An F factor can pick up chromosomal DNA as it exits a chromosome. (a) F is inserted in an Hfr strain at a repetitive element identified as IS1 (insertion sequence 1) between the ton and lac+ alleles. (b) The inserted F factor. (c) Abnormal “outlooping” by crossing over with a different element, IS2, to include the lac locus. (d) The resulting F′ lac+ particle. (e) F′ lac+/F− lac− partial diploid produced by the transfer of the F′ lac+ particle to an F− lac− recipient.
[Data from G. S. Stent and R. Calendar, Molecular Genetics, 2nd ed.]