Tandem repeats occur in DNA when a pattern of one or more nucleotides is repeated and the repetitions are directly adjacent to each other. Several protein domains also form tandem repeats within their amino acid primary structure, such as armadillo repeats. However, in proteins, perfect tandem repeats are unlikely in most in vivo proteins, and most known repeats are in proteins which have been designed.
An example would be:
in which the sequence ATTCG is repeated three times.
When exactly two nucleotides are repeated, it is called a dinucleotide repeat (for example: ACACACAC...). The microsatellite instability in hereditary nonpolyposis colon cancer most commonly affects such regions.
When three nucleotides are repeated, it is called a trinucleotide repeat (for example: CAGCAGCAGCAG...), and abnormalities in such regions can give rise to trinucleotide repeat disorders.
When the repeat unit copy number is variable in the population being considered, it is called a variable number tandem repeat (VNTR). MeSH classifies variable number tandem repeats under minisatellites.
The occurrence of tandem repeats can occur through different mechanisms. For example, slipped strand mispairing (SSM), (also known as replication slippage), is a mutation process which occurs during DNA replication. It involves denaturation and displacement of the DNA strands, resulting in mispairing of the complementary bases. Slipped strand mispairing is one explanation for the origin and evolution of repetitive DNA sequences.
Tandem repeat describes a pattern that helps determine an individual's inherited traits.
Tandem repeats can be very useful in determining parentage. Short tandem repeats are used for certain genealogical DNA tests. DNA is examined from microsatellites within the chromosomal DNA. Parentage can be determined through the similarity in these regions.
Polymorphic tandem repeats (alias VNTRs) are also present in microorganisms and can be used to trace the origin of an outbreak. The corresponding assay in which a collection of VNTRs is typed to characterize a strain is most often called MLVA (Multiple Loci VNTR Analysis).