Study Notes on Microbiology - Virus Life Cycles

Aims of Chapter 6A: Microbiology

  • Understand the terms lytic and latency.
  • Explain the lysogenic pathway and the lytic pathway.

Virus Life Cycles

  • Reproduction in host cells:
    • Viruses can only reproduce within a host cell.
    • They may enter a host cell via different methods:
    • Bacteriophages: Inject genetic material directly into bacteria.
    • Animal viruses: Enter cells through endocytosis, where viral envelopes fuse with the host cell membrane.
    • Plant viruses: Often use a vector (e.g., insects) to penetrate the plant cell’s cellulose wall.

Pathways After Viral Entry

  • Once within the host cell, viruses can follow one of two pathways:
    • Lysogenic pathway
    • Lytic pathway

Lysogenic Pathway

  • Viruses that enter this state do not immediately cause disease after infecting a host cell.
  • Characteristics:
    • Viral DNA is referred to as a provirus and integrates into the host DNA.
    • A specific viral gene codes for a repressor protein which inhibits the transcription and translation of viral DNA.
    • As the host DNA replicates, it also replicates the viral DNA (latent phase).
  • Latent period: The virus remains dormant, termed lysogeny.
  • Activation:
    • Viruses can transition to the lytic pathway due to:
    • Host cell damage.
    • Low nutrient levels inside the host cell.
    • Activation leads to a decrease in the repressor protein, thereby the virus enters the lytic pathway becoming virulent.

Lytic Pathway

  • Characteristics:
    • The viral genetic material is expressed, resulting in new viral DNA and proteins being produced.
    • Newly created viral components are assembled into mature viruses that accumulate within the host cell.
    • Eventually leads to lysis (bursting of the host cell), resulting in the release of numerous viruses which can infect new host cells.
    • This pathway typically causes disease.
Steps of the Lytic Cycle:
  1. Bacteriophage Attachment: A bacteriophage attaches to a bacterium.
  2. DNA Injection: Phage DNA is injected into the host cell, initiating the synthesis of viral enzymes.
  3. Bacterial DNA Inactivation: Phage DNA inactivates the host DNA and takes over the host's cellular biochemistry.
  4. New Phage Assembly: New phage particles are assembled, with new protein coats formed around phage DNA.
  5. Release: Enzyme lysozyme is synthesized or released, leading to the bacterial cell bursting and releasing up to 1000 phages for further infection.
  • The lytic and lysogenic pathways are critical for understanding viral behavior:
    • Lytic results in immediate viral reproduction and cell damage.
    • Lysogenic allows for long-term viral persistence without damage.

Retroviruses and Their Complex Life Cycle

  • Key Process: Retroviruses operate differently from typical viruses.
  • Steps involved:
    1. Viral RNA is released into the cytoplasm but cannot function as mRNA.
    2. Reverse Transcriptase converts the viral RNA into viral DNA.
    3. The viral DNA then enters the nucleus and integrates into the host DNA.
    4. Host transcriptase enzymes create viral mRNA and replicate viral genome RNA.
    5. New viral particles are assembled in the cytoplasm and exit the cell via exocytosis.
    6. The host cell continues to function as a virus production factory after release of new viruses.
Diagram Representation of Retrovirus Life Cycle
  1. Attack on host cell: Retrovirus attacks an animal cell.
  2. Viral RNA Entry: Viral RNA enters and is not immediately usable as mRNA.
  3. Reverse Transcription: Viral RNA gets transcribed into viral DNA in the cytoplasm.
  4. Host DNA Integration: Viral DNA incorporates into host DNA in the nucleus, directing the production of new viral RNA, mRNA, and coat proteins.
  5. Assembly of New Viruses: New viral particles are assembled and released via exocytosis; viral DNA remains in the nucleus for repeated cycles.

Implications — Viruses and Disease

  • Viral Infection Specificity:
    • Viral infections often target specific tissues. For example:
    • Adenoviruses cause colds by infecting the tissues of the respiratory tract.
  • Disease Examples:
    • Common viral diseases include flu, measles, and AIDS.
    • Viruses may also play a role in developing certain cancers.

Checkpoint Questions

  1. Differences Between Pathways: Describe the main differences between the lytic and lysogenic pathways.
  2. Structural Differences in Retroviruses:
    • (a) Identify the main structural difference between retroviruses and other types of viruses.
    • (b) Explain how this structural difference impacts their life cycle.

Classification of Viruses — Past Paper Questions

  • Classification is based on:

    • Type of nucleic acid they contain.
    • Arrangement of proteins in their capsid.
    • Presence or absence of an envelope.
    • Enzymes they contain.

  • Example Classification Task:

    • For structures, indicate the presence of features like DNA, helical capsid, envelope for viruses like Ebola and HIV.

  • Name an enzyme synthesizing DNA from RNA templates, commonly found in viruses (related to DNA polymerases in human cells).