Question

Which of the following best describes an HFR cell? Micriobiology

		A cell which has an F-plasmid
		A cell that has an F-plasmid that has integrated into nucleoid
		A bacterium infected with a bacteriophage
		A cell that is capable of homologous recombination
		None of the above

Answer 1

An HFR cell ( high frequency recombination cell) is a bacterium with a conjugative plasmid for example the F factor ( fertility factor) also called an F plasmid integrated into it's chromosomal DNA. Since all the genetic material including DNA and chromosomes are located in it's nucleoid it can be also stated that HFR cell is a cell that has an f plasmid that has integrated into nucleoid

A conjugative plasmid capable of chromosome integration is also called an episome (a segment of DNA that can exist as a plasmid or become integrated into the chromosome). When conjugation occurs, Hfr cells are very efficient in delivering chromosomal genes of the cell into recipient F⁻ cells, which lack the episome.

F+ cell Hfr cell Chromosome Homologous Recombination F plasmid Hfr cell F cell F factor Chromosome 0,0-o'o O O O Ö Hfr cell 6a 6b

1.The insertion sequences (yellow) on both the F factor plasmid and the chromosome have similar sequences, allowing the F factor to insert itself into the genome of the cell. This is called homologous recombination and creates an Hfr (high frequency of recombination) cell.
2.The Hfr cell forms sex pilli a pilus and attaches to a recipient F- cell.
3.A nick in one strand of the Hfr cell’s chromosome is created.
4.DNA begins to be transferred from the Hfr cell to the recipient cell while the second strand of its chromosome is being replicated.
5.The pilus detaches from the recipient cell and retracts. The Hfr cell ideally wants to transfer its entire genome to the recipient cell. However, due to its large size and inability to keep in contact with the recipient cell, it is not able to do so.
6.The F- cell remains F- because the entire F factor sequence was not received. Since no homologous recombination occurred, the DNA that was transferred is degraded by enzymes. In very rare cases, the F factor will be completely transferred and the F- cell will become an Hfr cell.