double helix, and this distortion is recognised by the DNA mismatch repair proteins, which then remove the newly
synthesised DNA. The gap in the newly synthesised DNA is replaced by a DNA polymerase that proofreads as it
synthesises and is sealed by DNA ligase. As shown in the figure, a nick in the DNA has been proposed as the signal that allows the mismatch repair proteins to distinguish the newly synthesised DNA (which contains the mistake) from the old DNA. Such nicks are known to occur in the lagging strands (see Figure 6-17) and are observed to also occur, although less frequently, in the leading strands. These nicks remain for only a short period after a replication fork passes so that mismatch repair must occur quickly.