28 bio

• Eukaryotes consist of Plant, Animal, Fungus kingdoms, and Protists which refer to all other eukaryotes
• Eukaryotes arose from Archaea or Archaea-like ancestors in the early Proterozoic, 2.5 to 2 bya
• Key features of eukaryotes include:

  • DNA in LINEAR chromosomes
  • Histone proteins inherited from a common ancestor with Archaea, allowing control of gene expression
  • Nucleus with a double membrane surrounding genetic material
  • Endomembrane system giving rise to organelles such as ER, Golgi, lysosomes, and peroxisomes
  • Well-developed cytoskeleton permitting complex cell shapes
  • Mitochondria
  • Ribosomes 80 S
• Proposed origin of endomembrane system by infolding of plasma membrane
• Ancient α-proteobacteria evolved a way to use O2 for oxidative respiration, leading to aerobic respiration
• Eukaryotes acquired the ability to carry out oxidative respiration through endosymbiosis with α-proteobacteria
• Mitochondria evolved from endosymbiotic α-proteobacteria within eukaryotic cells
• Evidence that mitochondria evolved from symbiotic bacteria:

  • Circular DNA with gene sequences similar to bacteria
  • Bacterial-type (70 S) ribosomes
  • 2 membranes similar to prokaryotes
  • Division through binary fission
• All known eukaryote lineages inherited mitochondria from endosymbiotic α-proteobacterium through horizontal gene transfer
• Origin of key features in eukaryotes:

  • DNA in LINEAR chromosomes
  • Histone proteins inherited from a common ancestor with Archaea
  • Nucleus with a double membrane
  • Endomembrane system giving rise to organelles
  • Well-developed cytoskeleton
  • Ribosomes 80 S
  • Evolution of mitochondria
  • Evolution of primary plastids (chloroplasts)
• All eukaryote lineages have oxygen-generating photosynthesis occurring in plastids (chloroplasts)
• Origin of photosynthesis in eukaryotes:

  • Light-harvesting reactions on thylakoid membranes inside plastids
  • Fixation of CO2 into glucose in the surrounding stroma
• Origin of the first chloroplast involved an amoeboid eukaryote ingesting a cyanobacterium that persisted as an endosymbiont
• Contemporary examples of eukaryotes hosting cyanobacterial endosymbionts include Fungus Geosiphon and sponges
• Origin of the first chloroplast:

  • Amoeboid eukaryote evolves increasing interdependence with endosymbiont over time
  • Most genes passed to host nucleus
  • Encoded proteins acquire signal sequence enabling transport back into symbiont
  • Endosymbiont eventually becomes an integral part of the host cell
• Primary plastids (chloroplast) evolved from endosymbiotic cyanobacterium
• Primary plastids of single origin occur in Archaeplastida:

  • Glaucophyte algae
  • Red algae
  • Green algae & land plants
  • Independent origin of primary plastid in Rhizarian Paulinella recently demonstrated
• Evidence that primary plastids originated from formerly independent cyanobacteria:

  • Plastids have their own DNA, circular chromosome, gene sequences similar to cyanobacteria
  • Have their own ribosomes (70S type, like bacteria)
  • Both membranes biochemically similar to cyanobacterial membranes
  • Divide by a process resembling bacterial binary fission
• Glaucohytes (Archaeplastida):

  • Few species of microscopic freshwater algae
  • Plastid pigments: chlorophyll a, phycobilins
  • Cellulose cell wall
  • Plastids surrounded by thin peptidoglycan layer between the two membranes
• Secondary plastids:

  • Evolved from an endosymbiotic, eukaryotic alga (green or red) with a primary plastid
  • Typically have three or four surrounding membranes
• Secondary plastids derived from red algal endosymbiont:

  • Stramenopile, Alveolate, Haptista & Cryptista plastids
  • Euglenids and Chlorarachniophytes have plastids derived independently from green algal endosymbionts
• In Chlorarachniophytes and Cryptophytes:

  • Plastid includes a remnant of the original endosymbiont nucleus called a nucleomorph
  • Nucleomorph has eukaryote DNA, surrounded by double membrane with pores
• Contemporary cyanobacterium most closely related to source of all these plastids: freshwater Gloeomargarita lithophora
• Eukaryote cytoskeleton provides finer control over cell shape compared to prokaryotes:

  • Microfilaments (actin & myosin)
  • Microtubules (tubulin)
  • Intermediate filaments
• Functional significance of cell and body shape:

  • Organism uses resources and produces waste in proportion to its volume
  • Diffusion of food, oxygen, and waste products into and out of cells depends on surface area
  • Larger surface area to volume ratio means more efficient cell/organism
• Maximizing surface area per volume:

  • Flattened and filamentous body forms maximize surface area per volume
• Slime mold plasmodium:

  • 2-D gigantic cell moves over surfaces feeding on bacteria
• Fungi:

  • 1-D filamentous growth within food resource maximizes surface in contact with food