ConceptsB

In a series of experiments with the bacteriophage T4, Seymour Benzer showed that recombination could occur within a single gene and created the first molecular map of a gene.

Complementation experiments At the time Benzer was conducting his experiments, the relationship between genes and DNA structure was unknown. A gene had been defined as a functional unit of heredity that coded for a phenotype. To test whether different rll mutations belonged to different functional genes, Benzer used the complementation (cistrans) test (see pp. xxx in Chapter 5).

Individuals heterozygous for two mutations may have the mutations in trans, a+ br a~ b+

meaning that they are located on different chromosomes, or in cis, meaning that they are located on the same chromosome b+ b+

(see p. xxx in Chapter 7). Suppose that the a tions occur at different loci, which proteins. In the trans heterozygote b-

and b muta-code for different a- b+

one chromosome has a functional allele at the a locus

( a+_b- ) and the other chromosome has a functional

cessive mutations, both A and B proteins will be produced. The two mutations complement each other, so the presence of wild type trait in the trans heterozygote indicates that these mutations belong to different complementation groups — they come from different loci.

Suppose the two mutations occur within a single locus that codes for one protein. In the trans heterozygote, one chromosome fails to produce a functional protein because it has a defect at the b site ( a+_b_) and the other chromosome fails to produce a functional protein because it has a defect at the a site ( a__b+ ). No functional protein is produced by either chromosome, and the trans heterozygote has a mutant phenotype — the mutations are unable to complement each other.

The heterozygous individual used in complementation testing must have the mutations in the trans configuration. When the mutations are in the cis configuration:

heterozygotes will have a wild-type phenotype regardless of whether the two mutations occur at the same locus or at different loci, because one chromosome ( a+_b+ ) is mutation free.

To carry out the complementation test in bacterio-phage, Benzer infected cells of E. coli K with large numbers of two mutant strains of phage ( FIGURE 8.32, step 1). We will refer to the two mutations as rlla ( a_b+ ) and rllb ( a+_bl). Cells infected with both mutants:

f Experiment)-

Question: How do we determine whether two different rll mutants occur at the same locus?

Mutant rII a- phage

Mutant rII a- phage

Mutant rII b-phage

[j'l E. coli K cells are simultaneously infected by two different rII mutants (a- and £r),...

Mutant rII b-phage

[j'l E. coli K cells are simultaneously infected by two different rII mutants (a- and £r),...

1 .making the cells

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