Role in mismatch repair Single-stranded nicks act as recognizable markers to help the repair machinery distinguish the newly synthesized strand (daughter strand) from the template strand (parental strand). Some sources of mismatched base pairs include replication errors and
deamination of 5-methylcytosine DNA to form thymine. MMR in most bacteria and
eukaryotes is directed to the erroneous strand of the mismatched duplex through recognition of strand discontinuities, while MMR in
E. coli and closely related bacteria is directed to the strand on the basis of the absence of
methylation. Nicking endonucleases introduce strand discontinuities, or DNA nicks, for both respective systems. Mut L homologues from eukaryotes and most bacteria incise the discontinuous strand to introduce the entry or termination point for the excision reaction. Similarly, in
E. coli, Mut H nicks the unmethylated strand of the duplex to introduce the entry point of excision. For eukaryotes specifically, the mechanism of DNA replication elongation between the leading and lagging strand differs. On the lagging strand, nicks exist between
Okazaki fragments and are easily recognizable by the DNA mismatch repair machinery prior to
ligation. Due to the continuous replication that occurs on the leading strand, the mechanism there is slightly more complex. During replication,
ribonucleotides are added by replication enzymes and these ribonucleotides are nicked by an enzyme called
RNase H2. It is possible that this is not a highly conserved process. Topoisomerase may cause short deletions when it cleaves bonds, because both full-length DNA products and short deletion strands are seen as products of topoisomerase cleavage while inactive mutants only produced full-length DNA strands. Nicks in DNA also give rise to different structural properties, can be involved in repairing damages caused by
ultraviolet radiation, and are used in the primary steps that allow for
genetic recombination. Nick idling is a biological process in which DNA polymerase may slow or stop its activity of adding bases to a new daughter strand during DNA replication at a nick site. This single strand is eventually transferred to the recipient cell during the process of bacterial conjugation. Before this cleavage can occur, however, it is necessary for a group of proteins to attach to the
oriT site. This group of proteins is called the relaxosome.
Role in meiosis DNA nicks promote
crossover formation during
meiosis, and such nicks are protected from ligation by
Exonuclease 1 (Exo1). ==References==