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Biomedical subjects

B Novák

Publications and source records attributed to B Novák.

At least 19 recordsLinked to original sources
PubMed2003

Modelling the controls of the eukaryotic cell cycle.

The eukaryotic cell-division cycle is regulated by three modules that control G(1)/S, G(2)/M and meta/anaphase transitions. By using mathematical modelling, we show the dynamic characteristics of these individual modules and we also assemble them together into a comprehensive model of the eukaryotic cell-division cycle. With this comprehensive model, we also discuss the mechanisms by which different checkpoint pathways stabilize different cell-cycle states and inhibit the transitions that drive cell-cycle progression.

Cell Cycle↗
PubMed2002

Regularities and irregularities in the cell cycle of the fission yeast, Schizosaccharomyces pombe (a review).

In an exponentially growing wild-type fission yeast culture a size control mechanism ensures that mitosis is executed only if the cells have reached a critical size. However, there is some scattering both in cell length at birth (BL) and in cycle time (CT). By computational simulations we show here that this scattering cannot be explained solely by asymmetric cell division, therefore we assume that nuclear division is a stochastic, asymmetric process as well. We introduce an appropriate stochastic variable into a mathematical model and prove that this assumption is suitable to describe the CT vs. BL graph in a wild-type fission yeast population. In a double mutant of fission yeast (namely wee1-50 cdc25 delta) this CT vs. BL plot is even more curious: cycle time splits into three different values resulting in three clusters in this coordinate system. We show here that it is possible to describe these quantized cycles by choosing the appropriate values of the key parameters of mitotic entry and exit and even more the clustered behavior may be simulated by applying a further stochastic parameter.

Cell Cycle↗
PubMed1999

Finishing the cell cycle.

The eukaryotic cell division cycle consists of two characteristic states: G1, when replication origins of chromosomes are in a pre-replicative state, and S/G2/M, when they are in a post-replicative state (Nasmyth, 1995). Using straightforward biochemical kinetics, we show that these two states can be created by antagonistic interactions between cyclin-dependent kinases (Cdk) and their foes: the cyclin-degradation machinery (APC) and a stoichiometric inhibitor (CKI). Irreversible transitions between these two self-maintaining steady states drive progress through the cell cycle: at "Start" a cell leaves the G1 state and commences chromosome replication, and at "Finish" the cell separates the products of replication to the incipient daughter cells and re-enters G1. We propose that a protein-phosphatase, by up-regulating the APC and by stabilizing the CKI, plays an essential role at Finish. The phosphatase acts in parallel pathways; hence, cells can leave mitosis in the absence of cyclin degradation or in the absence of the CKI.

Animals↗
PubMed1988

The kinetics of cell division in a synchronizing fermentor.

The kinetics of cell proliferation of Candida tropicalis synchronized by periodical glucose starvation was examined in a continuous synchronizing fermentor. The length of the deterministic (B) and stochastic (A) phase of the cell cycle was determined by cell density data analysis using the Transition Probability Model. The size distribution of the population shows that the absence of nutrient stops the cells at the beginning of the cell cycle but the succeeding cell divisions are not perfectly synchronized. The reason of this comes from the difference between the generation times of the cells which is the consequence of the different long "A" phases in their cycle.

Candida↗
PubMed1988

Changes of alternative respiration during the division cycle of Candida tropicalis.

The changes in cyanide-resistant and SHAM-sensitive respiration have been examined in the cell cycle of yeast cells synchronized by nutrient starvation. The changing of the cyanide-resistant respiration of cells proliferating in synchronizing fermentor, where the glucose concentration was continuously decreasing, was associated with the stage in cell cycle rather than with the alteration of environment. The cyanide-resistant alternative respiration may be attributed characteristic of a certain part of the cell cycle termed "A" phase. This hypothesis is supported by the observation that in cultures proliferating, synchronously under constant conditions, the cyanide-resistant respiration changes periodically and reaches its maximum in the "A" phase after the cell division.

