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Results of the Fluctuation Test
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To distinguish between these hypotheses, Luria and Delbr ck developed what is known as the uctuation test. They counted the mutants both in small ( individual ) cultures and in subsamples from a single large ( bulk ) culture. All subsamples from a bulk culture should have the same number of resistant cells, differing only because of
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Salvador E. Luria (1912 1991). (Courtesy of
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Dr. S. E. Luria.)
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III. Molecular Genetics
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12. DNA: Its Mutation, Repair, and Recombination
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The McGraw Hill Companies, 2001
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Mutation
Figure 12.1 Occurrence of E. coli Tonr colonies in Tons cultures. Ten cultures of E. coli cells were grown from a standard inoculum in separate test tubes in the absence of phage T1, then spread on petri plates in the presence of phage T1. The resistant cells grow into colonies on the plates. We expect a uniform distribution of resistant cells if the physiological induction hypothesis is correct (a) or a great uctuation in the number of resistant cells if the random mutation hypothesis is correct (b).
random sampling error. If, however, mutation occurs, the number of resistant cells among the individual cultures should vary considerably from culture to culture; the number would be related to the time that the mutation occurred during the growth of each culture. If mutation arose early, there would be many resistant cells. If it arose late, there would be relatively few resistant cells. Under physiological induction, the distribution of resistant colonies should not differ between the individual and bulk cultures. Luria and Delbr ck inoculated twenty individual cultures and one bulk culture with E. coli cells and incubated them in the absence of phage T1. Each individual culture was then spread out on a petri plate containing a very high concentration of T1 phages; ten subsamples from the bulk culture were plated in the same way. We can see from the results (table 12.1) that there was minimal variation in the number of resistant cells among the bulk culture subsamples but a very large amount of variation, as predicted for random mutation, among the individual cultures.
If bacteria have normal genetic systems that undergo mutation, bacteria could then be used, along with higher organisms, to answer genetic questions. As we have pointed out, the modern era of molecular genetics began with the use of prokaryotic and viral systems in genetic research. In the next section, we turn our attention to several basic questions about the gene, questions whose answers were found in several instances only because prokaryotic systems were available.
Genetic Fine Structure
How do we determine the relationship among several mutations that cause the same phenotypic change What are the smallest units of DNA capable of mutation and recombination Are the gene and its protein product colinear The answers to the latter two questions are important from a historical perspective.The answer to the rst question is relevant to our current understanding of genetics.
Tamarin: Principles of Genetics, Seventh Edition
III. Molecular Genetics
12. DNA: Its Mutation, Repair, and Recombination
The McGraw Hill Companies, 2001
Twelve
DNA: Its Mutation, Repair, and Recombination
Table 12.1 Results from the Luria and Delbr ck
Fluctuation Test
Samples from Bulk Culture* Tonr Colonies Found 14 15 13 21 15 14 26 16 20 13
Individual Cultures* Tonr Colonies Found 1 0 3 0 0 5 0 5 0 6 107 0 0 0 1 0 0 64 0 35 _____ 11.4
Culture Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mean (n)
Sample Number 1 2 3 4 5 6 7 8 9 10
____ 16.7 4.3
Standard deviation 27.4
Source: From E. Luria and M. Delbr ck, Genetics, 28: 491. Copyright 1943 Genetics Society of America. * Each culture and sample was 0.2 ml and contained about 2 107 E. coli cells.
Complementation
If two recessive mutations arise independently and both have the same phenotype, how do we know whether they are both mutations of the same gene That is, how do we know whether they are alleles To answer this question, we must construct a heterozygote and determine the complementation between the two mutations. A heterozygote with two mutations of the same gene will produce only mutant messenger RNAs, which result in mutant enzymes ( g. 12.2a). If, however, the mutations are not allelic, the gamete from the a1 parent will also contain an a2 allele, and the gamete from the a2 parent will also contain the a1 allele ( g. 12.2b). If the two mutant genes are truly alleles, then the phenotype of the heterozygote should be
mutant. If, however, the two mutant genes are nonallelic, then the a1 mutant will have contributed the wild-type allele at the A2 locus, and the a2 mutant will have contributed the wild-type allele at the A1 locus to the heterozygote. Thus, the two mutations will complement each other and produce the wild-type. Mutations that fail to complement each other are termed functional alleles. The test for de ning alleles strictly on this basis of functionality is termed the cis-trans complementation test. There are two different con gurations in which a heterozygous double mutant of functional alleles can form ( g. 12.3). In the cis-trans complementation test, only the trans con guration is used to determine whether the two mutations were allelic. In reality, the cis con guration is not tested; it is the conceptual control, in which wild-type activity (with recessive mutations) is always expected. The test is thus sometimes simply called a trans test. Functional alleles produce a wild-type phenotype in the cis con guration but a mutant phenotype in the trans con guration. This difference in phenotypes is called a cis-trans position effect. From the terms cis and trans, Seymour Benzer coined the term cistron for the smallest genetic unit (length of genetic material) that exhibits a cis-trans position effect. We thus have a new word for the gene, one in which function is more explicit. We have, in essence, re ned Beadle and Tatum s one-gene-one-enzyme hypothesis to a more accurate one-cistron-one-polypeptide concept. The cistron is the smallest unit that codes for a messenger RNA that is then translated into a single polypeptide or expressed directly (transfer RNA or ribosomal RNA). From functional alleles, we can go one step further in recombinational analysis by determining whether two allelic mutations occur at exactly the same place in the cistron. In other words, when two mutations prove to be functional alleles, are they also structural alleles The methods used to analyze complementation can be used here also. Crosses are carried out to form a mutant heterozygote (trans con guration) whose offspring are then tested for recombination between the two mutational sites. If no recombination occurs, then the two alleles probably contain the same
Seymour Benzer (1921 ).
(Courtesy of Dr. Seymour Benzer, 1970.)
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