Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA: Structure of chromosomes and genes
Eukaryotic DNAs contain highly repetitive base sequences

Prokaryotes typically contain only a single copy of DNA per Cell, and in almost all cases, each DNA molecule harbors only one copy of any given Gene. Aside from regulatory and signaling sequences, prokaryotic DNA contains very few silent, i.e., non-translated, regions. Moreover, a strict collinearity exists between each gene and the Amino Acid Sequence (or RNA sequence) encoded by that gene (Fig. 27-27).

The Organization of genes in eukaryotic DNA is far more complex, both structurally and functionally. In mouse DNA, for instance, fragments present in multiple copies have been discovered. When researchers attempted to determine how widespread such fragments are, the results were surprising. It turned out that ~ 10% of mouse DNA consists of short sequences (less than 10 Base Pairs) repeated millions of times. These are the so-called highly repetitive sequences. Another 20% of mouse DNA comprises segments repeated at least 1,000 times, designated as moderately repetitive sequences. The remaining ~ 70% of DNA consists of unique, i.e., non-repetitive sequences, as well as sequences repeated only a few times. The most highly repetitive sequences are referred to as satellite DNA and are generally considered to represent non-translated regions.

Similar experiments have been conducted on many other eukaryotic organisms. It now appears that all eukaryotic Chromosomes contain repetitive DNA sequences (repeats), whereas these are typically absent in prokaryotes. The proportion of highly and moderately repetitive sequences varies among different eukaryotic species.