Nucleosomes are the basic structures of chromatin and constitute a general repressor in Eukaryote due to the compaction of DNA which limits its accessibility to DNA-binding factors. The first level of compaction consists in wrapping the DNA 147 bp long fragments around a histone octamer, which makes this DNA less accessible to the DNA binding factors than the linker DNA. The unwrapped linker DNA is 2050 bp long. Nucleosomes further condense by linker histones H1 to form a 30 nm fiber. Differential compaction of the interphase chromatin is important for proper functioning of the genome.

Our understanding of the evolution of DNA content in angiosperm plants has improved in recent years. Genomes in angiosperms vary in DNA content over 1000-fold. Variations in plant genome size are primarily due to differences in ancestral polyploidy and the periodic burst in activity of retroele- ments. Genomic data show a strong correlation between the size of genome and the amount of retroele- ments. Gene duplication have contributed to the functional divergence of the duplicates, and have become a strong evolutionary force.