Candida↗
PubMed1980

Effect of starch and inorganic nitrogen on microbial transformations of organic compounds in soil.

It was found in long-term incubation experiments (100 weeks) that in soil non amended with organic compounds, mineralization proceeded in a steady state after an initial reactive respiration and corresponded approximately to the rate of mineralization of soil organic matter under natural conditions. The addition of nitrogen influenced this mineralization process only very slightly. The addition of starch decreased the stability of organic compounds in the soil at first but the newly formed organic substances were gradually stabilized again. The addition of nitrogen to starch increased the extent and the rate of these alterations. The overall extent of mineralization of the substrate after the addition of starch alone exceeds the amount of the added substrate; a small accumulation of the substrate could be observed after the addition of starch together with nitrogen.

Biotransformation↗
PubMed1979

Influence of fluctuating temperature on soil microflora.

The effect of different constant and "square-wave" fluctuating temperatures on the soil microflora was studied in laboratory experiments. The growth of soil microorganisms was higher at lower temperatures, indicating the higher degree of substrate immobilization and the better energy economy of the soil bionta. Microbial counts were slightly higher after five days of incubation at fluctuating temperatures than those obtained at a constant mean temperature. It was important whether the fluctuation started at lower or at higher temperatures. At a fluctuating temperature the amount of nitrates decreased compared to the amount at a constant temperature. The development dynamics of soil microorganisms was accelerated by the fluctuation of temperature as judged by nitrogen transformation and simultaneous changes in microbial counts.

Nitrates↗
PubMed1978

Effect of moisture level on nitrogen immobilization as affected by wheat straw decomposition in soil.

Samples of a slightly degraded chernozem soil were amended with 2 per cent of wheat straw at three moisture levels (10, 20, and 30 per cent) and were provided with 160, 240, and 400 ppm of ammoniacal nitrogen. Amounts of total, ammonia, and nitrate were determined at different periods of incubation at 28 degrees C and immobilization of the total available nitrogen in the soil was calculated. Substantial amounts of the added nitrogen were immobilized in the soil as a result of the straw decomposition during the first month of incubation. The amount of inorganic nitrogen (NH4 + -N + NO3 - -N), bound within 75 days of incubation, was almost three times less than that immobilized within 30 days. Maximum quantity of nitrogen was immobilized at 30 and 20 per cent moisture levels at 30 and 75 days of incubation, respectively. Minimum immobilization was observed at 10 per cent moisture at almost all levels of nitrogen. The absolute amount of nitrogen immobilized increased in proportion to the quantity of nitrogen added, but the relative immobilization tended to decrease with the increased inorganic nitrogen the soil.

Humidity↗
PubMed1978

Effect of moisture and nitrogen levels on the decomposition of wheat straw in soil.

Two per cent of wheat straw was mixed with samples of a slightly degraded chernozem soil, and its decomposition was studied at 10, 20, and 30 per cent moisture content of the soil with the addition of 160, 240, and 400 ppm of NH4 + -N. The overall decomposition, measured as CO2 production, and total carbon loss from the soil at 28 degrees C was enhanced by the added nitrogen at all levels of moisture in proportion to the quantity added. Maximum mineralization of the straw carbon was observed at 30 per cent moisture content but there was no significant difference between the amount of carbon mineralized at 20 and 30 per cent moisture levels. No stabilization of the substrate took place in the soil except at 240 and 400 ppm of applied nitrogen at 30 per cent moisture level towards the end of the incubation period. More straw carbon was mineralized when the soil samples were subjected to daily measurements of CO2 evolved than when CO2 measurements were made at intervals over the same period of incubation.

Fermentation↗
PubMed1976

Simple method for carbon determination in microbiological experiments.

A wet combustion method for the carbon determination was developed. It has been proved that the method can be successfully used for the determination of biochemical changes in microbe cultures, the soil, in composts, in farmyard manures etc. The samples are combusted in evacuated flasks by means of chromic sulphuric mixture, and the CO(2) evolved is determined interferometrically.

Carbon↗