Q.What are viruses parasitising bacteria called? Draw a well labelled diagram of the same.

Viruses that parasitise bacteria are known as bacteriophages. These viruses typically possess a polyhedral head, which encloses their genetic material, usually double-stranded DNA. Attached to the head is a helical tail structure, which facilitates attachment to the bacterial cell surface and injection of the genetic material.

A well-labelled diagram of a bacteriophage would show:

• Head (containing DNA)
• Collar
• Sheath …

Viruses are fascinating biological entities, unique in their obligate parasitic nature. Unlike other organisms, they lack cellular machinery and can only replicate by infecting a host cell, hijacking its metabolic processes. This parasitic lifestyle is not limited to animal or plant cells; viruses also specifically target and infect bacteria.

The viruses that parasitise bacteria are known as bacteriophages. The term "bacteriophage" literally means "bacteria eater," reflecting their ability to infect and often lyse (break open) bacterial cells. These phages are among the most abundant biological entities on Earth and play a significant role in bacterial population control in various environments.

A typical bacteriophage, such as the T-even phages (e.g., T4 phage) that infect Escherichia coli, exhibits a complex and highly specialised structure designed for efficient infection. This structure can be broadly divided into two main parts: the head and the tail.

• Head: This is usually polyhedral (often icosahedral) and contains the phage's genetic material, which is typically double-stranded DNA. The head is enclosed by a protein coat called the capsid, which protects the nucleic acid.
• Tail: Attached to the head is a complex tail structure. This tail is crucial for recognising and attaching to the bacterial host cell and injecting the genetic material. The tail typically consists of several components:
  • Collar: A short region connecting the head to the tail sheath.
  • Sheath: A contractile protein structure that surrounds a central hollow core. During infection, this sheath contracts, pushing the core through the bacterial cell wall.
  • Base Plate: A hexagonal plate at the end of the tail sheath, which helps in anchoring the phage to the bacterial surface.
  • Tail Fibres: Long, slender protein fibres extending from the base plate. These fibres are responsible for specific recognition and reversible attachment to receptors on the bacterial cell surface.
  • Pins (or Spikes): Short, sharp projections from the base plate that help in irreversible attachment and penetration of the bacterial cell wall.

A well-labelled diagram of a bacteriophage would clearly illustrate these components, typically showing a T-even phage structure.

Diagram of a Bacteriophage (e.g., T4 Phage): …

Go component-by-component through the T4 phage, stating the FUNCTION of each part first, rather than describing the diagram region by region.

• Head (capsid): protects the enclosed double-stranded DNA genome from the environment.
• Collar: connects the head to the tail assembly.
• Sheath: a contractile protein cylinder -- contracts during infection to drive the inner core through the bacterial cell wall.
• Core/tail tube: the hollow channel the DNA is injected through, once the sheath has punctured the host wall.
• Base plate: anchors the whole tail assembly to the bacterial surface.
• Tail fibres: recognise and reversibly bind specific receptors on the host bacterium -- this is what gives a phage its host specificity. …