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Discontinuity in DNA strand
A nick is a discontinuity in a double stranded DNA molecule where there is no phosphodiester bond between adjacent nucleotides of one strand typically through damage or enzyme action. Nicks allow DNA strands to untwist during replication, and are also thought to play a role in the DNA mismatch repair mechanisms that fix errors on both the leading and lagging daughter strands.

The diagram shows the effects of nicks on intersecting DNA in a twisted plasmid. Nicking can be used to dissipate the energy held up by intersecting states. The nicks allow the DNA to take on a circular shape. [[File:Effects of nicks on intersecting DNA forms. png|thumb|The diagram shows the effects of nicks on intersecting DNA forms. A plasmid is tightly wound into a negative supercoil (a). To release the intersecting states, the torsional energy must be released by utilizing nicks (b). After introducing a nick in the system, the negative supercoil gradually unwinds (c) until it reaches its final, circular, plasmid state (d).

Ligases are versatile and ubiquitous enzymes that join the 3’ hydroxyl and 5’ phosphate ends to form a phosphodiester bond, making them essential in nicked DNA repair, and ultimately genome fidelity. This biological role has also been extremely valuable in sealing the sticky ends of plasmids in molecular cloning. Their importance is attested by the fact most organisms have multiple ligases dedicated to specific pathways of repairing DNA. In eubacteria these ligases are powered by NAD+ rather than ATP. Each nick site requires 1 ATP or 1 NAD+ to power the ligase repair. One particular example of a ligase catalyzing nick closure is the E. coli NAD+ dependent DNA ligase, LigA. LigA is a relevant example as it is structurally similar to a clade of enzymes found across all types of bacteria. Ligases have a metal binding site which is capable of recognizing nicks in DNA. The ligase forms a DNA adenylate complex, assisting recognition. With human DNA ligase, this forms a crystallized complex. The complex, which has a DNA–adenylate intermediate, allows DNA ligase I to institute a conformational change in the DNA for the isolation and subsequent repair of the DNA nick.

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 the 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.

DNA nicks promote crossover formation during meiosis, and such nicks are protected from ligation by Exonuclease 1 (Exo1).