Evidence for control of the rate of nuclear DNA synthesis by the nuclear membrane in eukaryotic cells

An analysis of the mitotic cell cycle in the root meristem of Zea mays

A pulse labelling experiment was used to study the mitotic cell cycle of proliferating cells throughout the root meristem of Zea mays . Seventeen different regions were identified within the area of proliferative activity, extending from the initial cells of the cap columella up to the stele, cortex and epidermis 1000 μm from the cap-quiescent centre junction, and the data were analysed for each region separately. The analyses were made in terms of a mathematical model for cell proliferation and yield statistically efficient estimates of the cell-cycle parameters. The validity of the model is discussed in some detail. It appears that the main difference between the regions studied is in the mean duration of G 1 , that is, the average delay a newborn cell experiences before it begins to synthesize DNA. The mean durations of S and G 2 , the DNA-synthetic and post-DNA-synthetic phases of the mitotic cycle, are relatively constant. The one exception to this pattern is the quiescent centre; this region includes a relatively high proportion of slowly dividing and non-proliferating cells.

The time and duration of female meiosis in wheat, rye and barley

Few recent investigations have been made of female meiosis in cereals, and almost nothing is known about the duration of female meiosis in higher plants. Consequently, the time and duration of female meiosis in Triticum aestivum , Hordeum vulgare and Secale cereale have been studied. The appearance of the embryo sac mother cell (e. m. c.) and of the meiotic nuclei during female meiosis in Hordeum vulgare is described and illustrated. In the species studied, each floret contains only one ovary with a single e. m. c., and meiosis is almost synchronous in the pollen mother cells from all three anthers. Conse­quently, it is possible to make precise comparisons between the stages of male and female development within individual florets. Data from these comparisons, together with know­ledge previously determined of the duration of male meiosis in these species, allowed the estimation of the time and duration of female meiosis fairly accurately for T. aestivum and H. vulgare and approximately for S. cereale . The results showed that for H. vulgar and T. aestivum grown at 20°C, the duration of female meiosis was very similar to the duration of male meiosis. Furthermore, on average male and female meiosis occurred almost synchronously. In S. cereale however, male meiosis preceeded female meiosis by about 15 h. Growing T. aestivum under environmental stress induced asynchrony between male and female development at meiosis. Synchrony was not re-established after a long period under normal conditions. Nuclear DNA content and ploidy level are known to be important factors determining or affecting the duration of male meiosis. These factors appear to play an important role in controlling the duration of female meiosis also.

Early seed development in the Triticeae

The rates of early seed development were compared in several species in the Triticeae which play a major role in human nutrition, and in several related genotypes whose reproductive development is of current interest to plant breeders. Embryo and endosperm development during the first five days after pollination was studied in plants of 22 genotypes grown at 20 °C with continuous light. Spikes were emasculated before anther dehiscence and then pollinated once full female receptivity was reached. The numbers of embryo and endosperm nuclei or cells in individual florets were ascertained by using large samples of fertilized florets fixed at various known times after pollination. The pattern of early seed development was essentially the same in wheat, rye,Triticaleand barley, although some interspecific variation in the rate between genotypes was noted. Fertilization occurred in some florets of several genotypes studied within 40-60 min after pollination. Mitosis in the primary endosperm nucleus was completed about 6-7 h after pollination. During the next 24-48 h the number of endosperm nuclei increased geometrically, doubling about every 4-5 h. The endosperm was coenocytic at first but usually at about 72 h after pollination it became cellular. The rate of nuclear development in the endosperm declined on each successive day, the greatest fall occurring at the time of cell wall formation. Mitosis in the zygote occurred about 18-30 h after pollination which was later than mitosis in the primary endosperm nucleus. The cell cycle time in the embryo varied between species from about 12 to 18 h, and was similar to its duration in cells of other meristematic tissues in the same species. Cell cycle time in the embryo remained fairly constant during the first 5 days of seed development unlike the rate of nuclear development in the endosperm. Thus, at first the rate of embryo cell development was very slow compared with that of the endosperm nuclei, however, by the end of the fifth day the cell cycle time in the endosperm had increased to become equal to or longer than that of the cell cycle in embryo cells. The nature and possible cause (s) of rapid nuclear development in coenocytic endosperm is discussed. While embryo volume increased steadily over the period studied, the mean volume of embryo cells decreased about tenfold. This was because at first the rate of increase in embryo volume was lower than the rate of increase in embryo cell number. Eventually these two rates became similar and thereafter further development gave rise to embryo cells whose volume was constant and roughly equivalent to that of other meristematic cells in the same species. The rates of embryo and endosperm development were as a rule much faster in wheat species than in rye. By comparison, the rates in hexaploidTriticalegenotypes were usually much slower than in wheat, and sometimes even slower than in rye. Results for wheat-rye chromosome addition lines, disomic for each rye chromosome, show that most rye chromosomes apparently had a pronounced effect on slowing both embryo and endosperm development. Indeed, rye chromosomes VI and V II apparently had an effect equal to that of the presence of a whole rye genome. Comparison of the maximum rates of endosperm development in diploid and related polyploid species shows that there was no effect of polyploidy during the first 48 h of the coenocytic phase of endosperm development. Concurrently, during development of the cellular embryo there was a clear effect of ploidy level, with a positive relation between ploidy level and developmental rate. These results are compared with the effects of polyploidy on the rate of development in other tissues in the same species. The rates of embryo and endosperm development inHordeum vulgarewere much faster than in diploidH. bulbosum. This result is discussed with reference to the mechanism of chromosome elimination from embryo and endosperm tissues of Fj-hybrids between these two species. The present results provide a detailed picture of the course of normal early seed development in a wide range of cereal genotypes which varied with respect to several characters known to affect rate of development in other tissues. They provide, therefore, a baseline for comparative studies which aim both to describe abnormal early seed development and to quantify its extent, in for instanceTriticalewithshrivelled grain. At the same time they provide some indication of the factors which apparently influence or control the rate and extent of early embryo and endosperm development in these important crop species.

