In-Depth Notes on Genetics of Viruses and Bacteria

Viruses

• Definition:

  • Nonliving particles that are not composed of cells.
  • Cannot replicate independently or carry out metabolic processes.

• Characteristics:

  • Require a living cell to replicate.
  • Size in comparison:
  • Viruses: 20-400 nm (0.02-0.4 µm)
  • Bacteria: 1000-5000 nm (1-5 µm)
  • Eukaryotic cells: 40-60 µm.

• Host Range:

  • Broad Host Range: Can infect multiple species (e.g., Tobacco Mosaic Virus infects 150 plant species).
  • Narrow Host Range: Specific to certain species (e.g., Bacteriophages infect only bacteria).

• Virus Structure:

  • Capsid: Protein coat surrounding genetic material (DNA or RNA).
  • Genetic Material Types:
  • dsDNA: double-stranded DNA.
  • ssRNA: single-stranded RNA (includes +ve and -ve stranded).
  • Retroviruses: RNA acts as a template for DNA synthesis.
  • Viral Envelope: Some viruses have a membrane derived from host cells that includes glycoproteins for host recognition.

• Virus Reproduction:

  • Utilizes the host's cellular machinery for reproduction (organelles, enzymes).
  • Reproductive Cycles:
  • Lysogenic Cycle: Viral DNA integrates into host DNA and remains dormant until triggered, replicates with the host DNA without producing virus particles.
  • Lytic Cycle: Virus enters the cell, multiplies, and causes the host cell to lyse, releasing new viruses.
  • Steps in Reproduction:
  1. Attachment to the host cell.
  2. Entry of viral DNA/RNA.
  3. Integration into the host chromosome (if applicable).
  4. Synthesis of viral components.
  5. Assembly of new virus particles.
  6. Release from the host cell.

HIV Virus

• Overview:

  • HIV (Human Immunodeficiency Virus) is an ssRNA virus, classified as a retrovirus.

• Structure:

  • Contains a capsid, envelope with glycoproteins, and reverse transcriptase.

• Infection Process:

  • Binds to CD4 receptors on T-cells.
  • Viral envelope fuses with the host cell membrane.
  • Uncoats, and viral RNA is reverse transcribed into dsDNA.
  • Integrated into the host's DNA, which may remain dormant (provirus).
  • After activation, new viral components are synthesized, assembled, and released through budding.

• Treatment:

  • Integrase inhibitors, reverse transcriptase inhibitors, and protease inhibitors are various drug types used to combat HIV.

Bacteria

• Chromosomal Structure:

  • Bacteria contain a single, circular DNA chromosome located in the nucleoid region.
  • Additionally, they may have plasmids (small, circular DNA molecules).
  • Plasmids carry genes for antibiotic resistance, virulence, or fertility.

• Reproduction:

  • Occurs mainly via binary fission, allowing rapid multiplication (e.g., E. coli can divide every 20-30 minutes).

• Gene Transfer Mechanisms:

  • Conjugation: Direct transfer of DNA from one bacterium to another (sexual reproduction).
  • Transformation: Uptake of foreign DNA from the environment into a bacterium.
  • Transduction: DNA is transferred between bacteria via viruses (bacteriophages).

Emerging Viruses

• Definitions:

  • Newly identified viruses or strains that cause epidemics. Often arise from mutations or the change in host range.

• Examples:

  • SARS (2002-2003), coronaviruses including Covid-19, influenza strains with pandemic potential due to reassortment among species.

• Covid-19 Structure and Impact:

  • Caused by SARS-CoV-2; genome is a single positive-strand RNA molecule.
  • Infection triggers immune responses, potentially leading to cytokine storms and serious lung damage.

• Vaccination and Prevention:

  • Vaccines are created through cultivation in eggs or cells, take time to produce, and prime the immune system for a robust response.