Answer :
The F factor is integrated into the bacterial chromosome in all or most cells of an Hfr strain but in only a few cells in an F+ strain. Option d is correct.
Hfr strains of bacteria are different from F+ strains because the F factor is integrated into the bacterial chromosome in all or most cells of an Hfr strain but in only a few cells in an F+ strain. This means that Hfr strains are capable of transferring chromosomal genes during conjugation, while F+ strains are not.
During conjugation, the Hfr cell transfers a copy of its chromosomal DNA to the F- cell, but it is usually incomplete because the process is interrupted before the entire chromosome can be transferred. This results in a recombinant F- cell with new genes but still lacking the F factor. In contrast, an F+ cell can transfer only the F factor to an F- cell, not chromosomal genes. Thus, Hfr strains are an important tool for mapping bacterial chromosomes and studying gene transfer. Hence Option d is correct.
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