Article summary VII

Summary

Benzer (1955) described a functionally related region in the genetic material of a bacteriophage that was finely subdivisible by mutation and by genetic recombination. The case of the rII mutants of T4 bacteriophage offered a selective feature for the detection of small proportions of recombinants, which can best achieve a high degree of resolution. The sensitivity of rII mutants prompted the question of how closely the attainable resolution approaches the molecular limits of the genetic material. Benzer wished to translate linkage distances (derived from genetic recombination experiments) into molecular units, and he eventually used rII mutants to determine the fine structure of the gene to the nucleotide level. Most of these mutants were able to revert to wild type at measurable rates and thus were considered to carry point mutations. However, many mutants would give violently anomalous results; it was assumed that each mutation extends over a certain length of the chromosome, and production of the wild type required recombination within the space between those lengths. Standard crosses can then determine its position relative to other nearby point mutations. Benzer thus determined that the rII point mutations map at many sites, some so close together, he concluded, that the chromosomal elements separable by recombination were not larger than the order of a dozen nucleotide pairs (as calculated from the smallest non-zero recombination value) and that mutations involved variable lengths which may extend over hundreds of nucleotide pairs.

Reaction

The model described in this paper is capable of describing the observed properties of the rII mutants. Using the phenotype test, the pairs of rII mutants were divided into two functionally separable segments. It was feasible to extend genetic studies even to the level of the functional elements. By extension of these phenotype tests and “clustering” of similar mutants to still more closely linked mutations, Benzer characterized the sizes of the ultimate units of genetic recombination, mutation, and “function” in molecular terms. The findings indicated the units of recombination were not larger than the order of one dozen nucleotide pairs and that mutations might involve various lengths of “chromosome”, which were important to determine the fine structure of the gene to the nucleotide level. Yet many interesting questions are unresolved. Are there any correlations among the three parameters (the map position, the reversion rate, and the degree of phenotypic effect) of rII mutants? Why do the reversion rates for different mutants differ enormously? Whether there exist true “point” mutations (i.e., involving an alteration of only one nucleotide pair) or whether all mutations involve more or less long pieces of the chromosome? The answers to these questions will significantly advance our understanding of genetic studies at the molecular or nucleotide level.

Questions

1. What does Figure 5 (“Maps of microclusters”) mean?
2. What is the meaning of “function”? What are the two functionally separable segments of the rII region?
3. What are the impacts and applications of this study in the long run?