Multigene Families in Eukaryotes
In many eukaryotic genomes, the nuclear DNA generally consists of (i) a high proportion in the form of repeated DNA, whose presence can not be easily explained, except for a small fraction that constitutes genes like rRNA genes and tRNA genes and (ii) a very small fraction as unique DNA (sometimes less than 1%), that accounts for structural genes mainly meant for proteins. The presence of lengthy intron sequences in split genes (see Organization of Genetic Material 3. Split Genes, Overlapping Genes and Pseudogenes), accounts for upto as much as ten times the sequences (exons) coding for proteins, but this is still far short of the excess DNA that is found in eukaryotic genome.