Viruses, Bacteria, and Archaea

Characteristics of Life

  • Order

  • Regulation

  • Growth and Development

  • Uses Energy

  • Response to Environment

  • Reproduces

  • Potential to Evolve

Viruses

  • Not Cells

  • Structure: Nucleic Acid + Protein Coat (Capsid)

  • Simpler than Prokaryotic Cells

  • Cannot reproduce/metabolize outside host cells

  • Obligate Intracellular Parasites; limited host range

Viral Evolution

  • Do not fit definition of living organisms

  • Evolved from bits of cellular nucleic acid

Viral Infectious Agents

  • Viroids: Small circular RNA, infect plants

  • Prions: Infectious proteins causing brain diseases in mammals

Viral Genomes

  • Types: Double/Single-stranded DNA or RNA

  • Structure: Linear or Circular, 4-1,000 genes

Bacteriophages

  • Viruses that infect bacteria; complex capsids

  • Two reproductive cycles: Lytic (kills host) and Lysogenic (integrates into host DNA)

Animal Viruses Classification

  • Classified by genome type (RNA/DNA, single/double-stranded)

  • Common characteristics: presence of envelope, RNA genome

Retroviruses

  • Use reverse transcriptase (e.g., HIV)

  • Integrated DNA called Provirus, permanent in host genome

Viral Diseases

  • Can cause cell damage/toxins leading to symptoms

  • Not treatable by antibiotics; prevention via vaccines

  • Antiviral drugs offer treatment, not cures

  • Emerging viruses can cause global epidemics (e.g., pandemics)

Plant Viruses

  • Over 2,000 known types; primarily RNA

  • Spread via horizontal (damage) and vertical transmission (inheritance)

Prokaryotic Cells

  • Two Domains: Bacteria, Archaea

  • Thrive in extreme environments; unicellular most often

  • Characteristics: Small size (0.5-5 µm), various shapes (coccus, bacillus, spirillum)

  • Contain cell walls (bacteria: peptidoglycan; archaea: polysaccharides/proteins)

Classification - Gram Stain

  • Gram-positive: Simple walls, high peptidoglycan

  • Gram-negative: Less peptidoglycan, outer membrane

Antibiotic Resistance

  • Some prokaryotes form capsules and endospores for protection

  • R plasmids confer antibiotic resistance

Prokaryotic Genetic Diversity

  • Reproduction via Binary Fission

  • Genetic recombination through transformation, transduction, conjugation

Metabolism

  • Different nutritional types: Photoautotrophy, Chemoautotrophy, etc.

  • Oxygen-related metabolism: Obligate aerobes, obligate anaerobes, facultative anaerobes

Nitrogen Metabolism

  • Nitrogen fixation: conversion of atmospheric nitrogen to ammonia

Ecological Interactions

  • Symbiosis types: Mutualism, Commensalism, Parasitism

  • Importance of bacteria in human intestines and bioremediation