To express that the eukaryotes have large genomes, one must certanly be comparing them to organisms with much smaller genomes, the prokaryotes. It is the analysis between these organisms that may give conclusive evidence why the eukaryotes have large genomes.
Immediately one jumps to the proven fact that the eukaryotes are more complex and therefore they have to have a bigger genome, where would inevitably produce more genes. 基因组 This really is of course true, nevertheless the complexity of the genome doesn’t directly correspond to a growth in genome size. Wheat features a far larger genome than ours however, one would hesitate to call them more complex. Therefore, there must be more subtle underlying reasoned explanations why the eukaryotes have such large genomes.
Transposable elements add to the size of genomes by copying and inserting into different regions of the genome. However, transposable elements also occur in the prokaryotes and the affect of the transposons on eukaryotes in increasing how big the genome might be negligible. One reasons why the affect of transposons isn’t always detrimental to the organism is as a result of introns within a eukaryote. The gene rich DNA of the prokaryotes is in sharp contrast to genes scattered around eukaryotic genomes. The introns in the eukaryotic genome cause them to become much, much larger than in the prokaryotes. The gene number difference in eukaryotes is all about 10 fold whereas the base pair number could be 1000 fold bigger. The question therefore lies within the introns of the eukaryotes. Why are there so many introns in the eukaryotic genome; giving rise to its ‘large size’?
Since the eukaryotes evolved from the prokaryotes one must elucidate the purpose of the introns in the eukaryotes as, say a stream (against transposons, mutation etc). However, this may be construed to have the same equally beneficial properties in prokaryotes. A better approach is always to analyse the power output. Prokaryotes use 25% of their energy production in DNA copying and maintenance (the other 75% being that of protein production). Therefore, any upsurge in size of the DNA in prokaryotes would be too costly for the organisms to keep up and replicate and thus the DNA is stripped right down to a very gene rich molecule. Eukaryotes are profligate in pouring energy within their genomes (due for their greater energy production capacity as an immediate consequence of getting specific organelles devoted to this purpose) and consequently they can afford to be so disgracefully inefficient and allow accumulation of introns.
Another and probably the main consequence of to be able to pour large amounts of energy into the genome is that eukaryotes can allow gene duplication events, which produce proteins that differ very slightly. Not only this, however in plants especially, entire duplication of genomes and addition of different divergent genomes are commonplace within that kingdom. Giving these organisms such huge genomes.