Molecular Genetics

Fast reassociating DNA often consists of tandem highly repeated DNA.

Facts

• Since G-C base pairs are more dense that A-T base pairs, the density of DNA increases as the % GC content increases.
• Sheared nuclear DNA of Drosophila melanogaster was analyzed by isopycnic centrifugation in CsCl.
• The distribution of DNA revealed (right) a main band and several satellite peaks.
• DNAs in the satellite peaks have distinctive nucleotide sequences (below).
• Satellite DNAs are usually in heterochromatic regions of the chromosomes, often around centromeres and near telomeres. They are not transcribed into RNA.
• The satellite DNAs belong to the highly repetitive kinetic class of DNA sequences.

In rye molecular chromosomes, members of some highly repeated sequence families separate direct repeats of other repeated sequence families.  In the diagram below, each different pattern represents a different repeat family. The region consists only of highly repeated sequences.	Satellite	Sequence
	I	(AAT)n(AT)m
	II	(AACATAGAAT)n
	III	(378 bp sequence)n
	IV	90% AAGAG and 10% AAGAGAG

Interpretations

• "Satellite" actually refers to any DNA population whose density differs from that of the bulk of the DNA.
• Satellite DNAs consist of short sequences repeated many times, in the same orientation, and without intervening spacer sequences.
• Sequences arranged in this manner are tandem repeats.
• Not all tandemly repeated DNAs are density gradient satellites because some, by chance, have the same density as the bulk DNA. Some density gradient satellites are organelle DNA. Yet, the term "satellite DNA" is usually used as a synonym for tandemly highly repeated DNA.
• Repeated sequences may be interspersed with other repeated sequences.

Further information

• Human DNA has five satellites.
• Four ( I to IV) have short reiterated sequences of less than 100 bp containing divergent GGAAT repeats.
• The alphoid satellites, also found in other primates, are made of 171 bp repeats and are A-T rich.
• Human highly repeated DNA also includes SINES and LINES families, whose members are found interspersed among human globin gene and other loci.   The SINES family consists of a family called the Alu family.  Alu sequences are targets of binding a complex of a cohesin and a chromatin remodeling complex and may thus play a role in the cohesion of sister chromatids (ref)
• In maize, the highly repeated DNA consists of remnants of various families of transposable elements (rev).
• Other kinetically distinguished classes of DNA are the moderately repetitive and the unique DNA.

This is page 112284 of Molecular Genetics by Ulrich Melcher, © 1998-2000, 2002