Control of DNA synthesis in polyploid mammalian cells

To test the hypothesis that the duration of DNA synthesis is an inverse function of nuclear size or DNA content, the S phase was calculated from PLM analysis for pseudodiploid, tetraploid, and octaploid lines of Chinese hamster cells growing as a monolayer or in suspension. S phase times were found not to be significantly different between polyploid lines and the diploid lines from which they were derived, regardless of the conformation of the nucleus. There is no evidence, therefore, that would implicate the nuclear membrane, or nuclear surface area/volume relationships, in the control of DNA synthesis.

Chromatin and DNA synthesis associated with nuclear membrane in germinating cotton

The synthesis of nuclear DNA and possible attachment sites of chromatin in the cells of cotton (Gossypium barbadense) radicles during germination was investigated. Biochemical analysis of nuclear membrane fragments or Sarkosyl-magnesium-membrane complexes indicates that the DNA, including newly replicated DNA, is attached to the nuclear membranes during periods of active synthesis. Electron micrographs of nuclear membrane fragments indicate a physical association between chromatin fibers and the membranes. The attachment site appears to be proteinaceous, since the chromatin is released by protein degradative enzymes as evidenced by biochemical techniques and electron microscopic observations. Short-term labeling results in incorporation into a membrane-associated product indistinguishable from the bulk of nuclear DNA. DNA polymerase activity is also associated with nuclear membrane preparations in which [3H]thymidine triphosphate is incorporated into an acid-insoluble. DNase-sensitive product.

Nuclear envelopes. Structure and biochemistry of the nuclear envelope

The ultrastructure of the nuclear evelope is described in various cell types with special emphasis on its pore complexes (p.c.). The architecture of the p.c. is defined against the properties of other membranous pore formations. Evidence is presented that the non-membranous p.c. components contain ribonucleoproteins but do not represent the attachment sites of nuclear chromatin. The possible dynamic nature of the p.c. material is discussed in relation to nucleocytoplasmic translocation processes. DNA of the nuclear genome is firmly attached to interporous sections of the inner nuclear membrane. The stability of this attachment is demonstrated, and chemical and conformational characteristics as well as periods and kinetics of replication are given for both isolated membrane DNA and the corresponding chromatin in situ . The membrane-associated chromatin is dominated by a heterochromatinous character; it does not represent a transitory membrane interaction of replicating DNA. It is hypothesized that membraneattachment of specific regions of the chromosomes are a means to their ordered arrangement during interphase and prophase. Structure, lipid, protein and enzyme pattern of the nuclear membranes, as well as the incorporation kinetics, underline the relationship to the endoplasmic reticulum.

THE MITOTIC CYCLE IN AN AMPHIPLOID (TRITICALE) AND ITS PARENTAL SPECIES

The durations of the mitotic cycle and its phases were determined by 3H-thymidine labelling of root meristematic cells of a strain of a wheat-rye amphiploid (triticale), and its parents Triticum turgidum L. var durum 'Stewart' and Secale cereale L. 'Prolific'.The duration, in hours, of the various phases of the mitotic cycle were:[Formula: see text]Feulgen microspectrophotometry showed triticale to have 15% less DNA per 4C nucleus than the sum total of its parents Prolific and Stewart whose DNA contents were in the ratio of 2:3, indicating that the duration of the S period is independent of DNA content per nucleus. The rate of DNA synthesis, measured as number of silver grains over interphase nuclei, was correlated with the total area of the nuclear membrane.

Replication of nucleolus-associated DNA during "G2 phase" in Physarum polycephalum

In the myxomycete, Physarum polycephalum, the bulk of nuclear DNA replication occurs during a period of a few hours immediately following upon mitosis. During the remainder of the intermitotic period, incorporation of thymidine-(3)H continues at a low rate in the region of the nucleolus (radioautographs). A few nuclei incorporated thymidine-(3)H into the extranucleolar chromatin at a high rate at all times of the intermitotic period. These nuclei were exceptionally large and they frequently contained several small nucleoli of different sizes rather than the one, central nucleolus which is characteristic of a normal interphase nucleus.