Final answer:
Lederberg and Tatum used Hfr strains to map the genes of E. coli by mating them with F- cells, which allowed them to establish the order and relative sizes of the genes on the bacterial chromosome.
Step-by-step explanation:
Lederberg and Tatum utilized Hfr strains to construct the gene map of E. coli. An Hfr cell is a cell with an F plasmid integrated into its chromosome. When Hfr cells mate with F- cells, the integrated F plasmid initiates conjugation and begins the transfer of the chromosomal DNA into the recipient F- cell. This transfer starts at the integration site of the F plasmid and continues in a linear fashion.
By using different Hfr strains, each with the F plasmid inserted at various points in E. coli's genome, and mating them with F- cells with mutations in known genes, researchers were able to determine the order of genes on the chromosome. The timing of conjugation was also so precise that it could determine the relative size of genes being transferred.