MoBio Tandem Repeats Chapter 3

Tandem repeats are an array of consecutive repeats. They include three subclasses: satellites, minisatellites and microsatellites. The name "satellites" comes from their optical spectra.

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Figure 3-G-1. Illustration of satellite bands. By using buoyant density gradient centrifugation, DNA fragments with significantly different base compositions may be separated, and then monitored by the absorption spectra of ultraviolet light. The main band represents the bulk DNA, and the "satellite" bands originate from tandem repeats.

Satellites

The size of a satellite DNA ranges from 100 kb to over 1 Mb. In humans, a well known example is the alphoid DNA located at the centromere of all chromosomes. Its repeat unit is 171 bp and the repetitive region accounts for 3-5% of the DNA in each chromosome. Other satellites have a shorter repeat unit. Most satellites in humans or in other organisms are located at the centromere.

Minisatellites

The size of a minisatellite ranges from 1 kb to 20 kb. One type of minisatellites is called variable number of tandem repeats (VNTR). Its repeat unit ranges from 9 bp to 80 bp. They are located in non-coding regions. The number of repeats for a given minisatellite may differ between individuals. This feature is the basis of DNA fingerprinting.

Another type of minisatellites is the telomere. In a human germ cell, the size of a telomere is about 15 kb. In an aging somatic cell, the telomere is shorter. The telomere contains tandemly repeated sequence GGGTTA.

Microsatellites

Microsatellites are also known as short tandem repeats (STR), because a repeat unit consists of only 1 to 6 bp and the whole repetitive region spans less than 150 bp. Similar to minisatellites, the number of repeats for a given microsatellite may differ between individuals. Therefore, microsatellites can also be used for DNA fingerprinting. In addition, both microsatellite and minisatellite patterns can provide information about paternity. The most famous case is

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The term "polymorphism" describes the existence of different forms within a population, e.g., difference in the number of tandem repeats. All tandem repeat polymorphisms could result from DNA recombination during meiosis (Chapter 8 Section D). The microsatellite polymorphism could also be caused by replication slippage.