Two haploid cell lines have been established from androgenetic embryos of the frog, Rana pipiens; one line has been maintained in culture for 150 generations, the other for 200 generations. Karyotypes of the two lines agree well with the standard for the species although some chromosomes show small differences in length. The cells multiply in the same defined basal medium used for culture of other anuran cell lines; this medium consists of the usual amino acids, vitamins, and serum macromolecules plus an exogenous purine source. Both the haploids resemble "permanent" cell lines in their prolonged multiplication in culture. The two lines differ in their mode of growth, one being epithelial-like, the other forming an overlapping meshwork of fibroblast-like cells. Both have the low plating efficiency characteristic of "unaltered" cells. These two lines are exceptional in their ability to compete successfully with the diploid variants which arise by endomitosis or cell fusion and which usually overgrow the haploid population. The more vigorous line, RPH 68.2A, should provide the long-desired haploid material for genetic studies in cell culture.

Animals↗

PubMed1970

Evolution of the transcription unit of ribosomal RNA.

In eukaryotes the two principal RNA components of the ribosomes are initially synthesized as a large complex precursor molecule, which may be thought of as a transcription unit. The precursor is converted, via intermediates, to the mature forms of ribosomal RNA (rRNA). In order to assess the extent of variation in the size of this rRNA transcription unit among different organisms, and to infer its possible mode of evolution, we have determined its molecular weight in several selected species. Pulse-labeled and long-term labeled RNA's were extracted from various types of cells, and analyzed by electrophoresis on acrylamide gels. Identification of particular components as rRNA precursors was made according to several stated criteria. Our results, together with an analysis of previously published data, suggest that in plants and lower animals, up to and including reptiles, the unit of transcription of rRNA is a 2.7-2.8 million dalton molecule, which is only about 25 per cent larger than its combined rRNA products. In contrast, birds, marsupials and placental mammals, exhibit a seemingly less economical form of rRNA synthesis. Their transcription units are 4.0-4.2 million daltons, about 80 per cent larger than the rRNA products. In the organisms with the smaller transcription unit the major intermediate precursor of rRNA is 1.5-1.6 million daltons, as compared to 2.0-2.2 million daltons in birds and mammals. The significance of these findings is discussed in relation to evolutionary changes in the base composition of the ribosomal RNA genes.

Acrylates↗

PubMed1970

The association of a class of saltatory movements with microtubules in cultured cells.

Particulate structures in the cytoplasm of HeLa and other cultured cells in interphase undergo rapid individual linear displacements (long saltatory movements, LSM). By the use of time-lapse microscopy to locate saltating particles prior to fixation and histochemical examination of the cells, structures of several kinds have been shown to move in this manner. Elements that show LSM include lysosomes, pinosomes, ingested carbon particles, lipoidal granules, and unidentified particles that appear as bright objects in positive phase contrast. The pattern of movement of the particles suggests the presence of linear guiding elements radially disposed from the cytocenter (centriole region). The participation of microtubules in these movements is inferred from the observation that LSM cease after treatment with drugs which depolymerize microtubules, i.e., colchicine, Vinblastine, and podophyllin. The directions of the microtubules in the cytoplasm of HeLa cells found by electron microscopy are consistent with the aster-like configuration predicted from study of LSM. Further support for this arrangement of cytoplasmic microtubules is provided by light microscope observations of colchicine-sensitive radial arrays of acid phosphatase granules in the cytoplasm of some cell lines.

Animals↗

PubMed1969