Timings of DNA synthesis in the nucleoli of Allium cepa L

Abstract: This review addresses the problem of understanding nucleolar morphology in terms of nucleolar function. Nucleolar morphology and function are outlined to serve as a basis for reviewing in situ-cytochemical results that have, so far, not led to a generally accepted view on the structure-function correlation for nucleoli. Nucleoli in meiotic cells are dealth with in detail, because they illustrate the relationship between chromosomes and nucleoli particularly well. Spontaneous and induced changes in nucleolar morphology are presented and an attempt is made to explain particular morphologies in terms of functional states. The use of nucleolar morphology as a tool in diagnostics is critically evaluated.

Abstract: Nucleoli were noted by biologists very early (Fontana, 1781) and a lot of data has been accumulated on the structure of nucleoli in the last decades. Due to the work of Perry (1962; Perry et al., 1961) it is known that the nucleolus is the site of ribosome biogenesis. Its visibility both in the light-and the electron microscope is due to the fact that the genes for ribosomal RNA are not only transcribed but also processed and complexed with ribosomal proteins in nucleoli, rendering them morphologically different from the rest of the karyoplasm.

Abstract: In okadaic acid treated HeLa cells, the chromosomes sometimes condense without being accompanied by nuclear envelope breakdown. These cells show “persistent” nucleoli. Within these “persistent” nucleoli the intranucleolar chromatin condenses and can be observed in the region of the dense nucleolar component (DNC) of the nucleoli. Other nucleolar components, namely the fibrillar centre (FC) and the granular component (GC) remain unchanged. These observations strongly speak for the localization of nucleolar chromatin (ribosomal cistrons) within the dense nucleolar component of the interphase nucleolus.

Abstract: We have traced the nucleolar chromatin from early prophase to the metaphase stage. In prophase this chromatin begins to condense and in metaphase it is fully condensed. In mitotic chromosomes, this chromatin remains surrounded by achromatic materials resembling the fibrillar centre. As such this region of the chromosomes appears as a gap or constriction at the light microscope level. The possible role of this achromatic material in relation to nucleologenesis and satellite association has been discussed.

Abstract: A low-viscosity embedding medium based on ERL-4206 is recommended for use in electron microscopy. The composition is: ERL-4206 (vinyl cyclohexene dioxide) 10 g, D.E.R. 736 (diglycidyl ether of polypropylene glycol) 6 g, NSA (nonenyl succinic anhydride) 26 g, and S-1 (dimethylaminoethanol or DMAE) 0.4 g. The medium is easily and rapidly prepared by dispensing the components, in turn by weight, into a single flask. The relatively low viscosity of the medium (60 cP) permits rapid mixing by shaking and swirling. The medium is infiltrated into specimens after the use of any one of several dehydrating fluids, such as ethanol, acetone, dioxan, hexylene glycol, isopropyl alcohol, propylene oxide, and tert.-butyl alcohol. It is compatible with each of these in all proportions. After infiltration the castings are polymerized at 70°C in 8 hours. Longer curing does not adversely affect the physical properties of the castings. Curing time can be reduced by increasing the temperature or the accelerator, S-1, or both; and the hardness of the castings is controlled by changes in the D.E.R. 736 flexibilizer. The medium has a long pot life of several days and infiltrates readily because of its low viscosity. The castings have good trimming and sectioning qualities. The embedding matrix of the sections is very resistant to oxidation by KMnO 4 and Ba(MnO 4 ) 2 , compared with resins containing NADIC methyl anhydride. Sections are tough under the electron beam and may be used without a supporting membrane on the grids. The background plastic in the sections shows no perceptible substructure at magnifications commonly used for biological materials. The medium has been used successfully with a wide range of specimens, including endosperms with a high lipid content, tissues with hard, lignified cell walls, and highly vacuolated parenchymatous tissues of ripe fruits.

Abstract: Chromosomes in G1, S, G2 and early prophase of Allium cepa root tip nuclei are oriented in the same position as telophase chromosomes. The centromeric heteroehromatin is aggregated in a chromocenter at one side of the nucleus, the telomeres scattered at the opposite side. Telomeres appear to associate with other telomeres in interphase in a roughly two by two fashion. Telomere-centromere DNA is late replicating. These results support the conclusion that chromosomes in higher organisms frequently maintain their telophase orientation from the end of telophase, during interphase and well into the next prophase.

Abstract: Centromeres at premeiotic interphase are clustered and situated in a small area of the nucleus opposite to the nuclear envelope associated heterochromatic masses. The centromeres may occur singly or they may associate to form a structure composed of 2 or more centromeres. Many centromere associations are nonhomologous. Interphase centromeres are not attached to the nuclear envelope. — At zygotene and pachytene centromeres are no longer clustered at one pole of the nucleus but rather are distributed throughout the nucleus. Premeiotic associations appear to be resolved prior to meiotic pairing. Only homologous centromere associations occur during zygotene and pachytene. There is no indication that premeiotic centromere associations are involved in prezygotene alignment of homologous chromosomes.