Participation of a non-respiratory ferrous complex during mitosis in roots

Historical review: an energy reservoir for mitosis, and its productive wake

The realization that DNA is replicated during only a discrete part of interphase in most cells gave birth to the cell-cycle concept exactly half a century ago. But soon thereafter, a controversy arose as to whether a special reservoir of metabolic energy, built up at the end of interphase, was essential for the entry of cells into mitosis. After a period of intense debate, the 'energy reservoir' concept came to a sudden demise. But the notion that mitotic progression involves the cyclic build-up and consumption of key factors endured and, as we all now know so well, eventually triumphed.

Oxygen tension and the rates of mitosis and interphase in roots

The object of this work was to determine the influence of a wide range of oxygen tensions upon the relative rates of respiration, mitosis, and interphase in pea root tips, compared with the normal rates of these processes in air. From the rates of disappearance of mitotic figures in excised tips kept in various oxygen tensions, the relative rates of mitosis were found to decrease gradually from 122 per cent in 100 per cent oxygen to 24 per cent in 0.0007 per cent oxygen. From the mitotic indices of intact seedlings, the relative rates of interphase were found to decrease sharply from 82 per cent in 10 per cent oxygen to 6 per cent in 5 per cent oxygen. The data on relative rates of respiration, mitosis, and interphase in root tips were compared, and it was shown that the three processes are perfectly distinct in their quantitative relationships to low oxygen tensions.

Effects of carbon monoxide on cardiac muscle cells in culture

Embryonic rat cardiac muscle cells grown in the presence of various tensions of CO (5-95%) without the presence of O2 survived and exhibited reduced cell growth, which was concentration dependent. When cardiac muscle cells were grown in the presence of a mixture of CO (10-20%) and O2 (10-20%), the growth rate of these cells was comparable to that of the control cells. Cardiac myocytes continued to beat when exposed to varying tensions of CO, except in the case of 95% CO. The cells exposed to different concentrations of CO contained fewer myofibrils of different stages of differentiation compared with the control and the culture exposed to a mixture of 20% O2 and 20% CO, with cells that contained abundant, highly differentiated myofibrils. There was no significant difference in the structural organization of mitochondria between the control and the surviving experimental cells. It is evident from the present studies that O2 is required for the optimum in vitro cellular growth of cardiac muscle. Furthermore, CO in combination with O2 at a concentration of 10 or 20% can produce optimal growth of cardiac muscle cells in culture.

Metabolic inhibitors block anaphase A in vivo

Anaphase in dividing guard mother cells of Allium cepa and stamen hair cells of Tradescantia virginiana consists almost entirely of chromosome-to-pole motion, or anaphase A. Little or no separation of the poles (anaphase B) occurs. Anaphase is reversibly blocked at any point by azide or dinitrophenol, with chromosome motion ceasing 1-10 min after application of the drugs. Motion can be stopped and restarted several times in the same cell. Prometaphase, metaphase, and cytoplasmic streaming are also arrested. Carbonyl cyanide m-chlorophenyl hydrazone also stops anaphase, but its effects are not reversible. Whereas the spindle collapses in the presence of colchicine, the chromosomes seem to "freeze" in place when cells are exposed to respiratory inhibitors. Electron microscope examination of dividing guard mother cells fixed during azide and dinitrophenol treatment reveals that spindle microtubules are still present. Our results show that chromosome-to-pole motion in these cells is sensitive to proton ionophores and electron transport inhibitors. They therefore disagree with recent reports that anaphase A does not require a continuous supply of energy. It is possible, however, that anaphase does not directly use ATP but instead depends on the energy of chemical and/or electrical gradients generated by cellular membranes.

NON-IDENTICAL MECHANISMS OF MITOTIC ARREST BY RESPIRATORY INHIBITORS IN PEA ROOT TIPS AND SEA URCHIN EGGS

The adenosine triphosphate (ATP) content of pea root tips is about 0.40 mmole/kg fresh weight. The effects of partial and complete anaerobiosis, and of carbon monoxide and hydrogen cyanide, on the ATP level are described. The ATP content was shown to correspond closely to the oxygen uptake under these conditions. However, there was no relation between the ATP level and the rate of mitosis, a situation which is in contrast with that in sea urchin eggs. In anaerobic conditions, mitoses in pea root tips could continue at a reduced rate, even though the ATP content had fallen to 1.5 per cent of the normal value in air The ATP level in anaerobic conditions corresponded closely to the expected rate of ATP regeneration from known anaerobic sources of energy. Calculations show that even this severely restricted supply of energy would be more than adequate to drive the anaphase chromosome movements, so it is suggested that the concept of a mitotic energy reservoir is superfluous in root tips. No evidence could be found for the involvement during mitosis in sea urchin eggs of a non-respiratory ferrous complex such as occurs in pea root tips. Hence the dilemma remains, that whereas mitoses in both sea urchin eggs and pea root tips are arrested by respiratory inhibitors, yet the biochemical mechanisms of the arrest in the two types of cell are totally distinct.