Virus

Introduction to Viruses

• Viruses are infectious, obligate intracellular parasites.

• Composed of:

  • Nucleic acid core (DNA or RNA).

  • Protein capsid.

  • Sometimes an envelope from host cell membranes.

Virus Classification

• Different types of viruses based on morphology and genetic material:

  • Herpesvirus: 360 nm

  • Rabies: 200 nm

  • Measles: 180 x 80 nm

  • Variola: 150 nm

  • Other viruses include, but are not limited to:

    • HIV-1, SARS, Influenza, Adenovirus, Rotavirus.

    • Dengue, Hepatitis A/B/C, Zika, Poliovirus, and Ebola viruses.

Basic Structure of Viruses

1. Inner Components:

   • Viral genome (nucleoid):

     • Composed of either RNA or DNA.

   • Viral enzymes:

     • Require for replication:

       • DNA-dependent RNA polymerase.

       • RNA-dependent RNA polymerase.

       • RNA-dependent DNA polymerase (reverse transcriptase).

2. Outer Components:

   • Capsid:

     • A protein shell surrounding the nucleoid.

     • Composed of capsomers.

     • Structural patterns:

       • Icosahedral

       • Helical

       • Complex

   • Envelope:

     • Surrounds nucleocapsid (may or may not be present)

     • Composed of virus-specific proteins, lipids, and carbohydrates from the host membrane.

Viral Capsid Shapes

• The viral capsid can be:

  • Helical

  • Polyhedral

  • Complex

Classification of RNA Viruses

• Virus classification includes:

  • Naked Viruses:

    • Single-stranded and double-stranded RNA.

  • Enveloped Viruses:

    • Typically characterized as positive-sense (mRNA compatible) or negative-sense (requires RNA-dependent RNA polymerase).

Viral Identification

RNA Virus Identification:

• Classified as single or double-stranded RNA:

  • Positive-sense RNA:

    • Directly serves as mRNA.

  • Negative-sense RNA:

    • Requires transcription into mRNA via RNA polymerase.

Classification by Nucleic Acid Structure

• RNA Viruses:

  • Single-stranded (positive or negative sense)

  • Double-stranded

• DNA Viruses:

  • Generally double-stranded (except Parvoviridae); can be linear or circular.

Replication Cycle of Eukaryotic Viruses

1. Attachment:

   • Viral proteins attach to host cell.

2. Penetration:

   • Invasion of the host cell.

3. Uncoating:

   • Dismantling of the capsid to release genetic material.

4. Biosynthesis:

   • Synthesis of viral nucleic acid and proteins.

5. Assembly:

   • Assembly of new virions within the host cell.

6. Release:

   • Via lysis (naked viruses) or budding (enveloped viruses).

• Replication locations:

  • DNA Viruses: Replicate in the nucleus (except poxvirus).

  • RNA Viruses: Replicate in the cytoplasm (except influenza and retroviruses).

Conclusion

• Understanding viruses involves recognizing their complex structure, varying classifications, and replication cycle.