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101.
Using HeLa S-3 cells synchronized by selective detachment, in this paper we report a parallel study of nuclear morphology and autoradiography grain patterns between middle G1 and middle S phases: Our results show two distinct [3H]-thymidine labeling patterns. The first “peripheral” labeling pattern has a characteristic nuclear size distribution, in contrast to the heterogeneous and varying size distributions of Feulgen-stained nuclei, and apparently is characteristic of very early S phase. The sizes of the second labeling pattern—homogeneous or inhomogeneous grain distribution throughout the nucleus—are equal or larger than the first and vary with S phase progression. Together, the corresponding nuclear sizes of the labeled nuclei represent the larger extreme of nuclear areas, and the labeling index closely parallels the fraction of nuclei with areas larger than the minimum size of the labeled nuclei. These results suggest a characteristic nuclear size (reflecting unique intranuclear DNA distribution) as a necessary, if not sufficient, requirement for S phase initiation. Parallel experimentation with rat liver cells—synchronized in vivo by partial hepatectomy and analyzed by thin section autoradiography—confirms the existence of a peripheral labeling pattern in both the very early part and the very late part of S phase, which reconciles our data with previous results and points to the fact that both initiation and termination sites for DNA replication are near the nuclear periphery.  相似文献   
102.
Summary The E1 subgroup (E1, A, Ib, etc.) of antibacterial toxins called colicins are known to form voltage-dependent channels in planar lipid bilayers. The genes for colicins E1, A and Ib have been cloned and sequenced, making these channels interesting models for the widespread phenomenon of voltage dependence in cellular channels. In this paper we investigate ion selectivity and channel size—properties relevant to model building. Our major finding is that the colicin E1 channel is large, having a diameter ofat least 8 Å at its narrowest point. We established this from measurements of reversal potentials for gradients formed by salts of large cations or large anions. In so doing, we exploited the fact that the colicin channel is permeable to both cations and anions, and its relative selectivity to them is a functions and anions, and its relative selectivity to them is a function of pH. The channel is anion selective (Cl over K+) in neutral membranes, and the degree of selectivity is highly dependent on pH. In negatively charged membranes, it becomes cation selective at pH's higher than about 5. Experiments with pH gradients cross the membrane suggest that titratable groups both within the channel lumen and near the channel ends affect the selectivity. Individual E1 channels have more than one open conductance state, all displaying comparable ion selectivity. Colicins A and Ib also exhibit pH-dependent ion selectivity, and appear to have even larger lumens than E1.  相似文献   
103.
Summary mRNA from estrogen-stimulated rat myometrium, a tissue known to upregulate cell-cell channels in response to this hormone, was microinjected intoXenopus laevis oocytes. The oocytes had been freed from covering layers of follicle cells and vitelline to allow direct cell membrane interactions when paired. About 4 hours after the mRNA injection, paired oocytes become electrically coupled. This coupling was due to the presence of typical cell-cell channels characterized by size-limited intercellular tracer flux, the presence of gap junctions at the oocyte-oocyte interface, and the reversible uncoupling that occurred in the presence of carbon dioxide. The induction of new cell-cell channels in the oocyte membrane was observed against a zero background or a low level of endogenous coupling, depending on the maturation stage of the oocytes. The time course of development of cell-cell coupling after the microinjection of mRNA was determined. The mRNA capable of inducing cell-cell coupling was confined to an intermediate size class when fractionated on a sucrose gradient.  相似文献   
104.
J. R. Ellis  R. M. Leech 《Planta》1985,165(1):120-125
As part of an investigation into the control of chloroplast replication the number and size of chloroplasts in mesophyll cells was examined in relation to the size of the cells. In first leaves of Triticum aestivum L. and T. monococcum L. the number of chloroplasts in fully expanded mesophyll cells is positively correlated with the plan area of the cells. The linear relationship between chloroplast number per cell and cell plan area is also consistent over a fivefold range of cell size in isogenic diploid and tetraploid T. monococcum. In T. aestivum the chloroplast number per unit cell plan area varies among cells in relation to the size of the chloroplasts. Those cells containing chloroplasts with a relatively small face area have a correspondingly higher density of chloroplasts, and consequently, the total chloroplast area per unit cell plan area is very similar in all the cells. The results indicate that the proportion of the cell surface area covered by chloroplasts is precisely regulated, and that this is achieved during cell development by growth and replication of the chloroplasts.  相似文献   
105.
