Satellite DNA

Satellite DNA consists of highly repetitive DNA, and is so called because repetitions of a short DNA sequence tend to produce a different frequency of the nucleotides adenine, cytosine, guanine and thymine, and thus have a different density from bulk DNA - such that they form a second or 'satellite' band when genomic DNA is separated on a density gradient.

Types of satellite DNA

Satellite DNA, together with Minisatellite and Microsatellite DNA constitute the Tandem repeats.

Some types of satellite DNA are:

Type	Size of repeat unit (bp)	Location
α (alphoid DNA)	171	All chromosomes
β	68	Centromeres of chromosomes 1, 9, 13, 14, 15, 21, 22 and Y
Satellite 1	25-48	Centromeres and other regions in heterochromatin of most chromosomes
Satellite 2	5	Most chromosomes
Satellite 3	5	Most chromosomes

Length

A repeated pattern can be between 1 base pair long (a mononucleotide repeat) to several thousand base pairs long, and the total size of a satellite DNA block can be several megabases without interruption. Most satellite DNA is localized to the telomeric or the centromeric region of the chromosome. The nucleotide sequence of the repeats is fairly well conserved across a species. However, variation in the length of the repeat is common. For example, minisatellite DNA is a short region (1-5kb) of 20-50 repeats. The difference in length of the minisatellites is the basis for DNA fingerprinting.

Origin

Satellite DNA, at least the microsatellite variety, is thought to have originated by slippage of a replicated chromosome against its template.

Pathology

Microsatellites are often found in transcription units. Often the base pair repetition will disrupt proper protein synthesis, leading to diseases such as myotonic dystrophy.

http://en.wikipedia.org/wiki/Satellite_DNA