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At least 145 records · Page 8Linked to original sources
PubMed1973

Nuclear and mitochondrial DNA synthesis in gamma ray-resistant and -sensitive slime mold amebas.

Uptake of [methyl-(3)H]thymidine label from Escherichia coli 15T(-) into the DNA of Dictyostelium discoideum has been measured in control and [(60)Co]-gamma-irradiated cells of the resistant strain NC-4 (D(10), colony-forming survival = 300 krad) and two sensitive daughter strains, gammas-18 (D(10) = 75 krad) and gammas-13 (D(10) = 4 krad). Nuclear (n) and mitochondrial (m) DNA were resolved by isopycnic CsCl gradients. The uptake of label into n-DNA during the immediate postirradiation period was selectively inhibited by irradiation, compared with uptake into m-DNA. For all three strains, the gamma ray dose to reduce the uptake into n-DNA to 37% of the control during the first hour after irradiation was 3 krad, whereas for uptake into m-DNA it was 75 krad. After the initial dose- and strain-dependent lag, uptake into n-DNA resumed. gammas-18 showed longer lags in n-DNA synthesis and cell division than did NC-4. gammas-13 resumed n-DNA synthesis and cell division after slightly shorter lags than for NC-4. The early postlag uptake into n-DNA in this strain was almost at the control rate and was accompanied by division until the cell number had nearly doubled. The rate of label uptake then declined, division stopped, and gradual cell lysis ensued. The postdelay response of gammas-13 was almost independent of dose in the range of 10-100 krad. The response of gammas-18 in these and earlier experiments is consistent with the view-point that it is sensitive because of a decreased rate of repair of DNA damage. However, the basis for the sensitivity of gammas-13 seems to be more complex. This strain undergoes a premature but short-lived burst of n-DNA synthesis and division for what appears to be about one round of replication. Replication then ceases, even at very low doses, leading to greatly reduced probability of survival.

Animals↗
PubMed1997

Inversely correlated cycles in speed and turning in an ameba: an oscillatory model of cell locomotion.

Previous biophysical models of ameboid crawling have described cell movement in terms of a persistent random walk. Speed and orientation were treated in the latter model as independent and temporally homogeneous stochastic processes. We show here that, at least in the case of Dictyostelium discoideum, both speed control and reorientation processes involve a deterministic, periodic component. We also show that the processes are synchronized and negatively correlated, as was suggested by earlier findings. That is, increased turning correlates with periods of slow movement. Therefore, previous models are inconsistent with the behavior of cells. Using a heuristic approach, we have developed a mathematical model that describes the statistical properties of the cell's velocity and movement of its centroid. Our observations and the model are consistent with the phenomenological description of ameboid motility as a cyclic process of pseudopod extension and retraction.

Amoeba↗