"Microsatellites" are defined as loci (or regions within DNA sequences) where short sequences of DNA (nucleotides; adenine - A, thiamine - T, guanine - G, cytosine - C) are repeated in tandem arrays. This means that the sequences are repeated one right after the other. The lengths of sequences used most often are di-, tri-, or tetra-nucleotides. An example of a locus containing mixed lengths of repeats may be viewed by clicking here. This example contains a stretch of di-nucleotides (AC) that changes into a tri-nucleotide (CAA) repeat.
What makes microsatellites useful is the fact that at the same location within the genomic DNA the number of times the sequence (ex. AC) is repeated often varies between individuals, within populations, and/or between species. So one population may commonly have 13 AC's repeated in a row while another population has 18 AC's repeated at the same location within the genomic DNA. Different regions of the DNA contain sequences that mutate at various rates. Some regions have a high rate of mutation while others have a low rate of change. In areas of the genome with high rates of mutation there is a wider range in the number of repeats found within individuals of a population (some individuals have 10 repeats others 11, 13, . . .). Each sequence with a specific number of repeated nucleotides is designated as an allele. So, a locus (a specific region within the genomic DNA) with 8 repeats is one allele and within another individual the same locus that contains 9 repeats is another allele. Often the number of repeats and the type of repeat is designated in a formula. An example would be: (AC)14, where AC refers to the specific nucleotides that are repeated and the subscript (14) refers to the number of times that the sequence is repeated.
Microsatellites with different levels of variability are useful for different types of studies. Microsatellites with few variations (and subsequently a low mutation frequency) are useful for analysis of related species, while those with a few more variations are useful in identifying populations within a single species. If variability is high and there are a large number of alleles within a population a comparison of populations is difficult. The alleles shared between (found in both) the populations will be very high so identification of a single individual as belonging to either population becomes uncertain. Unless an allele is found in only one of the populations it does not provide any discriminatory information. It is not possible to determine which population an individual is from if the allele(s) present in the individual are found in both populations. The same is true for comparisons of species. If the alleles are shared between species the provide no information enabling segregation of individuals based upon the alleles found within their genomic DNA. On the other hand, when there are a large number of alleles within a population it may provide a lot of information enabling a distinction between specific individuals. When a large number of loci (plural of locus) are used a pattern specific to each individual may be identified. For example, individual #1 may have allele 2 at locus A, allele 5 at locus B, allele 1 at locus C, and allele 7 at locus D, while individual #2 may have allele 1 at locus A, allele 5 at locus B, allele 6 at locus C, and allele 5 at locus D. NOTE - each locus has its own set of alleles. So while both individuals #1 and #2 share allele 5 at locus B all of the other loci contain alleles that differentiate between the two individuals. Specific patterns that identify individuals are often referred to as fingerprints. Fingerprints are used in forensic and paternity/parentage work. When enough informative loci are used (i.e. loci that have a lot of variation and a lot of different alleles) each individual within a population may be identified.
In this research (with marine iguanas) a high degree of variation in number of repeats within microsatellites (lots of alleles found for each locus studied) will allow determination of paternity/parentage within a single population. Similarities and differences between the fingerprints of individuals within the same population will allow paternity/parentage to be determined. Individuals that are most closely related will have the most similar alleles at each, of the many examined, loci. Thus, similarity of fingerprints will provide the necessary information.
For descriptions of the various types of mutations that may occur within the genome [click here].
Here are two animated examples of the electrophoresis process (animated
GIF) where pieces of DNA are "run" on a gel in order to separate and
identify pieces of different lengths (Flash
animation), or for a simple descriptive diagram of gel attributes [click
here]. Or for a personal explanation and pictures visit this
page (electrophoresis pictures).
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