The skeletal function of non-genic nuclear DNA: new evidence from ancient cell chimaeras

摘要

DNA can be divided functionally into three categories: (1) genes, which code for proteins or specify non-messenger RNAs; (2) semons, short specific sequences involved in the replication, segregation, recombination or specific attachments of chromosomes, or chromosome regions (e.g. loops or domains) or selfish genetic elements; (3) secondary DNA which does not function by means of specific sequences. Probably more than 90percent of DNA in the biosphere is secondary DNA in the nuclei of plants and phytoplankton. The amount of genic DNA is related t the complexity of the organism, whereas the amount of secondary DNA increases proportionally with cell volume, not with complexity. This correlation is most simply explained by the skeletal DNA hypothesis, according to which nuclear DNA functions as the basic framework for the assembly of the nucleus and the total genomic DNA content functions (together with relatively invariant folding rules) in determining nuclear volumes. Balanced growth during the cell cycle requires that the cytonuclear ratio is basically constant irrespective of cell volume; thus nuclear volumes, and therefore the overall genome size, has to be evolutionarily adjusted to changing cell volumes for optimal function. Since bacteria, mitochondria, chloroplasts and viruses have no nuclear envelope, the skeletal DNA hypothesis simply explains why secondary DNA is essentially absent from them but present in large cell nuclei. Hitherto it has been difficult to refute the alternative hypothesis that nuclear secondary DNA accumulates merely by mutation pressure (whether "junk" or selfish DNA), and that selection for economy is not strong enough to eliminate it, whereas accumulation in mitochondria and plastids is prevented by intracellular replicative competition between their multiple genomes.