Viral Replication and Oncogenic

Introduction to Viral Replication

  • In previous discussions, replication mechanisms of viruses, lytic and lysogenic cycles, and comparisons of bacteriophage to animal cells were addressed.

  • Current focus is on how replication varies depending on genetic material of viruses.

Types of Viruses by Genetic Material

  • DNA Viruses
      - Characteristics:
        - Contain double-stranded DNA as genetic material.
        - Example: DNA viruses primarily discussed are animal viruses, not bacteriophage.
      - Attachment Phase:
        - Virions have specific proteins on their capsid that recognize receptors on the host cell, enabling attachment.
        - Different viruses have different proteins and cannot bind to incompatible receptors.
      - Entry Mechanisms:
        - Virus enters host cell via fusion or receptor-mediated endocytosis.
        - Capsid must be uncoated to release viral DNA into the host cell.
      - Transcription and Replication:
        - Viral DNA must enter the nucleus for transcription (process of creating mRNA).
        - Early transcription produces early viral proteins that exit to the cytoplasm.
        - Host cell's DNA polymerase replicates viral DNA.
        - Further transcription produces more mRNAs, leading to capsid protein translation.
      - Maturation and Release:
        - Capsid proteins return to the nucleus to assemble and mature the virus, which then buds from the nucleus and plasma membrane.

RNA Viruses

  • Overview of RNA Viruses
      - Numerous types of RNA viruses exist, warranting distinct replication strategies.
      - Need to understand sense and antisense RNA strands.

  • Sense Strand RNA Viruses:
      - Characteristics:
        - Genetic material consists of single-stranded, positive-sense RNA.
        - Recognized as the same sequence as mRNA and can be directly translated into proteins.
      - Replication Process:
        - Once inside the host, the RNA is uncoated and can be immediately translated to produce viral proteins.
        - RNA-dependent RNA polymerase is required to transcribe a negative strand from the positive strand, allowing additional positive strands to be produced.
        - Advantages of this system:
          - Rapid replication since translation can begin immediately after entry.
          - Examples of viruses: Picornavirus, Togavirus.
      - Diseases associated with sense strand RNA viruses:
        - Poliovirus:
          - Transmission: Contaminated water containing feces.
          - Symptoms: Flu-like symptoms progressing to CNS invasion, leading to paralysis.
          - Vaccines:
            - Salk vaccine (inactivated; provides weaker immune response).
            - Sabin vaccine (oral; uses attenuated virus for stronger immunity).
            - Associated risks with reversion to full strength in attenuated vaccines.
        - German Measles (Rubella):
          - Symptoms: Mild disease with a rash; poses serious risks of birth defects during pregnancy.
          - Vaccine integrated with MMR.
        - Dengue and Zika Viruses:
          - Vector-borne diseases spread via mosquitoes.
          - Symptoms of Dengue: High fever, severe headache, rash.
          - Zika's primary concern: Possible links to microcephaly in infants born to infected mothers.