Subsequent Cell Division Research Articles

Synthesis of purines and nucleic acids and catabolism of purines by rat liver and hepatomas

Summary Studies of the in vivo metabolism of formate-C 14 and adenine-8-C 14 by hepatomas and host liver, of the in vitro metabolism of adenine-8-C 14 and hypoxanthine-8-C 14 by minced hepatomas, host liver, and regenerating liver, and of the in vitro catabolism of adenosine-8-C 14 monophosphate (5′) and inosine-8-C 14 monophosphate (5′) by sonicates of hepatomas, host liver and regenerating liver have been carried out. The quantities of C 14 derived from the labeled substrate under the described conditions and present in a number of low molecular weight compounds and nucleic acids were determined. As a result of these studies it is suggested that control of cellular proliferation of liver and regenerating liver might be partially mediated through control of de novo synthesis of purine ribonucleotides and of DNA polymerase activity, while in hepatomas a faulty control of de novo synthesis permits purine ribonucleotides to be formed continuously and the levels of activity of catabolic enzymes and of DNA synthetase determine the rates of synthesis of nucleic acids and subsequent cell division.

On the question of renewal of differentiated cells

Studies of the incorporation of 3H-thymidine in cells of differentiated organs, especially the seminal vesicles of mice, provide evidence that this precursor is incorporated into the DNA (a) at a rate equal to that found in dividing tissues or (b) at a considerably lower rate. Comparison of labeling indices and mitotic indices shows that in many organs fewer cells divide than would be expected from the number of labeled cells. It is suggested that either all of the DNA or some part of the genetic material can be renewed without subsequent cell division.

Tritium Retention by Corneal Endothelium After Incorporation of H3--Thymidine

A program of study on DNA synthesis and mitosis in normal and injured tissues is being carried out in this laboratory. It has been demonstrated that injuries to the lens epithelium or corneal endothelium can stimulate the incorporation of tritium-labeled thymidine by many cells.1,2The accumulated evidence from other studies has suggested that thymidine is a specific precursor to DNA synthesis and is incorporated into cells that are undergoing chromosome duplication in preparation for cell division.3,4It is furthermore assumed that the tritium thus incorporated into cell nuclei is retained indefinitely by the cell5; a decrease in tritium per cell being accomplished, however, by subsequent cell divisions. Corneal endothelium apparently normally undergoes very little or no mitotic activity.6,10For a limited period of time after mechanical injury, however, a number of cells in the vicinity of the wound show thymidine incorporation and division. After this period,

Cyclic variations in the peptidase and catheptic activities of yeast cultures synchronized with respect to cell multiplication

The total “over-all” dipeptidase and proteinase A activities of normal and X-irradiated cultures of Saccharomyces cerevisiae were assayed during various physiological states. Yeast cultures first starved for some hours in succinate buffer were then made to undergo two or three somewhat synchronous cell divisions in a yeast extract growth medium. The cytological characteristics were followed as well as the content of UV-absorbing cytoplasmic materials. The enzymic activities were expressed on a per-cell basis and also per protein content. During the starvation phase the protein content per cell remained high but a marked decrease of dipeptidase activity per cell was noted, while the proteinase A activity showed a minor decrease. During the subsequent growth and cell division period marked cyclic variations occurred in the enzyme activities, more marked for the peptidase and less for the proteinase. The highest enzymic activities always coincide with stages preparatory to cell division, and the lowest levels were reached during each subsequent budding period. After 15,000 r of X-irradiation the first division occurred normally in timing as in protein and enzyme content. The second division was delayed and many of the cells inhibited from budding. After this division the nitrogen content per cell increased to about 150 per cent of the normal, and the enzyme content remained high. The cells were left in a state preparatory to cell division. No appreciable enzyme activity was detected in the culture medium during the various phases of normal yeast cell growth or in the post-irradiation period. The results indicate that cycles of increased proteolysis form essential steps during certain stages of cell growth preparatory to cell division. The idea is advanced that the proteinases and peptidases may be operative in the intracellular reactions aiming at the replenishment of the metabolic amino acid pool necessary for protein synthesis.

The DNA content of cells in the root of Vicia faba

The DNA content of individual cells in the root tip of Vicia faba has been measured by a cytochemical method. Roots were grown under standard conditions and divided into segments 0–2, 2–4, 4–8 and 12–20 mm behind the root cap, as in the biochemical experiments of Holmes et al. [7]. From the measurements on meristematic cells (0–2 mm) synthesis curves for DNA (from the 2C to the 4C value) are derived which are in broad agreement with that found earlier by the P 32 autoradiographic method [8]. Our results indicate that DNA synthesis occurs approximately during the middle third of interphase. Our results also indicate that there is negligible change in the total amount of DNA during the division of a cell. Measurements of the average DNA content per cell in each segment show an increase with distance from the root tip similar to that found by the biochemical method [7]. Cells in the elongation zone of the root appear to synthesise DNA from the 2C to the 4C value and from the 4C to the 8C value, without subsequent cell division.

