Figure 2.

Expansion of DNA tetranucleotide repeats during DNA replication in vitro. Reaction products were resolved on a denaturing 12% polyacrylamide gel containing 7.5 M urea. (A) Schematic of annealed primer and template used for DNA synthesis in vitro. U_a and U_b represent unique DNA sequences flanking the repeat tract in the template strand. (B) Expansion of the CCTG repeat-containing nascent strand. Lanes 1–8, replication with a (CCTG)₃ primer by h-polβ for 0–60 min, as indicated. Lane 9, 10 bp ladder. The arrow indicates the size of the fully replicated template (52 nt); all higher bands are due to repeat expansion. (C) Comparison of tetranucleotide repeat expansions of varying sequences. Expansion is plotted as the percentage intensity of the expanded region (defined as the amount of labeled products >52 nt divided by the total amount of labeled products in the lane) versus reaction time. Sequences shown indicate the sequence of the nascent-strand repeat.