5.3.1 Structure of Bacteriophage

Introduction

Bacteriophages are viruses that infect bacteria. They are among the most complex viruses in terms of structure. The bacteriophage is generally tadpole-shaped, with a head, neck, and tail. Its structure is highly specialized for infecting bacterial cells. Understanding the structure of bacteriophages is essential for understanding their life cycles and their use in genetic engineering.

Definition

Bacteriophage: The bacteriophage or simply phage is the virus that attacks upon bacteria.

In simple words: A bacteriophage is a virus that infects bacteria.

Core Concept

• The bacteriophage is generally a tadpole-shaped virus.
• It consists of:
• The head is icosahedral in shape.
• The inner core of the head consists of a single-stranded DNA genome (or double-stranded DNA, depending on the phage).
• Below the head is a narrow neck or collar which separates the head and tail.
• The tail is a hollow tube made up of proteins through which the nucleic acid passes during infection.
• The tail is surrounded by a contractile sheath, which contracts during infection of the bacterium.
• At the end of the tail, a base plate is present.
• The base plate possesses about six tail fibres around it and several tail pins or spikes at its lower surface.
• The tail fibres and tail pins are involved in the binding of the phage to the bacterial cell.
• At the bottom of the core tube of the tail, an enzyme, the lysozyme, is present which is released upon contraction of the tail.
• Lysozyme digests the portion of the host cell wall so that the core tube can be penetrated into the host cell during infection.

Detailed Explanation

Structure of Bacteriophage

A bacteriophage consists of the following parts:

Detailed Description of Each Part

1. Head

• Shape: Icosahedral (polyhedral)
• Contents: Inner core consists of a single-stranded DNA genome (or double-stranded DNA in some phages)
• Function: Protects the genetic material
• Size: About 100 nm in diameter

Key Point: The head contains the genetic material of the bacteriophage.

2. Neck (Collar)

• Location: Below the head
• Shape: Narrow
• Function: Separates the head and tail
• Structure: Made of proteins

Key Point: The neck connects the head to the tail.

3. Tail

• Shape: Hollow tube
• Composition: Made of proteins
• Function: The nucleic acid passes through the tail during infection
• Surrounded by: A contractile sheath

Key Point: The tail acts as a channel for the nucleic acid to enter the host cell.

4. Contractile Sheath

• Location: Surrounds the tail
• Function: Contracts during infection of the bacterium
• Effect: Helps inject the nucleic acid into the host cell

Key Point: The contractile sheath is like a spring that contracts to push the nucleic acid into the host.

5. Base Plate

• Location: At the end of the tail
• Shape: Hexagonal
• Function: Anchors the tail to the bacterial cell surface
• Attachments: Tail fibres and tail pins

Key Point: The base plate is the "landing pad" of the bacteriophage.

6. Tail Fibres

• Number: About six
• Location: Around the base plate
• Function: Involved in the binding of the phage to the bacterial cell
• Structure: Long, thin fibres

Key Point: Tail fibres help the phage recognize and attach to specific receptors on the bacterial cell.

7. Tail Pins (Spikes)

• Location: At the lower surface of the base plate
• Function: Involved in the binding of the phage to the bacterial cell
• Structure: Short, pointed structures

Key Point: Tail pins help the phage attach firmly to the bacterial cell.

8. Lysozyme

• Location: At the bottom of the core tube of the tail
• Function: Released upon contraction of the tail
• Action: Digests the portion of the host cell wall
• Result: The core tube can be penetrated into the host cell during infection

Key Point: Lysozyme is an enzyme that weakens the bacterial cell wall, allowing the phage to inject its nucleic acid.

