Buschiazzo and Gemmell (2006) reviewed the 'life cycle' of microsatellite DNA in eukaryotes, considering the processes whereby microsatellites may arise, expand in length, contract, and eventually become degenerate and die. Much remains unclear but it is thought that 'proto' microsatellites arise either de novo or through transposable DNA elements (Zhu et al. 2000; Wilder and Hollocher 2001). Single base substitutions may result in the generation of a two or three repeat locus, for example a G to A substitution altering the sequence from GACGCACG to GACACACG and thereby creating a run of three AC' dinucleotide repeats. Alternatively an indel (insertion/deletion) event may generate a new dinucleotide repeat, for example GCAT becomes GCACAT with a CA insertion. Such small repeats then serve as substrates for expansion. Messer and colleagues described the possible 'birth' of microsatellites within the n globin pseudogene based on primate sequence alignments, with single nucleotide substitutions leading to di- and tetranucleotide tandem repeats (Fig. 7.5) (Messier et al. 1996). The role of transposable elements (Chapter 8) may be to serve as both substrate, for example through reverse transcription errors involving poly(A) tracts at the 3' ends of long interspersed elements (LINEs) and short interspersed elements (SINEs), and the means of dispersal of microsatellites.
Microsatellites undergo a high rate of 'mutation' in that they are observed to expand or contract in length, through gain or loss of single, or less commonly, multiple repeat units. The major mechanism for this instability involves replication slippage during DNA synthesis, resulting in
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