Incorporation of [3H]Uridine and [3H]Thymidine During the Phase of Nucleolar Multiplication in Xenopus Laevis Oögenesis: A High-Resolution Autoradiographic Study

Concerted evolution of ribosomal DNA: Somatic peace amid germinal strife: Intranuclear and cellular selection maintain the quality of rRNA

Most eukaryotes possess many copies of rDNA. Organismal selection alone cannot maintain rRNA function because the effects of mutations in one rDNA are diluted by the presence of many other rDNAs. rRNA quality is maintained by processes that increase homogeneity of rRNA within, and heterogeneity among, germ cells thereby increasing the effectiveness of cellular selection on ribosomal function. A successful rDNA repeat will possess adaptations for spreading within tandem arrays by intranuclear selection. These adaptations reside in the non-coding regions of rDNA. Single-copy genes are predicted to manage processes of intranuclear and cellular selection in the germline to maintain the quality of rRNA expressed in somatic cells of future generations.

Transport of amino acids and nucleosides in metaphase-arrested unfertilized oocytes of Xenopus laevis

Metaphase-arrested, unfertilized shed oocytes of Xenopus laevis obtained after hormonal stimulation of the female are able to take up nucleosides (U, T) and amino acids (Ala, Gly, Glu, Gln, Tyr). For alanine, tyrosine, and glutamic acid the transport is uphill. The transport of the amino acids studied is activated by Na+, whereas the uptake of the nucleosides is independent of the Na+ concentration. Ouabain does not inhibit the uptake of amino acids significantly. The uptake of alanine and thymidine is not measurably affected by the presence of the jelly coat.

Quantitative ultrastructural characterisation of rabbit erythroblasts and of 3H uridine incorporation into their nuclear components

Electron microscopic-autoradiographic and morphometric investigations were carried out in erythroblasts from rabbit bone marrow. 1. to obtain quantitative data for the erythroblast classes. 2. to determine quantitative-morphometrically the incorporation rate of 3H uridine into the cell components of the different erythroblast classes. 3. to evaluate the influence of UV irradiation on the incorporation of 3H uridine. According to ultrastructural criteria erythroblasts are divided into 3 classes: basophilic erythroblasts (BE), polychromatic erythroblasts (PE) and orthochromatic erythroblasts (OE). The areas of the 3 classes of erythroblasts are in a ratio of 1:0.5:0.43. In all classes the portion of the nucleus is between 56 and 58.8%. The portion of mitochondria decreases from 13.3% (BE) to 9.3% (PE), and to 6.3% (OE) respectively. Simultaneously the portion of heterochromatin in the nuclear area rises from 24.2% (BE) to 49.8% (PE), and to 63.2% (OE) whereas the percentage of euchromatin decreased correspondingly. Nucleoli were found only in 78% of the BE and in 3.4% of the PE. For the quantitative determination of the silver grains and silver filaments, respectively, the morphometric procedures have been adapted to the particular conditions. So it was necessary to estimate the areas covered by silver grains of filaments. The silver grains or filaments localised in the nuclear area are in the range from 84.6 to 93.4%. The percentage of silver grains of the total cellular area is 14.4% in BE, 6.8% in PE and 4% in OE. In a similar way the relative values of the nucleolus (total) and the euchromatin and heterochromatin are reduced. UV irradiation of bone marrow in vitro for 10 to 20 min does not cause any significant changes in the incorporation of 3H uridine. This result is in contrast to other published experiments on the influence of UV on the RNA synthesis. It is assumed that the irradiation dose was too low to induce a measurable effect on the RNA synthesis or the point of investigation was too early.