Fundamentals of Viruses: Structure, Cultivation, and Replication

Viruses: General Characteristics

Acellular Nature

Viruses are unique infectious agents that lack cellular structure and cannot carry out metabolic processes independently.

• Acellular: Viruses are not true cells; they do not possess organelles or plasma membranes.

General Characteristics

• Extremely Small Size: Viruses are much smaller than most bacteria and can only be seen with electron microscopes.

• Obligate Intracellular Parasites: Viruses must infect living cells to reproduce. Some, called bacteriophages, infect bacteria.

• Lack of Cellular Components: Viruses do not have cytoplasm, organelles, ribosomes, or plasma membranes.

General Structure of Viruses

Viruses are composed of genetic material and a protective protein coat, with some possessing additional structures.

• Two Basic Parts:

  • Genetic material: DNA or RNA

  • Capsid: A protein coat that protects the nucleic acid, made up of subunits called capsomeres.

• Envelope: Some viruses are surrounded by a lipid envelope derived from the host cell membrane.

• Surface Projections: Some viruses have spikes (glycoproteins) that aid in attachment to host cells.

• Host Specificity: Viruses typically infect only specific types of cells in one host species.

• Morphology: Viruses exhibit three major shapes:

  1. Helical capsids

  2. Enveloped

  3. Icosahedron

  4. Complex

Cultivation of Viruses

Main Purposes of Cultivation

Viruses are cultivated for research, diagnosis, and vaccine production.

• Isolation and identification in clinical samples

• Research purposes

• Vaccine production

Methods of Virus Cultivation

• Living Cells: Viruses must be grown in living cells, as they cannot reproduce independently.

• Bacteria Viruses: Can be grown in either liquid or solid media using bacterial cultures.

• Animal Viruses: Grown in:

  1. Living animals

  2. Embryonated eggs

  3. Cell cultures (cells grown in controlled environments)

Viral Replication

Replication of Bacteriophages

Bacteriophages are viruses that infect bacteria and follow a specific replication cycle.

• Lytic Cycle:

  1. Attachment

  2. Penetration

  3. Biosynthesis

  4. Maturation

  5. Release

• Key Steps Explained:

  • Attachment: Virus binds to specific receptors on the bacterial surface.

  • Penetration: Viral DNA is injected into the host cell.

  • Biosynthesis: Host machinery is used to replicate viral components.

  • Maturation: New viral particles are assembled.

  • Release: Host cell lyses, releasing new viruses.

Bacteriophage Lysogenic Cycle

Some bacteriophages can integrate their DNA into the host genome, leading to a long-term relationship.

• Phage DNA integrates with host cell DNA, forming a prophage.

• Viral DNA is replicated along with bacterial DNA during cell division.

• Prophage may later exit the genome and enter the lytic cycle.

Replication of Animal Viruses

Animal viruses follow a similar pattern to bacteriophages, with some differences.

• Steps in Viral Replication:

  1. Attachment

  2. Entry

  3. Uncoating (unique to animal viruses): Removal of the capsid to release viral nucleic acid.

  4. Biosynthesis

  5. Maturation

  6. Release

     • Enveloped viruses: Bud from the host cell membrane.

     • Non-enveloped viruses: Cause cell lysis.

Latent Viral Infections

Some viruses can remain dormant in the host for extended periods, reactivating under certain conditions.

• Virus does not cause disease immediately; may remain in host without symptoms for years.

• Reactivation can occur due to immunosuppression or stress.

Table: Comparison of Bacteriophage and Animal Virus Replication

Step	Bacteriophage	Animal Virus
Attachment	To bacterial cell wall	To animal cell membrane
Penetration	Injection of DNA	Endocytosis or fusion
Uncoating	Not required	Required
Biosynthesis	In cytoplasm	In nucleus or cytoplasm
Maturation	Assembly in cytoplasm	Assembly in cytoplasm or nucleus
Release	Cell lysis	Budding (enveloped) or lysis (non-enveloped)

Key Terms and Definitions

• Capsid: Protein shell surrounding viral nucleic acid.

• Capsomere: Subunit of the capsid.

• Envelope: Lipid membrane surrounding some viruses.

• Prophage: Phage DNA integrated into bacterial genome.

• Lytic Cycle: Viral replication resulting in host cell lysis.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it.

• Uncoating: Removal of capsid in animal viruses to release nucleic acid.

Formulas and Equations

• Viral Growth Rate: $\text{Rate} = k[\text{Virus}][\text{Host Cell}]$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries inferred from standard microbiology knowledge.