An Hfr strain that is leuA+ and thiL+ was mixed with a strain that is leuA− and thiL−. The data points in the following graph were obtained when conjugation was interrupted at different time points and the percentage of recombinants for each gene was determined by streaking on a medium that lacked either leucine or thiamine.

A. Explain how these results are consistent with the idea that the bacterial chromosome is circular.
B. Draw a map of the bacterial chromosome that shows the order of genes and the locations of the origins of transfer among these different Hfr strains.