Prokaryotes

• There is compelling evidence that mitochondria and chloroplasts were once independent prokaryotic microbes
• Mitochondria and chloroplasts have their own DNA and replicate independently of the cell nucleus
• Mitochondria and chloroplasts were likely ingested by primitive host cells and became specialized organelles
• Mitochondrial DNA has a unique pattern of inheritance and provides clues about evolutionary relationships
• Bacteria are found in almost all habitats on Earth, from hot springs to deep ocean vents, and make up a large portion of the Earth's biomass
• Major groups of bacteria

  • Proteobacteria
  • Chlamydias (Gram-negative)
  • Spirochetes
  • Cyanobacteria
  • Gram-positive bacteria
• Proteobacteria


  • Free living and symbionts, some are pathogenic
• Chlamydias (Gram-negative)


  • Obligate intracellular parasites, cell wall with low peptidoglycan
• Spirochetes


  • Spiral shaped, free living, anaerobes, pathogenic, flagella present
• Cyanobacteria


  • Known as blue-green algae, chlorophyll present, present in aquatic environments
• Gram-positive bacteria


  • Pathogenic and decomposers, thick cell wall and without outer membrane
• Proteobacteria


  • Helicobacter pylori, Salmonella
• Gram-positive bacteria


  • Clostridium botulinum, Bacillus anthracis
• In recent past all living organisms are grouped into five kingdoms and Prokaryotes were placed into the kingdom Monera (Prokaryotae) but now the status of classification has been changed in late 90's
• The pioneer work had been conducted by Carl Woese (1990) and his colleagues proposed domain system of classification based upon the differences of subcellular structure ribosomes and cell membranes
• Domains


  • Archaea
  • Bacteria
  • Eukarya
• Two out of three domains' Eubacteria and Archaebacteria are prokaryotes
• The domain Eubacteria comprises all bacterial organisms and recently given status as kingdom bacteria, the domain Archaea comprises the rest of the prokaryotes, and the domain Eukarya includes all eukaryotes, including organisms in the kingdoms Protista, Fungi (Mycota), Plantae and Animalia
• Classification of organisms given by different scientist in different era

  • Linnaeus (1758)
  • Haeckel (1866)
  • Chatton (1925)
  • Copeland (1938)
  • Whittaker (1969)
  • Woese et al. (1990)
• Prokaryotes are the most primitive and first inhabitant on earth
• Prokaryotes may have evolved from protobionts ancestors
• Protobiont


  Enclosure of self-replicating RNA in a phospholipid membrane. The first cell is thought to have arisen by the enclosure of self-replicating RNA and associated molecules in a membrane composed of phospholipids
• The domain Bacteria of the Prokaryote also include cyanobacteria or blue-green algae
• Cyanobacteria


  Primitive prokaryotic alga that can perform photosynthesis and are called photoautotrophs
• Bacteria also exist as decomposers or saprotroptus and symbionts
• Prokaryotes have tremendous ability to adapt according to the changing environment
• Escherichia coli contains approximately 5,000 genes and on average, about one in every 200 bacteria is likely to have a mutation in at least one of the genes
• Archaea


  A very diverse group in a domain system of classification, initially classified as Archaebacteria but this classification is obsolete now they are a separate domain
• Archaea


  • They don't have peptidoglycan in their cell wall as it is present in bacteria and also some metabolic activities are different from bacteria
  • They reproduce asexual by binary fission, fragmentation or budding
• Archaea


  • Methanococcus
  • Halobacterium
• Archaea do not live as pathogens or parasites instead they develop useful associations with others as mutualists or commensals
• Comparison of domain system classification

  • Archaea
  • Bacteria
  • Eukarya
• Earth is formed around 4.5 billion years ago and the earliest undisputed evidence of life on earth dates at least from 3.5 billion years ago
• Mitochondria and chloroplasts were once primitive bacterial cells and over millions of years of evolution, they have become more specialized and today they cannot live outside the cell
• Mitochondrial DNA (mtDNA) has a unique pattern of inheritance and it accumulates changes much more slowly than other types of DNA
• Bacteria can be found in almost all the habitat on earth and the earth is estimated to hold at least 5 nonillion bacteria, and much of the earth's biomass is thought to be made up of bacteria
• Types of bacteria on the basis of mode of respiration

  • Aerobic bacteria
  • Anaerobic bacteria
• Major groups of bacteria

  • Proteobacteria
  • Chlamydias (Gram-negative)
  • Spirochetes
  • Cyanobacteria
  • Gram-positive bacteria
• Cyanobacteria


  • They have remarkable ability to survive in extremely high and low temperatures
  • They can survive in low CO2 environment and have RuBISCO enzyme that help to convert CO2 into sugars
  • Filamentous cyanobacteria can fix elemental nitrogen in their specialized cells
• 6.3.3. Cyanobacteria as photosynthetic bacteria