Viruses

26.1 The Nature of Viruses

• Basic structure: nucleic acid core surrounded by protein.

• Not considered cells; lack cytoplasm.

• Nucleic acid types: DNA or RNA; can be circular/linear, single/double-stranded.

26.2 Viral Diversity

• Virus Structure:

  • Forms capsid (protein sheath) around nucleic acid.

  • Some possess an envelope from host cell membrane with viral proteins.

  • May contain specialized enzymes like reverse transcriptase.

26.3 Bacteriophage: Bacterial Viruses

• Obligate intracellular parasites; rely on host cells for replication.

• Host range: specific virus types infect limited organisms.

• Tissue Tropism: infect specific tissues (e.g., Rhabdovirus targets neurons).

26.4 Viral Diseases of Humans

• Viral Replication:

  • Viruses hijack host cell machinery for replication.

  • No ribosomes/enzyes for protein synthesis.

  • Steps: Early genes (viral takeover), Intermediate genes, Late genes (assembly/release of virions).

• Human Viral Diseases:

  • Chickenpox: Varicella-zoster; double-stranded DNA; vaccine available.

  • Hepatitis B: Double-stranded DNA; fatality possibility, vaccine available.

  • HIV: single-stranded RNA; leads to AIDS.

  • Influenza: single-stranded RNA; annual vaccines evident.

26.5 Prions and Viroids: Infectious Subviral Particles

• Giant Viruses: Challenge definitions; gene coding for translation machinery, e.g., Mimivirus.

• Virus Classification: Based on taxonomy (ICTV) or Baltimore classification (genome structure).