S ribosomes. The comparisons were done by using polyacrylamide gel electrophoresis in urea with differentially labeled proteins. The data show that no major protein is common to both ribosomes.     

CHROMOSOME VELOCITY DURING MITOSIS AS A FUNCTION OF CHROMOSOME SIZE AND POSITION

Chromosome velocity has been studied in living Melanoplus differentialis spermatocytes by phase contrast cinemicrography. Melanoplus chromosomes (and bivalents) differ in length by as much as 1:3.5. As expected, no size-dependent velocity differences were detected in anaphase, and this is also shown to be true for the less predictable movements during prometaphase congression. The size of the X chromosome can change during observation following x-irradiation, but this is equally without influence on velocity. However, an effect of position on velocity is found in both prometaphase and in anaphase: the chromosomes furthest from the central interpolar axis move 25 per cent faster than more central chromosomes. A simple mechanical model relating frictional resistance and mitotic forces to chromosome velocity is discussed in detail. Calculations from the model suggest that a significant difference in the force acting on a large, as compared with a small chromosome is necessary to account for the observed similarity in velocity. Therefore, it is concluded that the mitotic forces are so organized or regulated that velocity is, within limits, independent of load. The implications of velocity-load independence in relation to the molecular origin of mitotic forces are discussed.     

Aspects of the life cycle of marine nematodes

Summary 1. Life cycles of approximately one month are recorded for laboratory maintained cultures of chromadorid, monhysterid and oncholaimid nematodes.2. Amphimictic and parthenogenetic reproduction occurs in the species investigated. Reproduction inMonhystera parelegantula is by parthenogenesis.3. The chromadoridChromadorina epidemos and the oncholaimidViscosia macramphida are able to reproduce by either amphimixis or parthenogenesis.4.Acanthonchus cobbi, Chromadora macrolaimoides andEuchromadora gaulica are amphimitically reproducing species.

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Aspekte des Lebenszyklus mariner Nematoden

Kurzfassung Bei einigen marinen Nematoden, die in Biscayne Bay (Florida) gesammelt und im Labor gezüchtet worden sind, vollzieht sich der gesamte Lebenszyklus ungefähr innerhalb eines Monats. BeiAcanthonchus cobbi, Chromadora macrolaimoides undEuchromadora gaulica ist zweigeschlechtliche Fortpflanzung festgestellt worden.Monohystera parelegantula vermehrt sich parthenogenetisch. Ein möglicherweise durch Umwelteinflüsse bedingter Wechsel von bisexueller und parthenogenetischer Fortpflanzung tritt beiChromadora epidemos undViscosia macramphida auf.