ROLES OF REPETITIVE SEQUENCES

Repetitive sequences are ubiquitous in the DNA of eukaryotes, some as tandem arrays and others interspersed widely in the genome. Repetitive sequences have special roles in genome evolution, which increasingly detailed sequence information is helping to elucidate. Processes, including meiotic crossing over (equal and unequal), unequal mitotic sister chromatid exchange, gene conversion and transposition, with or without multiplication, can foster homogeneity of the members of a repeat family (concerted evolution) and turnover of the whole genome. Some examples are considered. Tandem repeats, satellite and minisatellite sequences are considered as well as telomeric repeats. For a minisatellite locus, in which the frequency of length mutations has been measured, comparisons are made with expectations due to unequal sister chromatid exchange, and qualitative agreement is found. Interspersed repeats, of which Alu sequences are discussed as an important example, can through unequal recombination lead to a loss or duplication of the DNA between the recombination sites and hence to genetic disease or to gene duplication. It is argued that the rate of unequal Alu recombination may be quite high in the human genome.