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GATE 2023 BT – Question 20

General Biology · Microbiology: Microbial diseases, host-pathogen interactions, antibiotics and resistance, two-component system, bacterial communication, viruses · 1 mark · Multiple choice

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

  1. landing → attachment → tail contraction → penetration and unplugging → DNA ejection
  2. attachment → landing → penetration and unplugging → tail contraction → DNA ejection
  3. landing → tail contraction → attachment → DNA ejection → penetration and unplugging
  4. attachment → tail contraction → landing → penetration and unplugging → DNA ejection

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Correct answer: (A) landing → attachment → tail contraction → penetration and unplugging → DNA ejection

Explanation

The infection of a bacterial cell by a tailed phage (such as T4) proceeds in this order:

1. **Landing:** the phage reaches the cell surface.
2. **Attachment:** the tail fibres bind specifically to receptors on the bacterium.
3. **Tail contraction:** the sheath contracts, driving the tail tube toward the cell.
4. **Penetration and unplugging:** the tube pierces the cell envelope and the plug at its tip is removed.
5. **DNA ejection:** the genome passes through the tube into the cell.

This matches option **A**. The other sequences put contraction or penetration before attachment, which cannot happen because the tail is triggered only after binding.