General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    294 General cytogenetics §6.3 infrequently the right combination of mutants is available. They involve one normal strain of which only relatively few cells are mixed with a great majority of cells of a mutant strain. In a medium with aminopterin normal cells and hybrid cells proliferate but since both are low in number, they develop into separate colonies that can be distinguished. After it had been detected that the Sendai strain of parainfluen¬ za virus, even after UV inactivation, enhances hybridization by a factor of 100 to 1000, it became possible to isolate hybrid colo¬ nies without selective media, especially when the different types of cultures can be distinguished on the basis of morphological differences. Even then selective media may still be useful. Loss of chromosomes is relatively infrequent in hybrids be¬ tween cell strains derived from the same species. In cell hybrids from different species, however, the chromosomes of one of the two species usually are eliminated successively. In mouse—human hybrids elimination of the human chromosomes starts early in the development of the hybrid colonies and proceeds gradually. This elimination has been used to locate the gene for thymidine kinase on one of the human chromosomes. In a medium containing aminopterin, cell hybrids between thymidine kinase-deficient mouse cells and normal human cells will synthesize DNA as long as the human chromosome with the gene coding for the enzyme is present. When Weiss and Green in 1967 analysed several hybrid clones after 100 to 150 generations in a medium with aminopte¬ rin, two or three human chromosomes appeared to have remained. A small chromosome of the E group (compare fig. 1.21) was al¬ ways among them: it apparently carries the thymidine kinase gene. When these clones were transferred to a medium containing 5-bromo-deoxy-uridine, the presence of thymidine kinase was con¬ firmed: all cells carrying this chromosome were killed by the ac¬ tion of the enzyme on this substance, but cells that in the process happened to have lost this particular human chromosome, sur¬ vived. This and similar methods are very valuable for locating genes on chromosomes especially in those cases where formal genetic experimentation based on sexual reproduction is excluded or cumbersome. For a review see Ephrussi and Weiss (1969). Per-