The sensitivity of the cell to x-rays

1. 1) The effect of x-rays on plant cells has been studied in combination with indole-3-acetic acid and colchicine treatments. As material root tip cells of Allium cepa have been used. 2. 2) The greatest sensitivity to breakage and reunion was shown by meristematic cells in the root tip which had been arrested in interphase by indole-3-acetic acid. 3. 3) Cells which had been treated with colchicine revealed almost as many injuries as those irradiated during interphase. This is interpreted as depending on a lowered tendency to reunion due to the colchicine treatment. 4. 4) Differentiated cells, which had ceased to divide, were induced to undergo mitosis after irradiation by a treatment with indole-3-acetic acid. The chromosomes in these cells proved very refractory to x-rays. 5. 5) X-raying seems to strengthen the effect of indole-3-acetic acid in inducing cell division. The phenomenon became apparent with the lowest possible concentrations of indole-3-acetic acid inducing division. 6. 6) A hypothesis is advanced concerning the mechanism of the structural changes in the chromosomes induced by x-rays. It is based on the assumption that a chromosome consists of segments which act as units in reproduction. X-rays may either break the chromosome between these segmental units or destroy the reproductive power of a segment which leads to breakage in subsequent cell divisions.

The Effect of Hexachlorocyclohexane on Mitosis in Roots of the Onion (Allium Cepa) and Strawberry (Fragaria Vesca)

Experiments were carried out with a proprietary flea beetle dust containing crude BHC to ascertain whether, employed as an insecticide, it has any effect on cell division. Root-tips of onion seedlings and strawberry runners were used as test material and the doses were equivalent to 40–200 pounds per acre of flea beetle dust. By germinating onion seeds in direct contact with the insecticide it was shown that it has two effects on the chromosomes. The frequency of nuclear divisions was unaffected, but low doses may lead to the exclusion of nuclear fragments in subsequent cell divisions and higher doses may lead to polyploidy. Both effects were produced by the dust and the gamma and delta forms of BHC when used separately. The alpha and beta forms were only slightly active in high concentrations. The milder effect was particularly evident for a period of at least four days after removal from the insecticide.In a series of pot experiments it was shown that relatively high doses had only a slight effect on ce...

Nuclear and Cell Division in Allium cepa as Influenced by Slow Neutrons and X-Rays

1. Bulbs of Allium cepa, variety Ebenezer, were exposed to X-ray dosages of 100 r, 1000 r, or 10,000 r, or to show neutrons produced in the Argonne heavywater pile. The neutron-treated material was exposed to an average neutron flux of 5.8 x 1010 slow neutrons/sq cm/sec for periods of ~2, 4, 8, or 16 minutes. 2. Plants from treated bulbs in both series were markedly retarded in growth with respect to control plants. All plants from bulbs treated with slow neutrons died as did also those treated with the highest dosage of X-rays. 3. Bulbs from control and treated series were sprouted in soil. Cytological analysis of root tips collected at 48-, 52-, and 96-hour intervals showed that the effects of both types of treatment followed a pattern that was chronologically determined: a general, but temporary, inhibition of mitosis; abnormal nuclear division necessitated by structural chromosome aberrations; abnormal cell division directly following aberrant nuclear division; and, lastly, degeneration of nuclei and cells which had been severely damaged either by the original treatment or by disorientation in normal call processes during the preceding stages. 4. Chromosomal aberrations of all types-dicentrics, centric and acentric rings, acentric fragments, and dot deletions-were observed in root-tip cells from both treatments. Dosage had no apparent effect on type of aberrancy induced. Pycnosis of many nuclei at the higher dosages was of common occurrence. 5. Aberration frequency induced by 2-, 4-, or 8-minute exposures to slow neutrons was higher in all cases than that produced by either 100 r or 1000 r. Frequencies in the 10,000-r series approximated those found for the 16-minute dosage with slow neutrons. 6. The degree of fragmentation produced at the higher treatment levels by both X-rays and slow neutrons gave a more accurate picture of the shattering effect on chromosomes than did the extent of formation of dicentrics and rings. 7. The observation of pseudo-amitotic nuclei and daughter nuclei accompanied by micronuclei does not constitute evidence of induced amitosis as a result of treatment. These are stages in a time continuum of the general effects of radiation on chromosome breakage, abnormal nuclear division, and subsequent abnormal cell division. 8. The marked inhibition of mitosis found in the present study and the low dosage-curve exponents obtained for aberration frequency indicate that with regard to quantitative determination of the relationship between excessive treatment with slow neutrons and response of the organism, aberration frequency alone does not yield a wholly representative or valid index of the true picture of dosage effect.

THE GENETICS OF SEX IN LEPIDOPTERA

THE GENETICS OF SEX IN LEPIDOPTERA