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"Microsatellites" Home

Background Information

Why do microsatellites exist?

Do they have a function, or are they simply "junk" DNA?  Here are some current ideas:

1.  Microsatellites are "junk" DNA, and the variation is mostly neutral. They usually don't have any measurable effect on phenotype, and when they do mutate, it is generally harmful, not beneficial.  In humans, 90% of known microsatellites are found in noncoding regions of the genome.  When found in human coding regions, microsatellites are known to cause disease. Click here to learn more about how microsatellite expansions can cause disease in humans. Interestingly, when found in coding regions, microsatellites are usually trinucleotide repeats.  One possible explanation is that any other type of nucleotide repeat would be too detrimental to the coding region, because it would cause a frameshift mutation. 

2. Microsatellites provide a necessary source of genetic variation.   In bacteria, variation in microsatellite alleles in coding regions is thought to be adaptive in different environments.  In other words, a short allele may be adaptive in one environment, and a long allele with many repeats may be adaptive in a different environment. Specifically, a short protein filament may make a bacterium less "sticky," and a longer filament may make it more "sticky" and more pathogenic (see Moxon and Wills, 1999).  Therefore, having variation within the population would ensure the survival of the population of bacteria in varying environments.  Click here to see a lesson plan that addresses this possible function.

      Similarly, Kashi and Soller (1999) believe that microsatellite variation may be a way to compensate for loss of genetic variability due to genetic drift and selection.

3. Microsatellites may help regulate gene expression and protein function.  Kashi and Soller (1999) also suggest that microsatellites may have regulatory roles in gene expression.  They are systematically found near coding regions. Variation in microsatellite alleles have been shown to be associated with quantitative variation in protein function and gene activity.  For example, the Drosophila period clock gene per has two major variants, (Thr-Gly)₁₇ and (Thr-Gly)₂₀.  The two variants are distributed along a "highly significant" latitudinal cline and have been shown to be functionally adaptive in their respective latitudes (Kashi and Soller, 1999).

Moxon, E. R., Wills, C. 1999. "DNA microsatellites: Agents of Evolution?" Scientific American. Jan.,  pp. 72-77.

Kashi, Y. and M. Soller. 1999. "Functional Roles of Microsatellites and Minisatellites." In: Microsatellites: Evolution and Applications. Edited by Goldstein and Schlotterer. Oxford University Press.