Satellite DNA

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Satellite DNA consists of highly repetitive DNA, and is so called because repetitive DNA sequences tend to have a relatively high frequency of the nucleotides adenine and thymine, and thus have lower density - such that they form a second 'satellite' band when genomic DNA is separated along a density gradient.

There are several types of satellite DNA:

A: Simple sequence repeats (SSRs, also called short tandem repeats, or STRs): SSRs are dispersed throughout the genome, and form the basis for modern tests that verify paternity, identity etc. These are what the DNA lab on CSI: Crime Scene Investigation (the popular TV show) would use to identify or exclude a potential killer; unfortunately, in real life SSR analysis takes somewhat longer than on TV.

1. Minisatellite - base sequences of approximately 15 to 50 base pairs, repeated in tandem up to thousands of times. There are approximately 30 000 minisatellite loci in the human genome.
2. Microsatellite - base sequences of less than 15 base pairs, usually repeated anywhere from 10 to 100 times without interruption. There are approximately 200 000 microsatelite loci in the human genome.

B: Tandem repeat: These sequences are localized at telomeres (the ends of chromosomes) and centromeres (where kinetochores form), and may play a functional role in these regions (such as protein binding sites).

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.

Satellite DNA, at least the microsatellite variety, is thought to have originated by slippage of a replicated chromosome against its template. Microsatellites are often found in transcription units. Often the base pair repetition will disrupt proper protein synthesis, leading to diseases such as myotonic dystrophy.