Why the eukaryotic cell memory was needed.
WHEN REFERRING THIS MATERIAL, PLEASE CITE:
Ploompuu, T. Why the eukaryotic cell memory was needed. Biosüsteemide mälu teooria. Schola biotheoretica XXV. Tartu. Sulemees 1999 P. 51-56. (in Estonian)
(sorry, my English is not correct)
What we know:
* Eukaryotes have several chromosomes, prokaryotes have only one.
* Cell memory (DNA) of eukaryotes is always bigger than cell memory of prokaryotes (Lewin 1997).
* The effective of using of DNA of eukaryotes is lower than the effective of prokaryotes (Gold et al. 1997).
Three “why”-questions, based on aforesaid:
1. Why numerous chromosomes is “better” (better – i.e. this system formed later but it is more widespread) than one, how formed many chromosomes instead of one?
2. Why eukaryotes have more DNA than prokaryotes, why increasing capacity of DNA was impossible for prokaryotes?
3. Why eukaryotes (presumed as more complicated organisms) have more unusable information in DNA (have more noise in DNA)?
The steps in forming of eukaryote cell (grounded by environment, maybe some were parallel):
1. Forming of obligate heterotrophic organisms among (hyperthermophilic) litotrophic Archaea. It was possible only in the most dense life communities in areas of volcanic springs in oceans.
2. Forming of phagocytosis (it is the birth of animal type of feeding). It was possible only in periphery of life – in areas, where feed resources for fungus-type (the same as bacterial) digestion were too insufficient. Phagocytosis needs forming of cytoskeleton also. Forming of phagocytosis is also forming of amoeba-like moving.
3. The first heterotrophic organisms were food specialists (chemical oligophages) => the first phagocytotic organisms were also specialists. In very insufficient food conditions (in periphery of life, far from litotrophic communities) digesting of little part of compounds of swallowed feed was very uneconomic. Finds of feed were very rare and organisms were most of time dead-alive. In such dead-alive conditions was possible, that some genes and also whole chromosome of feed did not digest and it started work in digester’s cell. If the undigested chromosome issued from another type of heterotrophic cell, potential to digest food of new cell was greater. Such dead-alive cells, which did not digest their food correctly, gathered very variable complex of chromosomes. Maybe by that way have connected into eukaryotic cell both types of genes, genes of Archaea and genes of Eubacteria. Fore effective dividing of new multichromosomal cells started evolution of mitosis.
All this must have been fixed before developing of first Cyanobacteria (H2O-photosyntesis). The first multichromosomal monsters could survive only at very low bioproduction, where competition did not exist really (in very sparse life was the most important problem to survive period between swallowings).
4. Conjugation of gametes has obviously the same primary source – undigesting of another cell, cell of the same species. It released more effective recombination (and by this way potential enlarging menu of digestible compounds) of genome information than undigesting of unknown chromosome.
Multichromosomal cells have formed obviously before forming of nuclear membrane – in mitosis new complex of chromosomes forms without membrane.
5. Nuclear membrane started to form, when the most of “good” genes were gathered into protoeukaryotic cell – the opportunity get some useful information by a blind introduction of new alien chromosomes was not enough big yet for risk of spoiling of existing genome.
Protoeukaryotes were susceptible to UV – they formed in great area of deep dark oceans (aphotic zone) – in the most of water-room of world. It limited later their spreading to photosynthetic communities. The only UV resistant organisms were Eubacteria – the first (and only) photosynthetic organisms. Cyanobacteria were obviously UV-resistant (Walter 1994).
6. Endosymbiosis of pro-mitochondrion occurred after fixing structures of nuclear membrane. Probably It occurred more than 2,2 billion years ago (Tolk 1998). Then was not enough O2 for aerobic life in atmosphere (Holland 1994) – maybe mitochondrion formed in stromatolites in conditions of local high O2 concentration (Ploompuu 1998).
Answers to questions:
1. Holding of numerous chromosomes is better, because in this was only way to gather “good” genes of different bacteria to one unit. It formed in process of symbiosis (most of symbiosis formed by undigesting of food – lichens, mitochondrion, plastids, domestic animals).
2. Gathering of DNA by eukaryotic way was the most rapid opportunity to form multifunctional cell. Prokaryotes as monofunctional organisms can’t develop to econiche, which have occupied eukaryotes already.
3. Low effective of eukaryotes DNA (much noise) is relic of ages of completing of eukaryotic cell. In their later evolution was economy of nucleotide using not important already, as the most important eliminating force fixed friend (symbiont) or enemy (demolisher).