Question

A cross between a donor cell from an Hfr strain and a recipient cell from an \(F^{-}\) strain results in recipient which is:

• A. \(F^{+}\)
• B. \(F^{-}\)
• C. \(F'\)
• D. Hfr

Hint:

Remember:
• \(F^{+} \times F^{-} \rightarrow F^{+}\)
• Hfr \(\times F^{-} \rightarrow\) Usually recombinant \(F^{-}\) \[ \text{Hfr strains transfer chromosomal genes efficiently but rarely transfer the complete } F\text{-factor} \]

Answer 1

The Correct Option is B

Concept: In bacterial conjugation:
• \(F^{+}\) cells contain the fertility factor (\(F\)-plasmid).
• \(F^{-}\) cells lack the fertility factor.
• Hfr (High frequency recombination) strains have the \(F\)-factor integrated into the bacterial chromosome. During conjugation between an Hfr donor and an \(F^{-}\) recipient:
• Chromosomal genes begin transferring from the donor.
• The entire \(F\)-factor is rarely transferred completely. Hence, the recipient generally remains: \[ F^{-} \]

Step 1: Understanding conjugation involving Hfr strains. In Hfr cells: \[ F\text{-factor is integrated into the chromosome} \] DNA transfer starts from the integrated \(F\)-factor origin and proceeds into chromosomal genes. However, conjugation is usually interrupted before complete transfer of the entire \(F\)-factor. Therefore: \[ \boxed{\text{Recipient usually remains } F^{-}} \]

Step 2: Why the recipient does not become \(F^{+}\). To become \(F^{+}\), the recipient must receive: \[ \text{Complete } F\text{-factor} \] Since the entire integrated \(F\)-factor is rarely transferred from Hfr cells: \[ F^{-} \text{ recipient remains } F^{-} \]

Step 3: Eliminating the incorrect options.
• Option (A): Recipient generally does not become \(F^{+}\) because full \(F\)-factor transfer is incomplete.
• Option (C): \(F'\) cells arise when the \(F\)-factor excises incorrectly from the chromosome.
• Option (D): Recipient does not become Hfr because integrated transfer is usually incomplete. Hence, the correct answer is: \[ \boxed{F^{-}} \]