Model Answer
0 min readIntroduction
Viruses, obligate intracellular parasites, exhibit a remarkable diversity in their structure and genetic material. Their classification is based on several criteria, including the nature of their nucleic acid, mode of replication, and importantly, the symmetry of their capsid – the protein shell enclosing the genetic material. Viral symmetry dictates the arrangement of protein subunits that form the capsid, and is broadly categorized into helical, icosahedral, and complex symmetries. Understanding these symmetries is fundamental to comprehending viral structure and function. The T4 phage, a well-studied bacteriophage, exemplifies a complex symmetry, making it an ideal example for detailed structural analysis.
Viral Symmetry Types
Viruses are classified based on their symmetry into three main types:
- Helical Symmetry: Capsid proteins are arranged in a helical fashion around the nucleic acid. This results in a rod-shaped or filamentous virus. Example: Tobacco Mosaic Virus (TMV).
- Icosahedral Symmetry: The capsid is formed by 20 equilateral triangular faces. This is a highly stable and efficient structure. Example: Adenovirus, Poliovirus.
- Complex Symmetry: These viruses do not exhibit simple helical or icosahedral symmetry. They often have intricate structures with multiple parts. Example: Bacteriophages like T4, Poxviruses.
Structure of T4 Phage
The T4 phage, infecting Escherichia coli, is a classic example of a virus with complex symmetry. Its structure is composed of several distinct components:
Components of T4 Phage
- Head (Capsid): This is an icosahedral structure containing the viral DNA. It is approximately 130 nm long and 80 nm wide. The capsid is composed of protein subunits.
- Tail: A hollow, non-contractile tube extending from the head. It is about 80 nm long and 10 nm in diameter. It serves as a channel for injecting the viral DNA into the host cell.
- Head-Tail Connector: A hexagonal baseplate connecting the head to the tail. It plays a crucial role in DNA packaging and ejection.
- Baseplate: A complex structure at the end of the tail, possessing six tail fibers.
- Tail Fibers: These are long, thin appendages extending from the baseplate. They are responsible for recognizing and attaching to specific receptors on the host cell surface, initiating the infection process.
- Lateral Tail Spikes: These protein spikes help in the initial attachment to the host cell.
- Central Tail Spikes: These are longer spikes that aid in penetrating the host cell wall.
Mechanism of Infection: The tail fibers recognize and bind to receptors on the E. coli cell surface. Upon attachment, the tail contracts, driving the central spike into the cell wall. The viral DNA is then injected through the tail tube into the host cell cytoplasm, initiating replication.
| Component | Function |
|---|---|
| Head (Capsid) | Contains and protects the viral DNA |
| Tail | Acts as a channel for DNA injection |
| Tail Fibers | Recognize and bind to host cell receptors |
| Baseplate | Connects head and tail; aids in attachment |
Conclusion
Viral symmetry is a fundamental characteristic used in their classification, reflecting the underlying principles of capsid assembly and stability. The T4 phage, with its complex structure, exemplifies the sophisticated strategies viruses employ to infect and replicate within host cells. Understanding the intricate components and their functions is crucial for developing antiviral therapies and controlling viral infections. Further research into viral structures continues to reveal novel insights into their evolutionary history and mechanisms of pathogenesis.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.