Showing papers on "Nucleolus published in 1968"
TL;DR: The nuceloplasm and nucleolus of HeLa cells contain six distinct low molecular weight species of RNA that are long-lived and do not appear to be precursor to any cytoplasmic product.
443 citations
TL;DR: The ovarian DNA of Xenopus is described and it is demonstrated that it contains an excess of sequences coding for rRNA, which is particularly striking in ovaries of the toads Bufo and Xenopus, in which the differential synthesis occurs during pachytene.
Abstract: Molecular hybridization experiments have shown that the genes for ribosomal RNA are located in or near the nucleolus organizer in Drosophila melanogasterl' 2 and in the toad Xenopus laevis.3' 4 Under normal circumstances the number of organizers per genome will be characteristic of the organism, and DNA from various tissues should contain the same proportion of ribosomal genes. This conclusion has been confirmed for several tissues of the chicken.2 However, cytological evidence has long suggested that the nucleolus organizer undergoes a differential replication in oocytes of certain animals. Data supporting this view will be summarized later in this article. If such a differential replication occurs, oocyte DNA should be enriched with respect to the ribosomal genes. The cytological picture is particularly striking in ovaries of the toads Bufo and Xenopus, in which the differential synthesis occurs during pachytene. In recently metamorphosed toads the ovary contains a sufficiently high proportion of pachytene o6cytes to permit detection of the differential synthesis by biochemical means.5 This communication describes the ovarian DNA of Xenopus and demonstrates that it contains an excess of sequences coding for rRNA. Recently Brown and co-workers6' 7 have studied the DNA from older oocytes of Xenopus and have reached similar conclusions.
341 citations
TL;DR: The nucleic acid content of neuronal nucleoli and the total cell body nucleic Acid content of neurones of the hypoglossal nucleus were measured by ultraviolet absorption microspectrography and the results confirmed the predominance of Na6(CO3)(SO4) in neurons of the H2O2-positive type.
Abstract: 1. The nucleic acid content of neuronal nucleoli and the total cell body nucleic acid content of neurones of the hypoglossal nucleus were measured by ultraviolet absorption microspectrography.
2. After nerve injury both the nucleolar nucleic acid and the total cell body nucleic acid increased: nucleolar changes preceded those of the cell body.
3. The closer to the nerve cell body that the axon was injured the earlier was the onset and the decline of the nucleolar response.
4. Actinomycin D was given to prevent DNA-primed RNA synthesis, and the rate of `decay' of nucleolar RNA was measured. This rate varied after nerve injury and was closely related to the nucleolar nucleic acid content.
5. The apparent rate of transfer of labelled RNA from the neuronal nucleus into the cytoplasm changed after nerve injury in a manner closely related to the changes in nucleolar nucleic acid content.
6. It was demonstrated by making consecutive nerve injuries or by preventing or delaying nerve regeneration, that the nucleic acid changes were not induced by removal of contact between the neurone and its motor end-plate, and were not repressed by the restoration of such contact.
7. When regeneration was prevented the nucleolar nucleic acid content and the total cell body nucleic acid ultimately decreased to values less than normal: this decrease was greater when more of the axon was initially removed.
8. The results are discussed in relation to the factor responsible for derepression and repression of DNA cistrons for ribosome synthesis in injured nerve cells.
272 citations
TL;DR: This chapter discusses the electron microscopic morphology of oogenesis, which is concerned with the partial or complete perforation of the envelope, possibly facilitating the passage of nuclear material to the cytoplasm.
