Viruses and Prions: Structure, Classification, and Life Cycles

Viruses and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from living organisms. They are studied extensively in microbiology due to their medical importance and distinct biological properties.

• Very small size: Viruses are much smaller than bacteria and cannot be seen with a light microscope.

• Not made of cells: Viruses lack cellular structure and are considered acellular.

• Obligate intracellular parasites: They require a host cell to reproduce and cannot multiply independently.

• Genetic material: Viruses contain either DNA or RNA as their genetic material, but never both.

• Protein coat (capsid): The genetic material is enclosed within a protein shell called the capsid.

How Viruses Differ from Living Organisms

Viruses are not classified as living organisms due to several fundamental differences:

• Cannot reproduce independently: They must hijack a host cell's machinery to replicate.

• No metabolism: Viruses do not use or produce energy (no ATP generation).

• Acellular: They lack cellular components such as cytoplasm, organelles, and plasma membrane.

• Dependence on host: Survival and multiplication are only possible within a suitable host cell.

Structural Features of Viruses and Spikes

Viruses have a simple structure, but some possess additional features that aid infection.

• Genome: Can be DNA or RNA, single- or double-stranded.

• Capsid: Protein coat that protects the viral genome and gives shape to the virus.

• Envelope: Some viruses have a lipid membrane (envelope) derived from the host cell membrane.

• Spikes (glycoproteins): Surface projections that facilitate attachment and entry into host cells.

Shapes of Viruses

Viruses exhibit a variety of shapes, which are important for classification and infection mechanisms.

• Helical: Rod-shaped or spiral arrangement of capsid proteins (e.g., Tobacco mosaic virus).

• Icosahedral: Spherical or polyhedral shape with 20 triangular faces (e.g., Adenovirus).

• Complex: More intricate structures, such as bacteriophages with head and tail components.

• Enveloped: Surrounded by a lipid membrane, often with embedded spikes (e.g., Influenza virus).

Virus Genomes and Biosynthesis Pathways

Viruses use different strategies to replicate their genomes and produce proteins, depending on their nucleic acid type.

• DNA viruses: DNA is transcribed to mRNA, which is then translated into proteins.

• RNA viruses: RNA is directly or indirectly used to make mRNA, then proteins.

• Retroviruses: RNA is reverse-transcribed into DNA, which integrates into the host genome, then transcribed to mRNA and proteins.

Key Enzyme: Reverse transcriptase is used by retroviruses (e.g., HIV) to convert RNA into DNA.

Universal Requirement: All viruses must produce mRNA to synthesize viral proteins.

Major Virus Families Affecting Humans

Viruses are classified into families based on their genetic material and structure. The following table summarizes some important families:

Family	Nucleic Acid	Example
Retroviridae	RNA	HIV
Orthomyxoviridae	RNA	Influenza (flu)
Papillomaviridae	DNA	HPV
Herpesviridae	DNA	Herpes simplex virus (HSV), Chickenpox

Lytic vs. Lysogenic Cycles

Viruses can follow different replication cycles within host cells, especially bacteriophages (viruses that infect bacteria):

• Lytic Cycle:

  • Virus enters the host cell.

  • Viral genome directs synthesis of new viruses.

  • Host cell bursts (lysis), releasing new virions.

• Lysogenic Cycle:

  • Viral DNA integrates into the host genome (prophage).

  • Virus remains dormant and replicates with host DNA.

  • Can later switch to the lytic cycle under certain conditions.

Example: Lambda phage can alternate between lytic and lysogenic cycles in Escherichia coli.

Prions: Infectious Proteins

Prions are a unique class of infectious agents distinct from viruses and other pathogens.

• Definition: Prions are infectious, misfolded proteins that can induce abnormal folding of normal proteins in the brain.

• No nucleic acids: Prions lack DNA or RNA.

• Diseases caused by prions:

  • Mad Cow Disease (Bovine Spongiform Encephalopathy)

  • Creutzfeldt-Jakob Disease (CJD)

  • Kuru

  • Fatal Familial Insomnia (FFI)

Additional info: Prion diseases are collectively known as transmissible spongiform encephalopathies (TSEs) and are characterized by progressive neurodegeneration.