Microsatellites are short, repetitive sequences of DNA used as genetic markers to identify individuals from the same breeding population. They occur when a short sequence of base pairs repeats several times in a row and are often found in non-coding DNA. Mutations can occur, resulting in more or fewer units of the repeating segment. Microsatellites can disrupt normal cellular processes if they get too large.
A microsatellite is a short, repetitive sequence of DNA. Because 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 (STR) and simple sequence repeats (SSR).
If you think of a DNA molecule as a ladder, then each rung on 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 DNA strand its unique signature and forms 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 DNA strand shows a single microsatellite composed of the unit GTC in the upper half and CAG in the lower half, each repeating 4 times. Scientists would represent it as (GTC)4 or (CAG)4:
GTCGTCGTCGTC
| | | | | | | | | | | |
CAGCAGCAGCAG
These clusters of repeating sequences have been called “microsatellites” because, when DNA is separated by spinning it in a centrifuge, it tends to cluster into a large main band surrounded by smaller “satellite” bands. The researchers called the DNA they found in these bands minisatellites and microsatellites. Minisatellites are longer segments, which can 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 a child, resulting in more or fewer units of the repeating segment. So 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 occur infrequently enough that individuals within a single breeding group share certain characteristic sequences.
Most microsatellites are found in non-coding DNA – DNA that doesn’t have the “code” or instructions for making proteins. Consequently, they are not thought to play a significant role in cellular functioning. There is reason to believe, however, that a microsatellite can disrupt normal cellular processes if it gets too large. For example, in the case of Huntington’s disease, the number of repetitions of a certain sequence can be the difference between having the disease or being an unaffected carrier.
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