Lecture 33 Notes on Viruses and Viral Replication

Introduction to Viruses

  • Definition of a Virus:

    • An infectious particle consisting predominantly of genetic material encapsulated within a protein coat.

    • Historically, viruses were believed to be biological chemicals, leading to the term "virus," which translates to "poison" in Latin.

Discovery of Viruses

  • Historical Context:

    • In 1883, scientists studied the properties of tobacco mosaic disease (also known as spot disease).

    • Results:

    • Filtered sap from infected tobacco plants caused infection in healthy plants when rubbed on them.

    • This infection could be transferred from any infected plant, regardless of the source of the sap.

    • Conclusion Drawn:

    • The infectious agent could not be a bacterium, as it passed through the filter, suggesting a smaller replicating entity.

    • Hypotheses:

    • Original: Disease in tobacco plants caused by bacteria.

    • Second: Disease caused by REALLY SMALL bacteria.

    • Third: Disease caused by a replicating particle smaller and simpler than a bacterium.

    • Final Proof:

    • In 1935, the tobacco mosaic virus was crystallized and visualized under an electron microscope, confirming it as a virus.

Structure of Viruses

  • General Characteristics of Viruses:

    • Capsid:

    • A protein shell encapsulating the viral genetic material.

    • Made from protein subunits known as capsomers.

    • Forms various structures including:

      • Helical Capsid:

      • Example: Tobacco mosaic virus has a helical structure surrounding a single helix RNA molecule, appearing as a rigid rod.

      • Dimension: 40 nm in diameter.

      • Icosahedral Capsid:

      • Example: Adenoviruses possess an icosahedral shape with protein spikes at each vertex, containing double-stranded DNA.

      • Dimension: 40 nm in diameter.

      • Enveloped Viruses:

      • Example: Influenza virus has an outer membrane derived from the host cell, encasing viral components including glycoprotein spikes.

      • Dimensions: 80-200 nm in diameter, with spikes at variances.

      • Bacteriophage:

      • Example: Bacteriophage T4 has a complex structure featuring an icosahedral head, tail apparatus, and double-stranded DNA enclosed in the head.

      • Dimension: 40 nm in diameter.

  • Virus Sizes:

    • General measurements for diverse viruses include:

    • Tobacco mosaic virus: 40 nm

    • Adenovirus: 40 nm

    • Influenza virus: 70-90 nm (diameter)

    • Bacteriophage T4: 40 nm

Central Dogma of Molecular Biology

  • Framework:

    • The fundamental process of molecular biology can be summarized as:

    • DNA → Messenger RNA (mRNA) → Protein

  • Key Processes Involved:

    • Transcription: Process of synthesizing RNA from DNA.

    • Translation: Involves the conversion of mRNA to protein.

    • RNA Processing: Modifications made to RNA before it is translated.

  • Cellular Immortality:

    • Role of Telomerase RNA:

    • Acts as a non-coding RNA contributing to the mechanism of cellular immortality.

Viral Genomes and the Central Dogma

  • Variability of Viral Genomes:

    • Viruses exhibit different types of genomes that deviate from the traditional central dogma framework:

    • Parvovirus:

      • Single-stranded DNA.

    • Retrovirus:

      • Contains reverse transcriptase, converting RNA to DNA.

    • Hepatitis B Virus:

      • Double-stranded DNA.

    • Adenovirus:

      • Double-stranded DNA.

    • Herpes Simplex Virus:

      • Double-stranded DNA.

    • Poliovirus:

      • Positive single-stranded RNA.

    • Influenza Virus:

      • Contains negative single-stranded RNA.

    • Ebola Virus:

      • Negative single-stranded RNA.

  • Summary of Various Viral Genomes:

    • Single-stranded RNA: Parvovirus, Poliovirus, Reovirus.

    • Double-stranded RNA: Infectious agents like rotaviruses exhibit this structure.

Viral Capsids

  • Structure and Importance:

    • 3D protein shell encasing viral genetic material.

    • Constructed from protein subunits called capsomers.

Viral Envelopes

  • Functionality:

    • Envelopes facilitate entry into host cells.

    • Example:

    • The influenza virus has membranes derived from the host cell membranes, containing host phospholipids and proteins, as well as viral glycoproteins.

Classification of Viruses: Enveloped vs Non-Enveloped

  • Differences in Structure:

    • Enveloped Virus:

    • Consists of a lipid envelope surrounding the capsid and genetic material.

    • Non-Enveloped Virus:

    • Lacks a lipid envelope, encapsulating genetic material solely within a protein capsid.

Complexity of Viral Structures

  • Bacteriophage: T4 bacteriophage

    • Characteristics include:

    • Head, tail structure, collar, sheath, baseplate, long tail fibers, and DNA core with a protein coat.

Overview of Viral Replication

  • Viral Lifecycle Stages:

    • Virus enters host cell and releases its genome (DNA or RNA).

    • The viral genome is replicated using host enzymes.

    • Copies of the viral genome are transcribed into viral mRNA.

    • Viral mRNA is translated by host ribosomes into viral proteins.

    • Self-assembly of viral proteins and genome to form new viruses. - Some viruses cause the host cell to burst upon release, while others may leave the host cell intact.

Simplified Process of Viral Replication

  • Step-by-Step Process:

    1. Virus penetrates the cell and sheds, releasing viral DNA and capsid proteins.

    2. Host enzymes replicate the viral genome.

    3. Viral genome is transcribed into viral mRNA for protein synthesis.

    4. Capsid proteins and genomes self-assemble into new viral particles that exit the host cell.