Abstract: Publisher Summary This chapter discusses the electron microscopic morphology of oogenesis. The nucleus plays a very important role in regulation of cell life processes. In accordance with the theory that the nucleolus controls the synthesis of cytoplasmic RNA, it is noted in studies of oocyte nuclei that the nucleolus often shows signs of emitting granules into the nucleoplasm. In vitro nucleolar emission is observed in oocytes of the roach and this finding was correlated with a reticulate rim of the nucleolus as seen in the electron microscope. Two processes visibly affect the nuclear envelope during oogenesis. One is concerned with the partial or complete perforation of the envelope, possibly facilitating the passage of nuclear material to the cytoplasm; the other is the enlargement of the nuclear membrane area as well as its area relative to nuclear volume. 2. The literature on oogenesis makes little mention of smooth endoplasmic reticulum. It is generally accepted that this type of cytomembrane plays only a limited role in oocytes. The controversy in interpretation of Golgi structure and function may be equally applied to oogenetical studies. Electron microscopy has so far been the most reliable means for studying Golgi material.
188 citations
TL;DR: The demonstration by electron microscope autoradiography that the label in these cells was restricted to the periphery of the nucleus and nucleolus suggests that DNA synthesis in human cells may be initiated at the nuclear membrane.
171 citations
TL;DR: There is compelling evidence in amphibia and in the insect Tipula that this DNA is derived from the chromosomal nucleolar organizer region which contains the ribosomal cistrons, and this conclusion is supported by the recent finding that in ovarian tissue of Xenopus there is a disproportionately greater synthesis of ribosomes.
Abstract: DURING the development of the amphibian oocyte, the germinal vesicle becomes enlarged and can be seen to contain very many nucleoli1,2 instead of the four which would be expected from the tetraploidy of the egg nucleus. The RNA of these nucleoli is similar in base composition to ribosomal RNA3 and each nucleolus contains a small amount of DNA4–6. Amphibian oocyte nucleoli can be hypotonically disrupted to reveal DNP rings in the shape of beaded circles or necklaces2,6. There is compelling evidence in amphibia2,6,7 as well as in the insect Tipula8 that this DNA is derived from the chromosomal nucleolar organizer region which contains the ribosomal cistrons9,10. This conclusion is supported by the recent finding that in ovarian tissue of Xenopus there is a disproportionately greater synthesis of ribosomal cistrons11 which leads eventually to a higher proportion of them than that found in DNA of somatic tissues11,12. In Bufo, the nucleolar DNA migrates as granules from the chromosomal bouquet to the periphery of the nucleus where, at later stages, the granules give rise to nucleoli5. A similar process occurs in Xenopus where Feulgen-positive material appears in the oocyte nucleus as a “nuclear cap” at leptotene13. After zygotene this cap disperses and numerous nucleoli are formed which contain one or several DNA cores or rings14.
133 citations
TL;DR: Nucleoli isolated from mouse strain L-cells, L-5178Y lymphoblast tumor cells, and mouse liver and kidney cells showed a 3–4-fold enrichment in satellite DNA when compared with DNA from whole nuclei, suggesting that satellite DNA was more intimately bound to the nucleoli than main band DNA.
106 citations
TL;DR: The base composition of 45 s ribosomal precursor was found to be 70 mole % guanine plus cytosine compared to 64% predicted for a precursor containing one 28 s and one 18 s piece of Ribosomal RNA, in agreement with the previous finding that only one-half of the 45 s RNA eventually becomes mature ribosome RNA.
105 citations
TL;DR: It is postulate that the altered structure of the 45-S RNA does not permit this conversion of rRNA to tRNA, and synthesis of tRNA is not inhibited by toyocamycin, as shown by the ratio of methylation to nucleoside incorporation.
103 citations
TL;DR: The results indicate that the RNA polymerase is bound to the DNA in the aggregate enzyme as part of an initiation or transcription complex, and the small amount of A-U-rich RNA which is synthesized at 37° is immediately destroyed by degradative enzymes, whereas the G-C-richRNA is much more resistant.
95 citations
TL;DR: Cell fractionation studies revealed that, although the initial processing occurs in the presence of puromycin, the 16S moiety is immediately degraded, and no species of ribosomal RNA can be found to have emerged from the nucleolus.
