Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

    74/952 (page 54)
    CHAPTER 2 Conjugation Nature of Bacterial Variation Identification of Mutants For the first part of this century, bacteria were regarded as a unique life form because of two unusual properties: they did not appear to possess a complex nuclear apparatus analogous to that of eucaryotic cells; and they possess an evident ability to adjust to new environments with changes that are inheritable. But although the bacterial genome does not take the form of the deoxyribonucleoprotein complex typical of eucaryotes and is not surrounded by a nuclear membrane, it is now known to be directly comparable in structure and function to the eucaryotic chromosome and to suffer heritable change only by mutation. The first experiments to imply that the genetic apparatus of bacteria is organized on lines familiar from eucaryotic genetics were those of Luria and Delbrück (1943), who devised the fluctuation test that allows adaptation to be distinguished from spontaneous mutation. Before the development of this test, the nature of heritable variation in bacteria had been a long standing con troversy; for experiments which showed that only a small proportion of the bacteria in a culture may survive when transferred to new, selective conditions could be interpreted in either of two ways. Mutants might previously exist that are able to survive after the population is shifted to new growth conditions; or adaptation might be induced by exposure to the new conditions. To demonstrate that spontaneous mutation is the cause of variation, it was therefore necessary to show that the mutant bacteria are present in the population before exposure to the new condi tions; the statistical analysis of the fluctuation test allows prior existence and subsequent adaptation to be resolved. The system with which Luria and Delbrück worked was the acquisition of resistance to bacteriophage T1 by E.coli. When a culture of cells sensitive to the phage is plated in the presence of excess phage particles, virtually all the bacteria are infected and thus killed; but a few survive to yield colonies. The cells of these colonies are resistant because they have