In previous three topics, we discussed, structure, synthesis and packaging of DNA in prokaryotes and eukaryotes. During the last three decades, extensive survey has also been conducted for the DNA contents per genome in prokaryotes and eukaryotes. Large variation in this DNA content has been witnessed particularly in eukaryotes, where the quantity of DNA per cell is enormous, when compared with that in prokaryotes. Large proportion of this DNA in most eukaryotes has been shown to be present in the form of multiple copies of identical DNA sequences, this multiplicity ranging from a few copies to millions of copies within the same cell. These DNA sequences are together described as repetitive DNA. The remaining DNA in the cell is found in the form of single copy DNA sequences and is therefore described as unique DNA. While the function of repetitive DNA is largely unknown, the unique DNA contains most of the genetic information in the form of structural genes controlling phenotypic traits directly or indirectly. The repetitive DNA may be clustered at one place or interspersed with unique sequences, each repeat unit ranging from a few hundred to few thousand base pairs. These aspects of organization of DNA in the form of repetitive and unique sequences will be discussed in this section.