232 Biochimica et Biophysica Acta, 608 (1980) 232--242 © Elsevier/North-Holland Biomedical Press BBA 99686 NON-SATELLITE REPETITIVE HUMAN DNA FAMILIES SEQUENCE PROPERTIES AND EVIDENCE FOR OCCURRENCE IN CHIMPANZEE DNA KENNETH A. MARX * Institute of Animal Genetics, University of Edinburgh, Edinburgh (U.K.) (Received October 18th, 1979) Key words: Repetitive DNA sequence; DNA-cRNA hybridization; Kinetics; Thermal stability; CsCl equilibrium centrifugation; (Human DNA, Chimpanzee DNA) Summary Repetitive human DNA, fractionatedon CsCI gradientsfollowing hydroxy- apatite isolation,contains two complex DNA fractions,the 1.703 and 1.714 DNA families (Marx, K.A., Allen, J.R. and Hearst, J.E. (1976) Biochim. Bio- phys. Acta 425, 129--147). Biphasic Topt profiles, obtained in DNA excess hybridizations with cRNA tracers from each DNA family, have been shown to be the likely result of a fast kinetic component hybridizing at higher tempera- tures (67°C peak) and this fast plus a slow kinetic component both hybridizing at lower temperatures (47°C peak). Equilibrium CsC1 gradient DNA-cRNA hybrid distributions support previous interpretations of the sequence composi- tion of both DNA families. That is, the fast component is a relatively undiverged repetitive sequence of recent origin, while the slow component is a highly diverged, less thermally stabile, old primate sequence. This interpreta- tion is further strengthened by cRNA tracer hybridization experiments in chimpanzee DNA excess where the fast component appears to be absent and the slow component present. Introduction Repetitious DNA sequences have been shown to comprise a largefractionof the human genome [1--6]. And at leasthalfof the human genome isorganized into a grosspattern of short repetitousDNA sequences interspersedwith longer singlecopy sequences [ 4--6]. * Present address: Department of Chemistry, Dartmouth College, Hanover, NH 03755, U.S.A.