Viruses: morphology, classification, reproduction & cultivation

The study of viruses encompasses various aspects including their morphology, classification, reproduction/replication, and cultivation.

Morphology of Viruses

Structure of viruses:

• • Capsid:

    • The protein coat surrounding the viral genome, composed of protein subunits called capsomeres.
    • The capsid provides protection and aids in the attachment to host cells.
    • Helical: Capsids with a rod-like appearance (e.g., tobacco mosaic virus).
    • Icosahedral: Capsids with a roughly spherical shape (e.g., adenovirus).
    • Complex: Capsids with intricate structures (e.g., bacteriophage).

  • Envelope:

    • Some viruses have an outer lipid membrane derived from the host cell membrane, containing viral glycoproteins (e.g., influenza virus, HIV).

  • Genome:

    • The genetic material of virus, which can be DNA or RNA, single-stranded or double-stranded, linear or circular.
    • DNA Virus: Have DNA as their genetic material (e.g., herpesvirus).
    • RNA Virus: Have RNA as their genetic material (e.g., influenza virus).

Additional Structures:

• • Tegument:

    • A layer of proteins found between the envelope and capsid in some viruses (e.g., herpesviruses).

  • Tail Fibers and Spikes:

    • Structures used for attachment to host cells, commonly found in bacteriophages and some animal virus.

Classification of Viruses

• Virus is classified based on several criteria including the type of nucleic acid, replication strategy, morphology, and host range.
• The main classification systems include:

1. Based on Genetic Material:

• • DNA Virus:

    • Contain DNA as their genetic material.
    • Examples: Adenoviruses, Herpesviruses.

  • RNA Virus:

    • Contain RNA as their genetic material.
    • Examples: Influenza viruses, Coronaviruses.

2. Based on Replication Method:

• • Positive-Sense RNA Virus:

    • RNA serves directly as mRNA.
    • Examples: Poliovirus, Hepatitis C virus.

  • Negative-Sense RNA Virus:

    • RNA must be transcribed to mRNA.
    • Examples: Rabies virus, Ebola virus.

  • Reverse Transcribing Virus:

    • RNA virus that reverses transcribe into DNA.
    • Examples: HIV (retroviruses), Hepatitis B virus (hepadnaviruses).

3. Based on Morphology:

• Icosahedral Viruses:

  • Capsid with icosahedral symmetry.
  • Examples: Adenoviruses, Polioviruses.

• Helical Viruses:

  • Capsid with helical symmetry.
  • Examples: Influenza viruses, Tobacco mosaic virus.

• Complex Viruses:

  • Combination of icosahedral and helical structures or other complex shapes.
  • Examples: Bacteriophages, Poxviruses.

4. Based on Host Range:

• Animal Virus:

  • Infect animals.
  • Examples: Rabies virus, SARS-CoV-2.

• Plant Virus:

  • Infect plants.
  • Examples: Tobacco mosaic virus, Potato virus Y.

• Bacteriophag:

  • Infect bacteria.
  • Examples: T4 phage, Lambda phage.

Each of these groups can be further divided based on more specific characteristics and genetic differences

Reproduction/Replication of Viruses

The replication cycle of virus varies between different types, but generally includes the following stages:

• Attachment:

  • The virus binds to specific receptors on the surface of the host cell.

• Entry:

  • The viral genome enters the host cell through mechanisms such as fusion with the host membrane, endocytosis, or direct penetration.

• Uncoating:

  • The viral capsid is removed, releasing the viral genome into the host cell.

• Replication:

  • DNA Virus: Replicate in the nucleus using the host’s DNA polymerase (e.g., herpesvirus).
  • RNA Virus: Replicate in the cytoplasm using viral RNA polymerase.
    • Positive-sense RNA Viruses: Can be directly translated into proteins.
    • Negative-sense RNA Viruses: Must be transcribed into positive-sense RNA by RNA-dependent RNA polymerase.

• Transcription and Translation:

  • Viral mRNA is transcribed (if necessary) and translated into viral proteins using the host’s ribosomes.

• Assembly:

  • New viral particles are assembled from the replicated genome and synthesized proteins.

• Release:

  • Newly formed viruses are released from the host cell through lysis (breaking open the cell) or budding (enveloped viruses).

Replication Cycles:

• Lytic Cycle:

  • Virus replicates rapidly and lyses the host cell to release progeny viruses (e.g., bacteriophage T4).

• Lysogenic Cycle:

  • Viral DNA integrates into the host genome and replicates along with it without causing immediate lysis.
  • It can later enter the lytic cycle (e.g., bacteriophage λ).

Cultivation of Viruses

Host Systems:

• Cell Cultures: Viruses are grown in monolayers of cultured cells. Cell lines can be primary (directly taken from tissues) or continuous (immortalized).
• Cytopathic Effect (CPE): Observable damage to host cells due to viral infection.
• Embryonated Eggs: Used for the cultivation of certain virus (e.g., influenza virus). Different parts of the egg (chorioallantoic membrane, amniotic fluid, yolk sac) are used for different virus.
• Laboratory Animals: Some viruses require a whole organism for propagation.

Detection and Quantification:

• Plaque Assay: Measures the number of virus particles by counting plaques (clear zones) formed on a layer of host cells.
• Hemagglutination Assay: Measures the ability of virus to agglutinate red blood cells.
• ELISA (Enzyme-Linked Immunosorbent Assay): Detects viral antigens or antibodies against virus.
• PCR (Polymerase Chain Reaction): Detects and quantifies viral DNA or RNA.

Safety and Ethical Considerations:

• Proper biosafety protocols must be followed when working with virus, especially pathogenic ones.
• Ethical considerations are important when using animals for virus cultivation and research

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