In a genome-wide survey of simple sequence repeats, the highest density was found on chromosome 19 (Fig. 7.4) (Subramanian et al. 2003). Across the different chromosomes, mononucleotide repeats of poly(A) and poly(T) were found at a 300fold higher density than poly(G) or poly(C) ones (Subramanian et al. 2003). Dinucleotide repeats were most common in intronic DNA, with AC and AT most frequent and GC rarest. Trinucleotide repeats were two times more common in exonic DNA with AAT, AAC, AAG, and ACG most common; and ACG, ACT, and CCG rare. Among tetranu-cleotide repeats AAAT, AAAG, AAAC, AAGG were most common; the highest densities were seen on chromosomes 7 and 22; and AGAT was the most frequent repeat found on the Y chromosome. The density of hexanucleotide repeats was two to three times that of trinucleotides in exonic DNA.
Was this article helpful?