Diagrammatic Summary of Bacteriophage Structure

Summary Table of Bacteriophage Structure

Intermediate Level

Structure of Bacteriophage in Detail

Steps of Bacteriophage Infection

Comparison of Bacteriophage Structure with Other Viruses

Important Points

• 1. The bacteriophage or simply phage is the virus that attacks upon bacteria.
• 2. It is generally a tadpole-shaped virus.
• 3. It consists of head, neck and tail.
• 4. The head is icosahedral in shape.
• 5. The inner core of head consists of a single-stranded DNA genome.
• 6. Below the head is narrow neck or collar which separates head and tail.
• 7. The tail is a hollow tube made up of proteins through which the nucleic acid passes during infection.
• 8. The tail is surrounded by a contractile sheath, which contracts during infection of the bacterium.
• 9. At the end of the tail a base plate is present which possesses about six tail fibres around it and several tail pins or spikes at its lower surface.
• 10. The tail fibres and tail pins are involved in the binding of the phage to the bacterial cell.
• 11. At the bottom of core tube of tail, an enzyme, the lysozyme is present which is released upon contraction of tail.
• 12. It digests the portion of host cell wall so that core tube can be penetrated into the host cell during infection.

Remember

• - Bacteriophage = virus that attacks bacteria
• - Shape = tadpole-shaped
• - Head = icosahedral; contains DNA
• - Neck = connects head and tail
• - Tail = hollow tube; surrounded by contractile sheath
• - Base plate = anchors the tail
• - Tail fibres = help in binding
• - Lysozyme = digests the host cell wall
• - Function = infect bacteria

Common Mistakes

• 1. Confusing bacteriophage with animal viruses – Bacteriophages have a tail; animal viruses do not.
• 2. Thinking all bacteriophages have DNA – Some bacteriophages have RNA, but most have DNA.
• 3. Confusing the head and tail – Head = icosahedral, contains DNA; Tail = hollow tube, helps in infection.
• 4. Forgetting the role of lysozyme – Lysozyme digests the host cell wall.
• 5. Thinking the tail is solid – The tail is a hollow tube.
• 6. Confusing tail fibres and tail pins – Tail fibres = long, thin; Tail pins = short, pointed.
• 7. Thinking the contractile sheath is rigid – The contractile sheath contracts during infection.
• 8. Forgetting the base plate – The base plate anchors the tail to the bacterial cell.
• 9. Thinking the phage enters the bacterium – Only the nucleic acid enters; the rest of the phage remains outside.
• 10. Confusing the lytic and lysogenic cycles – Lytic = host lysed; Lysogenic = viral DNA integrates.
• 11. Thinking bacteriophages are harmful – Bacteriophages are used in phage therapy to kill pathogenic bacteria.
• 12. Forgetting the function of tail fibres – Tail fibres help in binding to the bacterial cell.

Quick Revision

• The bacteriophage or simply phage is the virus that attacks upon bacteria.
• It is generally a tadpole shaped virus.
• It consists of head, neck and tail.
• The head is icosahedral in shape.
• The inner core of head consists of a single stranded DNA genome.
• Below the head is narrow neck or collar which separates head and tail.
• The tail is a hollow tube made up of proteins through which the nucleic acid passes during infection.
• The tail is surrounded by a contractile sheath, which contracts during infection of the bacterium.
• At the end of the tail a base plate is present which possesses about six tail fibres around it and several tail pins or spikes at its lower surface.
• The tail fibres and tail pins are involved in the binding of the phage to the bacterial cell.
• At the bottom of core tube of tail, an enzyme, the lysozyme is present which is released upon contraction of tail.
• It digests the portion of host cell wall so that core tube can be penetrated into the host cell during infection.

60-Second Revision

• Bacteriophage = virus that attacks bacteria
• Shape = tadpole-shaped
• Head = icosahedral; contains DNA
• Neck = connects head and tail
• Tail = hollow tube; surrounded by contractile sheath
• Base plate = anchors the tail
• Tail fibres = help in binding
• Lysozyme = digests the host cell wall
• Function = infect bacteria
• Warning: Bacteriophages have a tail; animal viruses do not. The tail is a hollow tube. The contractile sheath contracts during infection. Lysozyme digests the host cell wall. Only the nucleic acid enters the bacterium. Tail fibres and tail pins help in binding. The head is icosahedral. Most bacteriophages have DNA.