Proc. Natl Acad. Sci. USA 105, 9936–9941 (2008)

In living cells, palindromes in a DNA sequence often stall the DNA replication machinery when their two halves bind, making the strand loop outwards.

Such arrangements, in which similar or identical sequences sit close to each other but run in opposite directions, are hotspots for chromosome breaks that can cause disease. Using gel electrophoresis to analyse DNA at various stages of its copying, Sergei Mirkin of Tufts University in Medford, Massachusetts, and his colleagues showed that hairpin structures are made this way in living bacterial, yeast and primate cells.

The researchers think that when a hairpin forms, the lagging strand is left uncopied. This makes it more prone to breakage, and thus at greater risk of elimination from the genome.