Circular Chromosome
 
All Hfr strains (for high frequency of recombination) of Escherichia coli originate from F⁺ strains (males, possess the F sex factor). When mated with F^– recipients (females), they generate about a thousand times more recombinants than the equivalent F⁺ strain under identical conditions (1). The recombinants obtained inherit a particular segment of the donor chromosome exclusively (1). The way in which Hfr donors transfer their chromosome to recipients was elucidated by violent agitation of a mixture of mating Hfr and F^– in a high-speed mixer. The mating was interrupted at different time intervals and the mixture was plated for detection of recombinants. It was found that recombinants acquire different selected characters from the donor cell at different times. The order in which these characters appear is the order of their arrangement on the chromosome. The only reasonable interpretation of these results is that the effect of agitation is a rupturing of the chromosome during the transit from the donor to recipient bacteria. Only those genes that have already penetrated the recipient bacteria at the time of treatment can appear in recombinants. Thus, the strain Hfr used comprises a homogeneous population of donor bacteria, all of which transfer their chromosome in the same specifically oriented and linear way, so that a particular extremity, designated O (for origin), is always the first to penetrate the recipient bacteria (2, 3).
 A number of Hfr stains of independent origin were analyzed with respect to the markers they transferred to recipients and to the orientation of the transfer. The outcome was remarkable and unforeseen. Although all of the bacteria of any one Hfr strain transfer a given set of markers in a particular sequence, different strains transfer different chromosomal segments, parts of which may overlap. Some strains transfer their genes in a direction reverse to others. The relationship of the various genes to one another however, is the same irrespective of the Hfr strain used to map them. This gives an unequivocal general linkage map for E. coli, consisting of a single linkage group. Each strain transfers its chromosome as a linear, oriented structure with a specific head and tail, for example, 

Another strain can always be found which transfers genes A and Z as closely linked markers, for example, 

Thus, in spite of this linear transfer, it is impossible to define any extremities on the chromosome. It is concluded that the chromosome of F⁺ donors is originally continuous or circular and that Hfr strains arise by opening up the circle at a point characteristic for each Hfr type to yield a linear, transferable structure (3.(
The circular nature of the E. coli chromosome has been confirmed by gentle liberation and dispersion of its DNA and visualization by autoradiography or by electron microscopy. Therefore there is indisputable, direct evidence that this chromosome is indeed a continuous, circular thread of double-stranded DNA (4, 5).
References
1. F. Hayes (1953) Cold Spring Harbor. Symp. Quant. Biol. 18, 75–93. 
2. E. L. Wollman and F. Jacob (1955) C. R. Acad. Sc. Paris 240, 2449–2451. 
3. E. L. Wollman and F. Jacob (1958) Ann. Inst. Pasteur 95, 641–666. 
4J. Cairns (1962) J. Mol. Biol. 4, 407–409. 
5. J. Cairns (1963) Cold Spring Harbor Symp. Quant. Biol. 28, 43–46.