The Origin of Eukaryotes

The Origins of Genome Architecture
Micheal Lynch

History - 4.6 BYs ago

1. First origin of life is prior to 3.8 BYs
2. RNA is primitive?
   1. DNA, though can self replicate, it is catalytically inert
   2. RNA is active
   3. RNA is proved to be selective of for variety of catalytic activities in various experiments
   4. As RNA can not replicate, primitive life forms existed as groups, involved in co-operative activities
   5. Cellular forms of life came into existence just few 100 MYs
   6. Prokaryotes - 0.5 - 1 BY
   7. Photosynthesis - 2.8 BY
   8. First algae - 2.1 BY
   9. Multicellular organisms - 0.5 BY - Cambrian Explosion
   10. Jawed Vertibrates and land plants - 0.4 BY
   11. Angiosperm - 0.14 BY
   12. Mammals - 0.1 BY
   13. Homo sepians - 0.002 BY

DNA world

1. Initial DNA had U instead of T as evident from many viruses
2. Why RNA based genomes were eradicated in the history?
   1. Probably DNA is more resistant to mutational vulnerability and stable relatively
3. Viral Origin of DNA
   1. Most initial Virus had DNA based genome
   2. Being the DNA based genome, virus can easily get transferred into host RNA genomes
   3. They don't need to establish the additional defensive mechanisms as DNA itself was more tolerant to mutations
4. Membranes - Early or Late?
   1. to the DNA world is not clear. But Koonin (That old man in SMBE) claims that cell membranes evolved independently subsequent to the establishment of DNA world

3 domains of cellular life

1. Archea, Eubacteria, and Eukaryotes
   1. as evident form 16s subunit of rRNA (still rooting was a problem)
   2. duplicated genes can be used to root the tree
2. ring of life
   1. Members of Archea and eubac. fused to form the eukaryotic domain

Enkaryotes

1. Almost all eukaryotic genomes contains one or more minor genome aquired from Archea or eubacteria (mitochon., chloroplast)
2. It is also clear that ancestral eukaryote could have had significant cell structure prior to origin of organelles
3. Eukaryotes differs in
   1. genome size expansion
   2. chromosome linearity
   3. trans-splicing - operon
   4. exons-introns structure - splicing
   5. nuclear membrane
4. Morphologically eukaryotes are classified as Unikonts and Bikonts
5. BigBang
   1. Major lineages occurred between 10-100 MY, but there are discrepency
   2. varying information from differenty type of moleculr scale (protein coding sequence, fossil, etc)

Eukaryotic genome repaterning

1. 1. subac and archea - 100,00 species
   2. eukaryotes - 1,500,000 species
   3. New species
      1. geopgraphical isolation
      2. varying environments
      3. Intergenomic transfer of genes
      4. Reproductive incompatibility - facilitates

Summary

1. RNA is first
2. Protein synthesis started (RNA for information storage and protein for catalytic activity)
3. Proteins assembled the DNA precursors
4. Biotic pathway of construction of cell membrane appeared -> Cellular forms of life -> metabolic and mophological diversification

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