The Mechanism of Cell-division

1. Developing eggs of the sea-urchin Psammechinus miliaris were subjected to carbon monoxide inhibition, which was controlled by changing from green to white light. The behaviour of the eggs was recorded by time-lapse photography. 2. If inhibition is applied before the eggs enter mitosis, their first cleavage is delayed by a time which is roughly equal to the period of the inhibition. 3. If the inhibition is applied when the cells have already entered mitosis, they complete mitosis and cleave with little or no delay, but their second cleavage is delayed by a time which is roughly equal to the period of the inhibition. 4. It is suggested that the necessary energy for the second mitosis and cleavage is being stored up during the first mitosis and cleavage, and that this energy store operates like a reservoir which is continually being filled but siphons out when it is full. Once the energy has siphoned out, it carries mitosis and cleavage through, even though the reservoir is not filling up because of carbon monoxide inhibition.

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Protoplasmic Structure and Mitosis

Journal of Experimental Biology ◽

10.1242/jeb.28.4.417 ◽

1951 ◽

Vol 28 (4) ◽

pp. 417-433

Author(s):

M. M. SWANN

Keyword(s):

Integral Equation ◽

Sea Urchin ◽

Time Lapse ◽

Mitotic Figure ◽

Psammechinus Miliaris ◽

Protoplasmic Structure ◽

Mitotic Figures ◽

Point To Point ◽

Oriented Material ◽

Time Lapse Photography

1. The present paper is the first of a series dealing with the birefringence of mitotic figures in the eggs of the sea-urchin Psammechinus miliaris. 2. Living eggs have been examined using time-lapse photography, and retardation curves for the mitotic figures constructed from densitometric measurements made on the film negatives. 3. In the case of the aster, an integral equation relating retardation and coefficient of birefringence can be formulated and solved exactly to give coefficient of birefringence. In the case of the spindle, coefficient of birefringence can only be calculated approximately. 4. In both asters and spindles, the coefficient of birefringence is nil at the centres,rises to a maximum at 5 or 6CL out, and then falls to a minimum at the equator of the spindle or the periphery of the aster. 5. The rise in coefficient of birefringence round the centre is not as sharp as might be expected, and there is some evidence that orientation is built up gradually over a distance of a few microns. 6. The fall in coefficient of birefringence away from the maximum is approximately an inverse square in the case of the spindle. In the aster it falls off somewhat more rapidly. Since the density of material does not vary from point to point, this fall must be due to changes in molecular and micellar arrangement, or to a decreasing proportion of oriented material. 7. The classical conception of the spindle and asters as structures built up of discrete fibrils radiating from the centres, would be expected, for geometrical reasons, to give an inverse square fall in proportion of oriented material. While, therefore, a homogeneous structure with varying molecular and micellar arrangement cannot be ruled out, it is possible that the mitotic figure consists of definite fibrils radiating from the centres. 8. Evidence from other sources supports this view, and suggests that the fibrils must be submicroscopic in size.

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Boursin, a sea urchin bimC kinesin protein, plays a role in anaphase and cytokinesis

Journal of Cell Science ◽

10.1242/jcs.114.3.481 ◽

2001 ◽

Vol 114 (3) ◽

pp. 481-491 ◽

Cited By ~ 3

Author(s):

I. Touitou ◽

G. Lhomond ◽

G. Pruliere

Keyword(s):

Cell Division ◽

Sea Urchin ◽

Paracentrotus Lividus ◽

Time Lapse ◽

Dominant Negative ◽

Dominant Negative Mutant ◽

Early Prophase ◽

Immunofluorescence Analysis ◽

Centrosome Separation ◽

Late Stages

We have isolated and characterized Boursin, a kinesin-related protein of the bimC family, from Paracentrotus lividus sea urchin eggs. Boursin is expressed at high levels in eggs and embryos during early cleavage stages. Boursin was found to be associated with different parts of the mitotic spindle from early prophase to telophase. Expression of a form of the protein predicted to act as a dominant negative mutant caused severe defects in cell division and resulted in the formation of embryos with polyploid and multiastral blastomeres. Immunofluorescence analysis indicated that these defects did not arise from failure in either centrosome separation or bipolar spindle formation. Time-lapse observations showed rather that these perturbations in cell division resulted from abnormal anaphase and failure to complete cytokinesis. These phenotypes differ from the phenotype described following perturbation of the function of bimC family members in other organisms. Our study has thus uncovered roles for a bimC kinesin in late stages of cell division.

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