Case II: Conjugation between Hfr cell and F- cell
What is Hfr?
A bacterial cell with F plasmid integrated to its chromosome is called an Hfr cell.
|Hfr cell formation|
Here F factor, or F plasmid of the donor is not a free plasmid but it is integrated to the donar bacterial chromosomal DNA as an episome. Thus F plasmid together with bacterial chromosomal DNA forms a recombinant DNA called as high frequency recombination DNA or Hfr DNA. Hfr strains can effect high rate of recombination as some portion of the donar bacterial DNA may also get transferred. Hence they are called Hfr strains.
|Conjugation between Hfr cell and F- cell|
Step 1: Hfr cells produces hair like appendenges called sex pili which facilitates cell to cell contact with F- strain by forming a conjugation tube. The formation of sex pili is governed by genes of F factor
Step 2: Hfr DNA of donor undergoes replication by ‘rolling circle mechanism’.
Step 3: Circular Hfr DNA breaks open and becomes linear. A portion of F plasmid occupies the leading end
Step 4: The leading end moves to the recipient cell via conjugation tube
Step 5: During this transfer, conjugation tube abruptly breaks up, so that only a part of the Hfr DNA is transferred to the recipient.
Step 6: The transferred donor DNA gets integrated to the recipient chromosome. This brings about genetic variability.
The third type which is rare is called sexduction, which we will be discussing in the next post