Minisatellites

Description

Minisatellite analysis, like RFLPs, also involves digestion of genomic DNA with restriction endonucleases, but minisatellites are a conceptually very different class of marker. They consist of chromosomal regions containing tandem repeat units of a 10-50 base motif, flanked by conserved DNA restriction sites. A minisatellite profile consisting of many bands, usually within a 4-20 kb size range, is generated by using common multilocus probes that are able to hybridize to minisatellite sequences in different species. Locus specific probes can be developed by molecular cloning of DNA restriction fragments, subsequent screening with a multilocus minisatellite probe and isolation of specific fragments. Variation in the number of repeat units, due to unequal crossing over or gene conversion, is considered to be the main cause of length polymorphisms. Due to the high mutation rate of minisatellites, the level of polymorphism is substantial, generally resulting in unique multilocus profiles for different individuals within a population. Minisatellite loci are also often referred to as Variable Number of Tandem Repeats (VNTR) loci. The term DNA fingerprinting was introduced for minisatellites, though DNA fingerprinting is now used in a more general way to refer to a DNA-based assay to uniquely identify individuals.

Strengths

The main advantages of minisatellites are their high level of polymorphism and high reproducibility.

Weaknesses

Disadvantages of minisatellites are similar to RFLPs due to the high similarity in methodological procedures. If multilocus probes are used, highly informative profiles are generally observed due to the generation of many informative bands per reaction. In that case, band profiles can not be interpreted in terms of loci and alleles and similar sized fragments may be non-homologous. In addition, the random distribution of minisatellites across the genome has been questioned.

Applications

Minisatellites are particularly useful in studies involving genetic identity, parentage, clonal growth and structure, and identification of varieties and cultivars, and for population-level studies. Minisatellites are of reduced value for taxonomic studies because of hypervariability.

Suggested reading

Hypervariable "minisatellite" regions in human DNA Jeffreys, A.J., Wilson, V. and S.L. Thein (1985). Nature, 314: 67-73.

Individual-specific "fingerprints" of human DNA Jeffreys, A.J., Wilson, V. and S.L. Thein (1985). Nature, 316: 76-79.