Virus Structure, Classification, and Replication Cycles

Virus Characteristics

Viruses are unique pathogens, not classified as living organisms. A virus particle, a submicroscopic infectious agent, has a single nucleic acid type (DNA or RNA) within a protein coat called the viral capsid, forming the nucleocapsid.

Some viruses have a lipid-protein envelope. The capsid's protein organization determines the virus's shape, usually helical or polyhedral. Viruses are genetic elements in transit, switching between extracellular and intracellular states.

Extracellular Phase (Virion)

In the extracellular phase, the virion is metabolically inert. It carries viral genetic material from one cell to another.

Intracellular Phase

Inside a cell, the virus replicates, using the cell's machinery to synthesize new viral particles. The nucleic acid within the capsid (DNA or RNA, single or double-stranded) directs this process. All viruses are intracellular parasites, redirecting the host cell's metabolism through a viral cycle.

Virus Classification

Viruses have a narrow host range and are classified into:

• Animal viruses
• Plant viruses
• Bacteriophages (infect bacteria)

Viral Replication Cycles

Bacteriophages have two main cycles:

Lytic Pathway

The viral genetic material is active, transcribed, and translated to assemble new virions, leading to cell lysis (destruction).

Lysogenic Pathway

The viral genetic material integrates into the host cell's DNA. Viral gene expression is minimal, but the virus retains the potential to produce new virions.