Genetic Analysis 277proportions as low as 10−^8 ” by “crossing” pairs ofrIImutants in the bacterial
strain B, in which they may replicate and recombine, and plating the progeny on a
bacterial strain K12(λ) on which mutants cannot produce plaques [Benzer, 1955,
345]. Benzer was not committed to Muller’s original conception of the material
gene as the composite unit of function, mutation, as well as recombination, and
defined the functional unit, thecistron(according to thecis-transtest, although
only many years later thecis-combinations of mutants were isolated and directly
tested) as not necessarily identical to both the unit of mutation, themuton,and
the units of recombination, therecon[Benzer, 1955; 1957]. Utilizing what ap-
peared upon genetic analysis to be intra-cistronic “deletions” — mutant that did
not recombine with (at least) two other mutants that recombined with each other
- Benzer greatly improved the efficiency of the mapping analysis and constructed
a consistent linear map of therIIgene of the bacteriophage T4 that was divisible
into two adjacent cistrons. Although it was implicitly presumed that the sited of
the mutons and recons would converge to those of individual nucleotides of the
cistrons’ DNA sequence, neither recons nor mutons corresponded to individual
nucleotides. Furthermore, the failure to isolate the protein coded by therIIgene
did not allow testing the colinearity of the gene’s DNA sequence and the protein
product’s amino-acid sequence. The colinearity of mutations sites in the linear
genetic map and changes in amino-acid in the sequences of the protein produced
was, however, demonstrated for the tryptophane synthetase B gene, or cistron of
E. coli[Yanofskyet al., 1964).
The continuity of intra-geneic and inter-genic DNA sequences was indicated
by mutations (such asrII 1589), which genetically were deletions starting in one
cistron and ending at an adjacent one (unless some non-DNA matter was deleted
with the DNA sequences). Support for thestructural non-discreteness of the gene
(or cistron) was, however, obtained when it was shown that such a deletion com-
bining what-was-left of the two genes (cistrons) could be “read”continuouslyas
long as the welded parts were in the same “reading-frame” (see section 5.1). The
gene, or cistron, was shown to be a sequence along the continuous DNA, with no
inbuilt structural punctuation signals. To the extent that a unique sequence could
be identified, this was byfunctionalcriteria, such as providing an attachment site
for the RNA-polymerase machinery at transcription, or having an open reading
frame (ORF) for the ribosomal translation machinery of a nucleotide sequences to
an amino-acid sequences.
Arguably, mapping the gene “down to the ground” and demonstrating that
the genetic map is collinear with that of the translated polypeptide product of
this gene appeared finally to end an epoch: It was not necessary to refer to the
gene instrumentally by its product, to infer the genotype from the phenotype,
whether eye color or polypeptide. The sequence of nucleotides was the source
of “information” for the phenotype. Once the techniques of manipulating DNA
were developed, it would be conceivable to refer to a sequence of DNAper se,
irrespective to its function.
The fundamental unit of classical genetics is an indivisible and ab-