Viral Biology: Structure, Classification, and Life Cycles

Introduction to Viruses

Viruses are acellular organisms that exist at a level of organization below the cell. To live and multiply, they must parasitize a host cell. They present two distinct phases: the extracellular and the intracellular phase.

Extracellular and Intracellular Phases

In the extracellular phase, viruses are in a latent state and are known as virions. They consist of a nucleic acid enveloped by a protein structure. In the intracellular phase, the virus exists solely as nucleic acid. Animal viruses can contain either DNA or RNA, whereas plant viruses typically carry RNA. Viruses that infect bacteria, known as bacteriophages, can contain either DNA or RNA.

Viral Morphology and Structure

Viruses are classified by the shape of their protein coat, or capsid:

• Helical/Cylindrical: These feature a hollow cylindrical arrangement of capsomeres following a helical path.
• Icosahedral: These possess a polyhedral structure.
• Enveloped: These are viruses surrounded by a membrane envelope.

Complex Viruses: Bacteriophages

Complex viruses have a specialized structure consisting of an icosahedral region called the head, which houses the DNA, and a tubular tail. The tail axis is connected to a base plate equipped with caudal fibers and spines.

Classification of Viruses

Viruses are often classified by their capsid shape and genetic material:

• Helical: Examples include RNA viruses like Influenza and DNA viruses like Smallpox.
• Polyhedral: Examples include RNA viruses like HIV and DNA viruses like Herpes.
• Complex: This category includes bacteriophages such as T2 and T4.

The Viral Multiplication Process

Viruses are obligate parasites that can only multiply when their nucleic acid penetrates a host cell. They hijack the cellular machinery of the host to produce multiple identical copies of themselves. Once these copies are produced, each is surrounded by its own capsid before leaving the cell to infect others.

Infection Mechanisms

Animal viruses carry lipoproteins in their capsids, allowing them to be recognized by the host cell as its own. Plant viruses typically enter through cracks or pores in the cell wall. There are two primary types of reproduction cycles: the Lytic Cycle and the Lysogenic Cycle.

The Six Stages of the Lytic Cycle

1. Fixation: The bacteriophage attaches to the bacterial wall using specific enzymes.
2. Penetration: The viral DNA is injected from the head of the virus into the bacteria.
3. Replication: The viral nucleic acid halts bacterial activity and degrades the host DNA to create copies of itself.
4. Production: Viral DNA synthesizes messenger RNA (mRNA), which binds to bacterial ribosomes to produce viral proteins.
5. Assembly: Once the DNA molecules and capsids are synthesized, they assemble; the DNA enters the head and the tail is added.
6. Release: Approximately 100 new bacteriophages are released as the bacterial cell wall breaks (lysis) due to enzymatic action.

The Lysogenic Cycle and Prophages

Lysis does not always occur immediately. In some cases, temperate phages integrate their DNA into the bacterial genome. This process involves the incorporation of the viral DNA into the host's DNA, where the integrated viral fragment is referred to as a prophage.