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GATE 2023 BT Q.20: solution and answer

GATE 2023 · BTQ.20 · 1 mark · MCQ

The correct sequence of events during bacteriophage infection of a bacterial cell is

  1. (A)

    landing → attachment → tail contraction → penetration and unplugging → DNA ejection

  2. (B)

    attachment → landing → penetration and unplugging → tail contraction → DNA ejection

  3. (C)

    landing → tail contraction → attachment → DNA ejection → penetration and unplugging

  4. (D)

    attachment → tail contraction → landing → penetration and unplugging → DNA ejection

See the question in the paper
GATE 2023 BT question 20Tap to open full size

Answer and solution

ANSWER

The correct sequence is landing → attachment → tail contraction → penetration and unplugging → DNA ejection, which is option A.

Official answer: ASource: Official answer key, GATE 2023 BT (IIT Kanpur)

  1. For a contractile-tailed phage such as T4, infection follows a fixed order: landing, attachment, tail contraction, penetration and unplugging, then DNA ejection.

  2. Landing comes first. The long tail fibres touch the cell surface and reversibly recognise receptors such as lipopolysaccharide or OmpC. Firm attachment cannot happen until the virion has landed.

  3. Attachment follows. The short tail fibres and baseplate bind irreversibly, and this changes the baseplate shape and triggers the sheath.

  4. Tail contraction comes next. The sheath shortens and drives the rigid inner tail tube forward. Contraction is triggered by a firmly attached baseplate, so it cannot come before attachment.

  5. Penetration and unplugging follow. The tube crosses the outer membrane and peptidoglycan, and the plug at the tip is released, which opens the channel.

  6. DNA ejection is last, because the genome can leave the head only through the open tube.

  7. Option A gives exactly this order. B and D put attachment before landing. C puts contraction before attachment, and DNA ejection before penetration and unplugging. So only A is correct.

KEY IDEA

Phage infection proceeds from reversible landing to irreversible attachment, which triggers sheath contraction. The tube then penetrates and the plug opens, and only then is the DNA ejected.

WATCH OUT FOR

Swapping landing and attachment, or putting contraction before attachment. Contraction is the result of firm baseplate binding, not its cause. Another slip is to eject DNA before the channel is open.

From the GATE 2023 BT paper. Every question of this paper · Download the full paper · All 11 Pathogens & antibiotics questions, with notes

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