Mammalian cell nucleolus

D. Ranitidine is an H2-receptor antagonist. H2-receptors are found in the cell membrane of parietal cells, not in the nucleus, nucleolus, or cytoplasm. Mammalian cells do not have cell walls. 

Which of the following statements correctly describes the nucleolus of a mammalian cell ... 

Two of the most highly methylated proteins described to date are found in the nucleolus of mammalian cells and include a 34 kDa scleroderma antigen (Lischwe, Ochs, Reddy, Cook, Yeoman, Tan, Reichlin, and Busch, 1985) and the 110 kDa... 

Two types of relationships between nucleolus and chromosomes have been considered an indirect relationship in which the size and number of nucleoli are assumed to be dependent on the chromosomal stock of the cells, and a direct relationship in which the nucleolus is thought to be derived directly from chromosomal material. Studies on plants seem to have established conclusively that the number and size of nucleoli are characteristic of a species and, therefore, under genetic control. The number of nucleoli was found to be directly related to the number of chromosomes in the heteroploid nuclei of amphibians. Although no such conclusive relationships have been satisfactorily established in mammalian tissue, mammalian chromosomes possibly control nucleolar composition in a more subtle manner than by affecting their number and size. 

RNA polymerase has now been purified in various laboratories from various mammalian sources [188], and RNA polymerase of eukaryotic nuclei always exists in multiple forms that can be distinguished by the cation requirement, their sensitivity to a-amanitin, and their ability to react with specific templates. One enzyme (polymerase I) is found in the nucleolus, the others in the nucleoplasm. All mammalian ribonucle-ases are complex proteins formed of several subunits. Three types of RNA polymerases have been purified from ascites tumor cells by chromatography on car-boxymethyl-cellulose. Two are nucleolar and one is nucleoplasmic. Protein factors of unknown nature that stimulate all three enzymes have been found in calf thymus, rat liver, and ascites cells. These factors can be separated into two classes heat stable and heat labile. Both types stimulate the activity of the Novikoff RNA polymerase several-fold, but only with native DNA as templates. The factors have no effect on E. coli RNA polymerase [266-267]. For further information, refer to the review of Jacobs [189]. 

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