T4 rII system
The T4 rII system is an
Origin
One type of mutation in the T4 bacteriophage identified by researchers in phage genetics by the 1950s was known as r (for rapid), which caused the phage to destroy bacteria more quickly than normal. These could be spotted easily because they would produce larger
From there, Benzer saw that it would be possible to generate many independent r mutants, and by measuring the
Benzer's concept was quite controversial within classical genetic thought, in which each gene is treated as a singular point along a chromosome, not a divisible stretch of nucleic acids (as implied by the work of Watson and Crick). Initially, Max Delbrück—a respected phage geneticist and leader of the so-called phage group of which Benzer was a part—found Benzer's idea outrageous.[3]
Benzer's work
Beginning in 1954, Benzer put the T4 rII system to use, creating and crossing hundreds of r mutants and developing an increasingly detailed map of the structure of the rII gene. In his early work, he identified two separate but very close loci within the rII region, which he suggested were
Benzer identified a number of different types of r mutants. Some he classified as
In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities, indivisible by recombination and arranged like beads on a string. The experiments of Benzer using mutants defective in the T4 rII system, during 1955-1959, showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA[6][7] (see also Phage group).
Work by others
After Benzer demonstrated the power of the T4 rII system for exploring the fine structure of the gene, others adapted the system to explore related problems. For example, Francis Crick and others used one of the peculiar r mutants Benzer had found (a deletion that fused the A and B cistrons of rII) to demonstrate the triplet nature of the genetic code.[8]
The principal that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4[9][10] (also see Crick, Brenner et al. experiment).
Notes
References
- R Jayaraman. "Seymour Benzer and T4 rII: Running the Map into the Ground." Resonance, October 2008, pp. 898–908.
- Jonathan Weiner. Time, Love, Memory: A Great Biologist and His Quest for the Origins of Behavior. Knopf. ISBN 0-679-44435-1