33 
The principal microorganism used with recombinant DNA techniques 
is a laboratory strain, K-12, of the species, Escherichia coli. The 
genetic map of E. coli may be represented as a circle: rEs chromosome 
is actually a single, circular DNA molecule. About 650 out of an 
estimated total of 3000-4000 of the organism's genes have been assigned 
locations on the chromosome. The map provides considerable infor- 
mation--e. g. , it shows how some functionally related genes are clustered 
together and whether the information in a gene is read in a clockwise 
or counterclockwise direction. 
In some regions of the chromosome, the resolution of the map 
permits us to identify genes that regulate expression of other genes 
as well as those that code for proteins. One example is a set of genes 
that govern E. coli 's ability to metabolize the milk sugar, lactose. 
Such models - now guide our search for answers about expression and 
regulation of the genes of higher organisms. 
More recently, recombinant DNA experiments have themselves 
amplified our understanding of bacterial chromosomes and how they 
work. Recent experiments demonstrated that a segment of DNA can, 
in living cells, be excised from a chromosome, be turned around, 
and then reinserted in the opposite orientation. One such inversion 
controls the type of flagellum (whip -like appendage for locomotion) 
that the cell makes (24). There are indications that similar mech- 
anisms may operate in higher organisms. 
Complexity of Eukaryote DNA. Our extensive knowledge of E. 
coli 's chromosorne - contrasts sharply with our ignorance about the 
molecular anatomy of human and other mammalian genomes. As of 
October 1975, fewer than 150 human genes had been assigned locations 
on one or another of the 2 3 chromosome pairs. There could be 5, 000- 
10, 000 unknown genes between any two mapped human genes! Although 
research in this field is flourishing, the acquisition of information about 
mammalian genomes is slow, costly, and very unlikely to provide the 
level of resolution achieved with E. coli. 
Deciphering the molecular details of E. coli 's chromosome (and 
those of several viral genomes) would not have been accomplished 
without the ability to isolate specific regions of its DNA in pure form 
and in large enough quantities for analysis. Scientists confronted with 
the incredibly complex mixture of DNA fragments obtained from human 
and other mammalian chromosomes are severely handicapped. But if 
the complex assortment of DNA fragments could be reassembled in 
the proper order and the genetic signals deciphered, it would provide 
a new vision of our genome's structure and would have profound signif- 
icance for improving human health. Recombinant DNA techniques offer 
a feasible approach. 