Abstract: Inhibition of protein synthesis by puromycin (100 γ/ml) is known to inhibit the synthesis of ribosomes. However, ribosomal precursor RNA (45S) continues to be synthesized, methylated, and processed. Cell fractionation studies revealed that, although the initial processing (45S → 32S + 16S) occurs in the presence of puromycin, the 16S moiety is immediately degraded. No species of ribosomal RNA can be found to have emerged from the nucleolus. The RNA formed in the presence of puromycin is normal as judged by its ability to enter new ribosomal particles after puromycin is removed. This sequence of events is not a result of inhibition of protein synthesis, for cycloheximide, another inhibitor of protein synthesis, either alone or in combination with puromycin allows the completion of new ribosomes.
TL;DR: The specific arrangement and association between the Golgi complexes and endoplasmic reticulum strongly suggest that the two organelles play a role in the elaboration of proteid yolk granules and cortical granules.
TL;DR: It is suggested that nucleoli show at least two basic reaction patterns to inhibitors of RNA synthesis, one typified by actinomycin D and one by ethionine.
Abstract: The structure of nuclei and nucleoli of hepatic cells after short-term ethionine administration was investigated with the electron microscope. By 1½ hr after the injection, a distinct alteration occurred in the nucleoli which was characterized by the appearance of electron-opaque masses in the nucleolonema. After 6–8 hr, the nucleoli showed partial fragmentation into small, dense masses. Large aggregates of interchromatinic granules appeared in the nucleoplasm. Condensation of chromatin became prominent in the nucleoplasm particularly along the nuclear membrane. By 12 hr almost complete fragmentation of nucleoli had occurred. The administration of adenine or methionine at 4 hr prevented the development of nucleolar changes. Also, adenine administration at 8 hr after ethionine completely reversed the nucleolar lesion by 12 hr. After methionine administration at 8 hr, many nucleoli showed incomplete reconstruction with many twisted ropelike structures when viewed 4 hr later. Identical structures were found when adenine was given at 8 hr, and animals were sacrificed 2 hr later. On the basis of this observation, the simplified structures of nucleoli found 2 hr after adenine or 4 hr after methionine appeared to be precursors of the nucleolonema. It is suggested that nucleoli show at least two basic reaction patterns to inhibitors of RNA synthesis, one typified by actinomycin D and one by ethionine.
TL;DR: Rat liver nuclei were freed of cytoplasmic contamination by washing with Triton-X-100 and subsequent centrifugation through 2.2 M sucrose and obtained nuclei obtained in yields of about 70% and were suitable for the isolation of nucleoli.
Abstract: Rat liver nuclei were freed of cytoplasmic contamination by washing with Triton-X-100 and subsequent centrifugation through 2.2 M sucrose. Electron microscopic examination showed that the outer membranes of the nuclei had been removed, but that the nuclei otherwise resembled the nuclei of intact liver. Morphological studies, chemical estimations of DNA, RNA, and protein and the estimation of cytoplasmic "marker" enzymes suggested that contamination of nuclei by cytoplasmic components was limited. These nuclei were obtained in yields of about 70% and were suitable for the isolation of nucleoli. Nucleoli were isolated by the breaking of the nuclei by ultrasound and subsequent differential centrifugation. In ultrastructural appearance, the isolated nucleoli resembled nucleoli in intact tissue. However, at high magnifications the "granular" component of isolated nucleoli appeared to consist of tightly twisted fibers. The nucleoli could be obtained in yields of at least 30%, and the values for the chemical composition of the isolated nucleoli agreed with values previously reported.
TL;DR: It was found that 3 h after cycloheximide treatment the 32 P-labelled RNA which remained in the nuclei did not show the characteristic [ 32 P]base composition of ribosomal RNA, and results suggest that protein synthesis is required for both continual formation of ribsomal RNA and for maturation of Ribosomes.
