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Of the information stored in DNA molecules, what most interests us are the genes coding for proteins. In eukaryotes, an average of only 2% of the genome codes for proteins, whereas the rest is mainly repetitive sequences and introns (non-coding sequences that interrupt the coding sequences of genes). The genomes of prokaryotes and virus are, however, much more compact and they are essentially organized in genes that code for proteins. For our purpose it is enough to have in mind the simplest genomes, since they are already extremely complex life machines.