Chapter *36 



BACTERIA: CLONES 

 AND MUTATION 



Yi 



'ou MAY have noticed that our 

 present understanding of the 

 mechanisms involved in the 

 biological replication of DNA in vivo (Chap- 

 ter 34) and in vitro (Chapter 35) has been 

 very significantly advanced by means of ex- 

 periments with bacteria. Such studies used 

 the DNA as well as the DNA polymerase of 

 these organisms. The presumption was 

 made, in evaluating that work, that the bac- 

 terial DNA employed was typical "chromo- 

 somal" DNA. Though microscopic ex- 

 amination of bacteria reveals a nuclear body, 

 it has not been possible, so far, to clearly 

 delineate any chromosome structure within 

 it. Nevertheless, the DNA within the bac- 

 terial nucleus is similar to typical chromo- 

 somal DNA in (1) basic chemical content, 



(2) primary and secondary organization, 



(3) mechanism of synthesis, and (4) con- 

 servation. We are, therefore, justified in 

 considering bacterial DNA as being pri- 

 marily chromosomal DNA. 



Since bacteria contain chromosomal DNA, 

 we would expect them also to contain chro- 

 mosomal genes, in accord with the hypothe- 

 sis, for which much indirect support has al- 

 ready been presented, that DNA is genetic 

 material. How suitable are bacteria as ex- 

 perimental material for the study of muta- 

 tion? In the case of a particular bacterium, 

 Escherichia coli, microscopic observation 

 reveals that each cell contains one to four 

 nuclei, usually two or four (Figure 36-1). 

 Although the nature of the morphological 

 329 



mechanism of nuclear division is still contro- 

 versial, the exact replication of DNA occurs 

 at each nuclear division, and it may be con- 

 cluded the daughter nuclei are genetically 

 identical, just as they are following typical 

 mitosis. After nuclear replication the bac- 

 terium divides to produce daughter bacteria. 

 This means of increasing bacterial cell num- 

 ber is called vegetative reproduction, and is 

 an asexual process. We shall make the 

 assumption, until such time as experimental 

 evidence to the contrary may be presented, 

 that genetic recombination cannot occur within 

 or between bacterial cells. In the absence of 

 genetic recombination we shall, of course, be 

 unable to study the recon. (It should be re- 

 called that in Chapter 2, p. 8, the choice 

 was made to study the properties of the ge- 

 netic material as revealed from a study of sex- 

 ually reproducing, cross-fertilizing species. 

 That pathway led to our present concept of the 

 recon based upon the recombinations which 

 the genetic material undergoes consequent to 

 segregation, independent segregation, cross- 

 ing over, and fertilization.) 



Starting with a single bacterium, continu- 

 ous vegetative reproduction results in the 

 production of a population of cells called a 

 clone. Since a clone is a population of indi- 

 viduals all derived from a single cell by 

 asexual reproduction, all clonal members are 

 genetically identical, barring mutation. If 

 mutation occurs during clonal growth, the 

 mutant will be transmitted to all the progeny 

 of the mutant cell. This would produce a 

 genetically mosaic clone, whose proportion 

 of mutant individuals would vary depending 

 upon the time the mutation occurred and the 

 relative reproductive potential of mutant and 

 nonmutant cells. You may recall (p. 245) 

 that except for meiosis, its products, and fer- 

 tilization, all cells of a sexually reproducing 

 organism are also clonal in origin, so that 

 muhicellular organisms can also be mosaic 

 for a mutant. Let us consider the character- 

 istics of bacteria and their clones which 