TL;DR: Evidence exists that favors the RNP nature of this material, and double-labeling studies of RNA and protein reveal that both types of particle incorporate uridine-3H, but that the 50-55S material of immature oocytes does not incorporate 14C-labeled amino acids.
Abstract: Studies of the sedimentation properties of RNP1 material from the nucleus of the amphibian oocyte have indicated (1) that there are few, if any, 78S ribosomes in the nucleus, (2) that there are smaller particles sedimenting at 50-55S and 30S, and (3) that the larger of these is the precursor of the 60S subunit of the cytoplasmic ribosomes. Although the nature of the 30S material is not completely clear, it probably includes precursor particles to the 40S ribosomal subunit. Heavy (50-55S) particles are predominant in immature oocytes of Triturus viridescens, whereas in immature oocytes of Triturus and Amblystoma mexicanum they are reduced greatly in amount, but are still detectable. Double-labeling studies of RNA and protein reveal that both types of particle incorporate uridine-3H, but that the 50-55S material of immature oocytes does not incorporate 14C-labeled amino acids. However, other evidence exists that favors the RNP nature of this material. Sedimentation analyses after SDS extraction show that 50-55S particles contain 40 and 30S RNA, whereas 30S particles contain 20S RNA. These types of RNA represent at least 80% of all the extractable nuclear RNA. The 50-55S particles are probably heterogeneous, including both particles containing mostly 40S RNA and particles containing only 30S RNA.
TL;DR: A tentative model for the conversion process whereby 45-S RNA undergoes a single scission with an endonuclease followed by a stepwise trimming with exonucleasing is suggested.
TL;DR: The peripheral region of the nucleus was found to be more active than the rest of the nucleoplasm, even after a period of chase and particularly when the cell population is in the second part of the S period.
TL;DR: The changes in fine structure of the growing female germ cell of Arbacia lixula and Paracentrotus lividus were examined by light and electron microscopy, finding strong evidence that they do not represent patent openings.
Abstract: The changes in fine structure of the growing female germ cell of Arbacia lixula and Paracentrotus lividus were examined by light and electron microscopy. During the previtellogenic growth stage of the oocyte, when the ribosome population in the cytoplasm increases several thousand times, the nucleolus also increases considerably in volume. A large number of ribosome-like granules are concentrated in these nucleoli. Although particularly during this stage a passage of “informative” material from the nucleus to the cytoplasm is to be expected, at present, evidence of any kind of extrusion has not been observed in these species. The only indirect morphological evidence for passage of material out of the nuclei might be the many newly formed pore complexes in the nuclear envelope. There is, however, strong evidence that they do not represent patent openings, since a dense, almost homogeneous, material is observed within the pore. Thus it is assumed that small molecules might pass through the pores freely, but not large particles the size of ribosomes.
Dense aggregates of two different types have been observed in secondary oogonia and in oocytes at the growth stage: one shows densely packed fibrils of about 80 A and the other granules of 300 to 400 A.
At the end of vitellogenesis, the major nucleolus becomes vacuolated and disappears. At the same time several minor “agranular” nucleoli are formed. At meiosis these too disappear. In the mature egg few “agranular” nucleoli are seen. During cleavage the several “agranular” nucleoli are fibrillar, without ribosome-like granules. From the blastula stage on, at a time when the cell divisions stop almost completely, the one or two nucleoli which can be observed in each nucleus have a small number of granules. During the following development the number of granules increases gradually up to gastrula, whereas the number of fibrils becomes more and more reduced.
TL;DR: The greater labeling of the fibrillar components of the nucleoli suggests that initial biosynthetic reactions occur in these regions as suggested by Granboulan and GranBoulan [4] and by Geuskens and Bernhard [3].
TL;DR: The significant reduction in nucleolar number between G1 and mitosis was found to be due largely to fusion, and increase in size resulted from both fusion and synthetic activity within the nucleolus.
