Satellite DNA

When fractionated human DNA is centrifuged
in a cesium chloride density gradient, the main
portion of DNA (1) forms a band at buoyant
density 1.701 g · cm"3. Three additional bands
(satellites) appear at 1.687, 1.693, and 1.697 g ·
cm"3, respectively. These are less dense because
their CG content differs from that of the main
DNA. One distinguishes classic satellite DNA (2)
made up of repeats of 100–6500 bp, minisatellites
(3) of 10–20 bp repeats, and microsatellites
(4) of 2–5 bp repeats. AT-rich and CG-rich segments
can be recognized. Microsatellites are the
most frequent form of repetitive DNA. Their
general structure is (CA)n where n equals about
2–10. The human genome contains 50000–
100000 polymorphic (CA)n blocks.