A microsatellite is a short, repetitive sequence of DNA. Since they tend to vary little between closely related organisms, microsatellites are often used by scientists as genetic markers to identify individuals that come from the same breeding population. They are also known as short tandem repeats (STRs) and simple sequence repeats (SSRs).
If one thinks of a molecule of DNA as resembling a ladder, then each rung in the ladder is made up of a pair of smaller molecules called nucleotides. The four nucleotides that appear in DNA are adenine (A), guanine (G), thymine (T) and cytosine (C). Adenine pairs with thymine, and guanine pairs with cytosine. The order in which these base pairs appear gives a strand of DNA its unique signature and constitutes a code that stores genetic information.
A microsatellite occurs when a short sequence of base pairs, usually between 1 and 6, repeats several times in a row. The following diagram of a short strand of DNA shows a single microsatellite composed of the unit GTC on the top half and CAG on the bottom half, each repeated 4 times. Scientists would represent this as (GTC)4 or (CAG)4:
G T C G T C G T C G T C
| | | | | | | | | | | |
C A G C A G C A G C A G
These groups of repeating sequences were named "microsatellites" because, when DNA is separated by spinning it in a centrifuge, it tends to group into a large main band surrounded by smaller, "satellite" bands. Researchers named the DNA they found in these bands minisatellites and microsatellites. Minisatellites are longer segments, which may consist of up to about 100 repeating base pairs.
Microsatellite markers are often useful for identifying individuals from the same breeding population. Rarely, mutations occur when a genetic sequence is passed from a parent to child, resulting in more or fewer units of the repeating segment. Thus, in our example above, (CAG)4 could become (CAG)3 or (CAG)5. These mutations occur often enough that a wild breeding population is likely to have different microsatellites than other breeding groups, but they occur infrequently enough that individuals within a single breeding group are likely to share certain characteristic sequences.
Most microsatellites are found in non-coding DNA — DNA which does not have the "code" or instructions to manufacture protein. Consequently, they are not believed to play a significant role in cell functioning. There is reason to believe, however, that a microsatellite can disrupt normal cell processes if it grows too large. For instance, in the case of Huntington's disease, the number of repeats of a certain sequence can mean the difference between being affected by the disease, or being a non-affected carrier.