TL;DR: The nucleolus appears to initiate incorporation of 3H-5-uridine into the peripheral region at which the chromatin is associated, and during gastrulation, the nucleoli acquires particulate components and becomes positive to cytochemical tests for RNA.
TL;DR: Results show that any substance that inhibits nucleolar biosynthesis also produces segregation of nucleolar granular and fibrillar elements.
TL;DR: The nucleoli purified in high salt concentration conserve the capacity to carry on in vitro the initial stages of processing of ribosomal RNA precursors as well as retain structural and functional properties by nucleoli after purification.
TL;DR: These findings strongly support the earlier proposition that androgens exert a discriminatory action on RNA formation by prostatic nuclei and selectively enhance the RNA synthesizing activity at non-nucleolar chromatin regions.
Abstract: RNA synthesis from 3H-nucleoside triphosphates by isolated prostatic nuclei of control and testosterone-treated castrated rats was analyzed by autoradiographic technique. RNA synthesis by nuclei of control castrates was found to localize all over the nuclear chromatin regions except nucleolar regions, where RNA synthesis appeared feeble. In the nuclei of testosterone- injected animals, there was vigorous synthesis of RNA at nucleoli and their immediate vicinity. The androgen manipulation of animals did not significantly alter the RNA synthesizing activity at non-nucleolar chromatin regions. AdditionAddition of a low concentration of actinomycin D to the RNA synthesizing system in vitro can selectively suppress the androgen effect on the nucleolar RNA synthesis without any dramatic effect on the RNA synthesis at non-nucleolar chromatin regions. These findings strongly support the earlier proposition that androgens exert a discriminatory action on RNA formation by prostatic nuclei and selectively enhance th...
TL;DR: The present experiments suggest that the Vinca alkaloid drugs inhibit both the synthesis and processing of the nucleolar RNA precursors of rRNA.
Abstract: The Vinca alkaloids vinblastine sulfate and vincrystine sulfate, which are mitotic poisons, inhibit RNA synthesis in human (HEp-2) cells cultured in vitro. Analyses of RNA synthesis by cells treated with these drugs by acrylamide gel electrophoresis show that 28s rRNA and to a lesser extent 18s rRNA are preferentially inhibited. The synthesis of tRNA is affected much less than that of rRNA. The present experiments suggest that the drugs inhibit both the synthesis and processing of the nucleolar RNA precursors of rRNA. An explanation is also given for previous reports that these alkaloids preferentially inhibit the synthesis of tRNA in animal cells in vitro.
TL;DR: Findings show that DNA is present in the ring-shaped nucleoli of mature human lymphocytes, and this effect represents a repression of activity of nucleolar DNA rather than its loss from the nucleolus.
TL;DR: It is suggested that protein component of the ribosome is necessary for the stabilization of ribosomal RNA in HeLa cells and therefore for its appearance in the cytoplasm.
TL;DR: The absence of any special ultrastructure in the chromatin from spermatogonia, and the small mass of the Chromatin condensations, show that the human X chromosome and perhaps the Y chromosome are not heteropycnotic in the interphasic nuclei of human sperMatogonia.
Abstract: The nuclear structure of human spermatogonia has been studied with electron microscopical and histochemical methods. Type B spermatogonia have chromatin clumps without any special ultrastructure and several nucleoli. Five different types of nuclear bodies, and besides, a nuclear vacuole, have been observed in type A spermatogonia. Type I bodies are typical nucleoli consisting of three regions: amorphous, fibrillar and granular. Type II, III and V are considered to be atypical nucleoli. Type IV bodies are small chromatin condensations. Type I bodies are the only ones in which RNA was demonstrated by light histochemical techniques and no PAS positive material was found inside the nuclei. The absence of any special ultrastructure in the chromatin from spermatogonia, and the small mass of the chromatin condensations, show that the human X chromosome and perhaps the Y chromosome are not heteropycnotic in the interphasic nuclei of human spermatogonia.