Q.20 The correct sequence of events during bacteriophage infection of a bacterial cell is
(A) landing → attachment → tail contraction → penetration and unplugging → DNA
ejection
(B) attachment → landing → penetration and unplugging → tail contraction → DNA
ejection
(C) landing → tail contraction → attachment → DNA ejection → penetration and
unplugging
(D) attachment → tail contraction → landing → penetration and unplugging → DNA
ejection

Correct Answer: (A)

The sequence in option (A)—landing → attachment → tail contraction → penetration and unplugging → DNA ejection—accurately reflects the standard lytic cycle of tailed bacteriophages like T4.

Option Analysis

Option (A): Starts with the phage landing on the bacterial surface via initial contact, followed by firm attachment to specific receptors using tail fibers. Tail contraction then drives a needle-like tube through the cell wall (penetration and unplugging), enabling DNA ejection into the host cytoplasm. This matches established phage infection mechanisms.

Option (B): Places attachment before landing, which reverses the logical order since non-specific landing precedes receptor-specific attachment. Tail contraction occurs too late, after penetration, which is incorrect as contraction facilitates penetration.

Option (C): Begins with landing then jumps to tail contraction before attachment, ignoring that attachment must precede sheath contraction. DNA ejection before full penetration disrupts the sequence.

Option (D): Sequences tail contraction immediately after attachment and before landing, which is illogical since landing initiates the process and contraction follows attachment.

The bacteriophage infection sequence represents a precise series of events where tailed viruses like T-even phages hijack bacterial cells during the lytic cycle. Understanding this bacteriophage infection process is crucial for exams like GATE Biotechnology, where questions test the exact order: landing, attachment, tail contraction, penetration and unplugging, and DNA ejection. This knowledge aids in grasping viral replication mechanisms in microbiology and molecular biology.

Key Stages Explained

• Landing: Phage particles first make reversible contact with the bacterial surface, scanning for proximity.

• Attachment: Tail fibers bind irreversibly to specific lipopolysaccharide receptors on the cell wall.

• Tail Contraction: Sheath muscles contract, thrusting a lysozyme-tipped tail tube through the peptidoglycan layer.

• Penetration and Unplugging: The tube breaches the inner membrane; a pilot protein unpluggs to clear the path.

• DNA Ejection: Phage genome is forcefully injected into the cytoplasm, initiating replication.

Exam Relevance

This sequence frequently appears in competitive exams like IIT JAM and GATE BT, emphasizing lytic over lysogenic cycles. Incorrect options often swap attachment with landing or misplace contraction, highlighting the need for step-by-step recall.