Tandem repeats are stretches of DNA in which a short sequence motif is repeated directly adjacent to itself. Each copy, called a repeat unit or motif, lies end-to-end in the same orientation. Simple examples include a motif such as ATTCG repeated three times to form ATTCGATTCGATTCG. Tandem repeats are a common and dynamic component of eukaryotic and prokaryotic genomes.

Types and characteristics

Repeat loci are often grouped by the length of their repeat unit. Common categories include:

  • Microsatellites or short tandem repeats (STRs): motifs of about 1–6 base pairs repeated many times.
  • Minisatellites: longer motifs, often around 10–60 base pairs, with variable repeat counts.
  • Satellite DNA: larger tandem arrays that form blocks often found near centromeres and telomeres.

The number of repeat copies can vary between individuals and between chromosomes. This variability makes tandem repeats highly polymorphic markers in population and forensic studies.

Origin and mutational processes

Tandem repeats arise and change in copy number through mechanisms such as replication slippage, unequal crossing over during meiosis, and DNA repair errors. Repeats are prone to instability because repeated motifs can misalign, producing insertions or deletions of whole repeat units. Over evolutionary time this instability contributes to rapid sequence divergence and local structural variation.

Biological roles and significance

Tandem repeats affect genomes in several ways: they can influence chromatin structure, gene regulation, and recombination rates. Expansions of particular repeats are responsible for a number of inherited disorders: for example, expansions of trinucleotide repeats are implicated in conditions such as Huntington's disease and fragile X syndrome. Because repeat length can alter gene function or expression, these loci are medically and biologically important.

Applications and detection

Practical uses of tandem repeats include genetic mapping, paternity testing, and forensic profiling, where microsatellite length differences help distinguish individuals. Laboratory techniques for detecting and sizing repeats include PCR amplification, gel or capillary electrophoresis, Southern blotting for large arrays, and high-throughput sequencing methods. Computational tools and databases also exist to locate and annotate tandem repeats across genomes; see further resources for tools and references.

Notable distinctions

Unlike dispersed repetitive elements (such as transposons), tandem repeats are contiguous copies at a single locus. Their high mutation rate and polymorphism set them apart from most single-nucleotide variants, giving them particular value in studies of recent evolution, demographic history, and identity testing. However, their repetitive nature also complicates genome assembly and variant calling using short-read sequencing.