Question

1. We know that an F+ cell can donate its F plasmid to a recipient cell to create a new F+ cell.

But what about an Hfr? Can an Hfr transfer its DNA as well? (yes/no) Please explain your answer. Why did you answer yes or no?

2. Let’s just assume,, that an Hfr cell could transfer DNA. What would the recipient cell look like? What would it be called? F+? F-? Hfr+? Something else altogether? Please explain your answer.

Answer 1

1. A high frequency recombination cell (Hfr strain) ia a bacteria with conjugative plasmid (F plasmid) integrated in to its chromosomal DNA. The integration of Fplasmid occur through homologous recombination. When conjugation occur, Hfr cell are very efficient in transfering chromosomal DNA into an recepient F- cell (F- cells lack F plasmid).

2. Yes, Hfr cells can transfer its chromosome. But only a portion of chromosome is transfered to the recepient F- cell. Due to the large size of chromosome and time required for the transfer is too long for the cells to maintain their physical contact, it is very rare to transfer the entire chromosome to the F- cell. Hence the F- is not completely transfered in to the recepient and the F- never become an F+.

The interepted mating results in two possibilies.

1. since their is no homologous recombination occured, the small piece of DNA tranfered to the recepient later get degraded.

2. in very rare case, the f plasmid is transfered completely and F- becomes Hfr. (probability is very low)