Microsatellite DNA, also known as Simple Sequence Repeats (SSRs) or Short Tandem Repeats (STRs), is a short segment of DNA consisting of a repeating sequence of 1 to 6 base pairs, which are repeated multiple times in tandem at a specific location in the genome.
Characteristics of Microsatellite DNA
- Short Repeating Units: The defining feature is the short, repetitive nature of the DNA sequence. Common examples include repeats of dinucleotides (e.g., CA-CA-CA-CA), trinucleotides (e.g., CAG-CAG-CAG), and tetranucleotides (e.g., GATA-GATA-GATA).
- Tandem Repeats: The repeating units occur one after another in a consecutive sequence. This distinguishes them from other types of repetitive DNA, such as interspersed repeats.
- Genomic Location: Microsatellites are found throughout the genome, both in coding and non-coding regions. However, they are more commonly found in non-coding regions.
- Polymorphism: A key characteristic is their high degree of polymorphism. The number of repeats at a particular microsatellite locus often varies greatly among individuals. This variation makes them valuable markers for genetic studies.
- Mutation Rate: Microsatellites have a relatively high mutation rate compared to other parts of the genome. This is primarily due to "slippage" during DNA replication, where the DNA polymerase can lose its place and either add or skip repeats.
Applications of Microsatellite DNA
The polymorphic nature of microsatellites makes them widely used in various applications, including:
- DNA Fingerprinting: Because the number of repeats at different microsatellite loci varies between individuals, microsatellites are a powerful tool for DNA fingerprinting and forensic analysis.
- Genetic Mapping: Microsatellites can be used as genetic markers to map genes and identify quantitative trait loci (QTLs).
- Population Genetics: The variability of microsatellites makes them useful for studying population structure, gene flow, and evolutionary relationships.
- Disease Diagnostics: Changes in microsatellite repeat numbers can be associated with certain diseases, such as Huntington's disease (trinucleotide repeat expansion).
- Cancer Research: Microsatellite instability (MSI), characterized by changes in microsatellite length, is a hallmark of some cancers, particularly colorectal cancer.
Example
Consider a microsatellite locus with the sequence (CA)n, where "n" represents the number of CA repeats. In one individual, this locus might have 10 CA repeats (CA)10, while in another individual, it might have 15 CA repeats (CA)15. This difference in repeat number is what makes microsatellites polymorphic and useful as genetic markers.
In summary, microsatellite DNA is a short, repetitive DNA sequence characterized by high polymorphism, making it a valuable tool in various areas of genetic research and applications.
