Why does conjugation between an $H_{\mathrm{fr}}$ strain and an $F^{-}$ strain not result in two $H_{\mathrm{fr}}$ strains?

• A. The $F^{-}$ strain does not receive the entire $H_{\mathrm{fr}}$ chromosome during conjugation.
• B. The $F^{-}$ strain lacks the necessary enzymes to integrate the $H_{\mathrm{fr}}$ DNA.
• C. The $H_{\mathrm{fr}}$ strain loses its fertility factor during the process.
• D. Conjugation between $H_{\mathrm{fr}}$ and $F^{-}$ strains results in the formation of an $F^{+}$ strain.

Answer: (A)

Verified step by step guidance

1. Understand the concept of Hfr and F- strains: Hfr (High frequency of recombination) strains have the F factor integrated into their chromosome, allowing them to transfer genetic material to F- strains during conjugation.
2. Recognize the process of bacterial conjugation: During conjugation, the Hfr strain attempts to transfer its entire chromosome to the F- strain through a pilus. However, this process is often interrupted before the entire chromosome is transferred.
3. Identify the limitations of the F- strain: The F- strain typically does not receive the entire Hfr chromosome, including the F factor, due to the interruption of the conjugation process.
4. Consider the role of enzymes: The F- strain lacks the necessary enzymes to integrate the partial Hfr DNA it receives into its own chromosome, preventing it from becoming an Hfr strain.
5. Conclude the outcome of the conjugation: Since the F- strain does not receive the complete F factor, it cannot become an Hfr strain. Instead, it may become an F+ strain if it receives enough of the F factor to form a plasmid.