Human erythrocytes were incubated in isotonic solutions of different monovalent cations. The apparent size of the red cells measured on scanning electron microscopic pictures decreases in the order Li+>Na+=K+>Rb+. These differences in size are abolished after pretreatment with trypsin, which removes a large part of the charges associated with membrane glycoproteins. Shape alterations are also observed. Normal biconcave shapes are visible after Na+ or K+ incubation, whereas Li+ leads to flabby, flattened cells with a certain tendency to crenation, and Rb+ causes more pronounced biconcavity with a certain tendency to cupping. The overall effects of pretreatment with trypsin are similar to those of Li+. Our results provide evidence that the electrostatic repulsion of glycoproteins and other charged membrane components may play an essential role in maintaining red cell shape.  相似文献   
106.
Probabilistic models of the cell cycle maintain that cell generation time is a random variable given by some distribution function, and that the probability of cell division per unit time is a function only of cell age (and not, for instance, of cell size). Given the probability density, f(t), for time spent in the random compartment of the cell cycle, we derive a recursion relation for n(x), the probability density for cell size at birth in a sample of cells in generation n. For the case of exponential growth of cells, the recursion relation has no steady-state solution. For the case of linear cell growth, we show that there exists a unique, globally asymptotically stable, steady-state birth size distribution, *(x). For the special case of the transition probability model, we display *(x) explicitly.This work was supported by the National Science Foundation under grants MCS8301104 (to J.J.T.) and MCS8300559 (to K.B.H.), and by the National Institutes of Health under grant GM27629 (to J.J.T.).  相似文献   
107.
Paolo Madoni 《Hydrobiologia》1984,111(3):201-206
A technique is described for the estimation of the size of ciliate populations based on replicated counts. Sample drops were taken by automatic pipettes of different volume from activated sludge and from a small oligotrophic stream. Specific aims were: 1) the estimation of the number of species; 2) the estimation of the number of individual in each species; 3) the selection of a suitable sub-sample size and number of replicates. For each volume the cumulative increase in species taken in successive sub-samples was determined. For each species the minimum permissible sub-sample volume and the number of replicates required for an allowable error of 10% and 25% were determined. Lastly, the relationship between the mean number of individuals counted for each species and the relative coefficient of variation was visualized.  相似文献   
108.
Manipulations of substrate size and components of heterogeneity were designed to test their independent effects and interactions on the abundance and species richness of stream macroinvertebrates. Two components of substrate heterogeneity, variation in size class proportions and number of size classes, had no independent effect on abundance or richness; and in general did not interact with median particle size. Median particle size, stream current, and detritus accounted for most of the significant variation in macroinvertebrates colonizing the experimental substrates. Rocks with high surface heterogeneity (roughness) were colonized by more individuals (but not taxa) than rocks with low surface heterogeneity.  相似文献   
109.
Predator-mediated habitat use: some consequences for species interactions   总被引:6,自引:0,他引:6  
Synopsis Behavioral responses to predators can have a major impact on a fishes' diet and habitat choice. Studies with the bluegill sunfish, Lepomis macrochirus, demonstrate that bluegills undergo pronounced shifts in diet and habitat use as they grow in response to changes in their vulnerability to predators. Other species of fish exhibit similar habitat shifts with body size, presumably also in response to changing predation risks and/or foraging gains. An important but little appreciated consequence of this type of predator-mediated habitat use is that predation risk, by structuring size and/or age-specific resource use, may also indirectly affect species interactions. This paper discusses some of the ways in which behavioral responses to predators may affect intra- and interspecific competition in fish. Observational and experimental studies with sunfish (Centrarchidae) provide most of the examples. These studies suggest that the nonlethal effects of predators may be as important as the actual killing of prey.  相似文献   
110.
Six closely related taxa of the sect.Eusideritis of the genusSideritis (S. leucantha, S. pusilla, S. flavovirens, S. granatensis, S. biflora andS. osteoxylla) are analysed to elucidate their phylogenetic relationships and position within the sect.Eusideritis. Meiotic behaviour, karyotype features, size and fertility of pollen grains, DNA amounts and seed protein profiles are reviewed. A polyploid origin of the group (from x = 7) and the further diversification through dysploidy and chromosome repatterning is postulated.S. osteoxylla is apparently of hybrid origin.  相似文献   
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