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Rat-liver polysomes have been studied in CsCl buoyant density gradients. 1. Membrane-bound polysomes band at a density of about 1.53 g/cm3, whereas free polysomes are recovered from the gradients a t two higher densities (about 1.56 and 1.545 g/cm3, called, respectively, type I and 11). Both types of free polysomes appear heterogeneous on sedimentation.They are found in all regions of the polysomal gradient.2. Monoribosomes obtained after mild ribonuclease digestion of either free or membrane-bound polysomes band a t a unique density (1.55 g/cm3).3. The two types of free polysomes have quite different sedimentation behaviors in the preparative discontinuous sucrose gradient. After 6 h of centrifugation a t 113000 xgSv, the polysomal pellet is mainly composed of the type I. Type I1 remains with rough membranes at the lower interphase and is recovered in the pellet only after a 20-h centrifugation.4. During starvation, it appears that the proportion of type I polysomes increases, while the proportion of type I1 decreases.Possible interaction with membranes of the type I1 polysomes is discussed.Isopycnic centrifugation in CsCl gradients provides a second dimension of analysis of ribonucleoprotein such as ribosomes, after sucrose gradients. Buoyant density of these particles can be measured after stabilization by formaldehyde [l] or glutaraldehyde [2] to prevent deproteinization [3]. Since it has been previously shown that buoyant density is an indication of the RNA to protein ratio [l], such a technique permits some approach to the problem of structural differences between free and membranebound ribosomes. Two classes of membrane-bound monoribosomes have been distinguished by this method in HeLa cells [4]. In rat liver cells, reports have mainly considered the stability of the binding of the ribosomes to membrane [5], the biological role [6-€41 and the metabolism of RNA of free and membrane-bound polysomes [9,10]. Structural differences between free and membrane-bound polyaomes remains obscure.This report presents the results of analysis in CsCl gradients of free and membrane-bound polysomes from rat liver. Since the choice of a detergent remains critical, effects of ionic and non-ionic detergents were first investigated.Abbreviation. RNA -protein, ribonucleoprotein.Enzyme. Ribonuclease (EC 2.7.7.16). Definition. A,,, unit, the quantity of material contained in 1 ml of a solution which has an absorbance of 1 at 260 nm, when measured in a 1-cm path-length cell. MATERIALS AND METHODSMale albino rats of the Wistar strain, weighing 200-250 g, were used in all experiments. Unless otherwise specified, they were fed ad libitum until sacrifice.[5-3H]Orotic acid (24 Ci/mmol, 0.5 mCi per rat) or [6-14C]orotic acid (34 Ci/mmol, 0.1 mCi per rat) (C.E.A. Saclay, fiance) were administered intraperitoneally in 0.9O/, NaCl solution, 16 h before the rats were killed. Under these conditions, polysomes were uniformly labeled [ll]. All centrifugations were carried out in Spinco L2 65 B ultracentrifuges (Beckman). SolutionsBuffer I: 50...
An ultrastructural study of the nucleus was carried out, during oogenesis of Perinereis cultrifera, accompanied by an autoradiographic and biochemical study of the syntheses of RNA. The nucleus encloses formations deriving from the dispersal of meiotic chromosomes and a voluminous nucleolus. The latter undergoes morphological development of which each stage is characteristic of a stage of oogenesis. The autoradiographic study shows that the synthesis of RNA of extra-nucleolar origin is highly intense in young oocytes (during the stages of previtellogenesis and vitellogenesis) and that it decreases in older oocytes. The synthesis of RNA of nucleolar origin is very weak during previtellogenesis, increases during vitellogenesis, which is the stage at which it reaches its peak, and then decreases during the stages of the development of cortical alveoli and of maturity. These autoradiographic results are confirmed by a biochemical study which shows that once an oocyte diameter of 80 Jlm is reached (mid-vitellogenesis), the specific radioactivity of 18 and 28 S rRNA and of 4 and 5 S RNA decreases progressively up to the end of oogenesis.
It is necessary, in order to teach Biology and Biochemistry at university nowadays, to deeply adapt the methods and behaviours of the teachers and those of the students. Some original devices, contributing to preparing or making easier the students'' professional integration, have been set up at the University of Sciences and Technology in Lille (USTL), at the first degree and also at the second and third one, in the Departments of Biology and Biochemistry. They are brought up, analysed and lightened with the thinking over that there are new conditions at university for most of the people to be able to get a genuine scientific and technical culture.
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