Fold-back structures at the distal end influence DNA slippage at the proximal end during mononucleotide repeat expansions

Polymerase slippage during DNA synthesis by the Klenow fragment of DNA poly merase across A, C, G and T repeats (30 bases) has been studied. Within min utes, duplexes that contain only repeats (30 bp) expand dramatically to sev eral hundred base pairs long. Rate comparisons in a repeat duplex when one strand was expanded as against that when bath strands were expanded suggest a model of migrating hairpin loops which in the latter case coalesce into a duplex. Moreover, slippage (at the proximal or 3'-end) is subject to posi tive and negative effects from the 5'-end (distal) of the same strand. Grow ing T and G strands generate T,A:T and G-G:C motif fold-back structures at the distal end that hamper slippage at the proximal end. On the other hand, growing tails at the distal end upon annealing with excess complementary t emplate accentuates proximal slippage several-fold.