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.
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| 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.
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| 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
Practice Questions:
1. Hfr bacterial strain is involved in:
A) Complete transfer of F plasmid
B) Total lack of F plasmid transfer
C) Integration of Plasmid and Bacterial chromosome
D) Complete transfer of Bacterial Chromosome
Answer: C) Integration of Plasmid and Bacterial chromosome
Learn more:
Tags:
Bacterial Conjugation
conjugation
episome
Hfr
Hfr cell and F- cell
Hfr cell x F- cell
plasmid
recombinant DNA
sex pili