Nick (DNA)

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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. [1]

Contents

Formation of nicks

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. [2]

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). Effects of nicks on intersecting DNA forms.png
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).

Nicked DNA can be the result of DNA damage or purposeful, regulated biomolecular reactions carried out in the cell. During processing, DNA can be nicked by physical shearing, over-drying or enzymes. Excessive rough handling in pipetting or vortexing creates physical stress that can lead to breaks and nicks in DNA. Overdrying of DNA can also break the phosphodiester bond in DNA and result in nicks. Nicking endonuclease enzymes can assist with this process. A single-stranded break (nick) in DNA can be formed by the hydrolysis and subsequent removal of a phosphate group within the helical backbone. This leads to a different DNA conformation, where a hydrogen bond forms in place of the missing piece of the DNA backbone in order to preserve the structure. [3]

Repair of nicks

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. [4] Each nick site requires 1 ATP or 1 NAD+ to power the ligase repair. [4]

Minimalistic mechanism of DNA nick sealing by DNA ligase Ligase nick repair mecanism.jpg
Minimalistic mechanism of DNA nick sealing by DNA ligase

In order to join these fragments, the ligase progresses through three steps:

  1. Addition of an adenosine monophosphate (AMP) group to the enzyme, referred to as adenylylation,
  2. Adenosine monophosphate transfer to the DNA and
  3. Nick sealing, or phosphodiester bond formation. [5] [6]

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. [7]

Ligases have a metal binding site which is capable of recognizing nicks in DNA. The ligase forms a DNA-adenylate complex, assisting recognition. [8] 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. [9]

Biological implications

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). [1] DNA mismatch repair (MMR) is an important DNA repair system that helps maintain genome plasticity by correcting mismatches, or non Watson-Crick base pairs in the a DNA duplex. [10] 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. [11] 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. [1] Together, the presence of a nick and a ribonucleotide make the leading strand easily recognizable to the DNA mismatch repair machinery.

Nick translation is a biological process in which a single-stranded DNA nick serves as the marker for DNA polymerase to excise and replace possibly damaged nucleotides. [3] At the end of the segment that DNA polymerase acts on, DNA ligase must repair the final segment of DNA backbone in order to complete the repair process. [4] In a lab setting, this can be used to introduce fluorescent or other tagged nucleotides by purposefully inducing site-specific, single-stranded nicks in DNA in vitro and then adding the nicked DNA to an environment rich in DNA polymerase and tagged nucleotide. The DNA polymerase then replaces the DNA nucleotides with the tagged ones, starting at the site of the single-stranded nick.

Role in replication and transcription

Nicked DNA plays an important role in many biological functions. For instance, single-stranded nicks in DNA may serve as purposeful biological markers for the enzyme topoisomerase that unwinds packed DNA and is critical to DNA replication and transcription. In these instances, nicked DNA is not the result of unwanted cell damage. [2]

Topoisomerase-1 preferentially acts at nicks in DNA to cleave adjacent to the nick and then winds or unwinds the complex topologies associated with packed DNA. Here, the nick in the DNA serves as a marker for single strand breakage and subsequent unwinding. [12] 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. [13]

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. [14]

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. [4] This is particularly relevant to Okazaki fragments in lagging strand in double stranded DNA replication because the direction of replication is opposite to the direction of DNA polymerase, therefore nick idling plays a role in stalling the complex as it replicates in the reverse direction in small fragments (Okazaki fragments) and has to stop and reposition itself in between each and every fragment length of DNA.

DNA structure changes when a single-stranded nick is introduced. [14] Stability is decreased as a break in the phosphodiester backbone allows DNA to unwind, as the built up stress from twisting and packing is not being resisted as strongly anymore. [12] Nicked DNA is more susceptible to degradation due to this reduced stability.

In bacteria

The nic site or nick region is found within the origin of transfer (oriT) site and is a key in starting bacterial conjugation. A single strand of DNA, called the T-strand, is cut at nic by an enzyme called relaxase. [15] 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. [15] It is thought that portions of the oriT site are bent in a way that creates interaction between the relaxosome proteins and the nic site. [15]

Cleaving the T-strand involves relaxase cutting a phosphodiester bond at the nic site. [15] The cleaved strand is left with a hydroxyl group at the 3' end, which may allow for the strand to form a circular plasmid after moving into the recipient cell. [16] [17]

Role in meiosis

DNA nicks promote crossover formation during meiosis, and such nicks are protected from ligation by Exonuclease 1 (Exo1). [18]

Related Research Articles

<span class="mw-page-title-main">DNA ligase</span> Class of enzymes

DNA ligase is a type of enzyme that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It plays a role in repairing single-strand breaks in duplex DNA in living organisms, but some forms may specifically repair double-strand breaks. Single-strand breaks are repaired by DNA ligase using the complementary strand of the double helix as a template, with DNA ligase creating the final phosphodiester bond to fully repair the DNA.

<span class="mw-page-title-main">DNA replication</span> Biological process

In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part of biological inheritance. This is essential for cell division during growth and repair of damaged tissues, while it also ensures that each of the new cells receives its own copy of the DNA. The cell possesses the distinctive property of division, which makes replication of DNA essential.

<span class="mw-page-title-main">Primer (molecular biology)</span> Short strand of RNA or DNA that serves as a starting point for DNA synthesis

A primer is a short, single-stranded nucleic acid used by all living organisms in the initiation of DNA synthesis. A synthetic primer may also be referred to as an oligo, short for oligonucleotide. DNA polymerase enzymes are only capable of adding nucleotides to the 3’-end of an existing nucleic acid, requiring a primer be bound to the template before DNA polymerase can begin a complementary strand. DNA polymerase adds nucleotides after binding to the RNA primer and synthesizes the whole strand. Later, the RNA strands must be removed accurately and replace them with DNA nucleotides forming a gap region known as a nick that is filled in using an enzyme called ligase. The removal process of the RNA primer requires several enzymes, such as Fen1, Lig1, and others that work in coordination with DNA polymerase, to ensure the removal of the RNA nucleotides and the addition of DNA nucleotides. Living organisms use solely RNA primers, while laboratory techniques in biochemistry and molecular biology that require in vitro DNA synthesis usually use DNA primers, since they are more temperature stable. Primers can be designed in laboratory for specific reactions such as polymerase chain reaction (PCR). When designing PCR primers, there are specific measures that must be taken into consideration, like the melting temperature of the primers and the annealing temperature of the reaction itself. Moreover, the DNA binding sequence of the primer in vitro has to be specifically chosen, which is done using a method called basic local alignment search tool (BLAST) that scans the DNA and finds specific and unique regions for the primer to bind.

<span class="mw-page-title-main">DNA polymerase</span> Form of DNA replication

A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create two identical DNA duplexes from a single original DNA duplex. During this process, DNA polymerase "reads" the existing DNA strands to create two new strands that match the existing ones. These enzymes catalyze the chemical reaction

DNA topoisomerases are enzymes that catalyze changes in the topological state of DNA, interconverting relaxed and supercoiled forms, linked (catenated) and unlinked species, and knotted and unknotted DNA. Topological issues in DNA arise due to the intertwined nature of its double-helical structure, which, for example, can lead to overwinding of the DNA duplex during DNA replication and transcription. If left unchanged, this torsion would eventually stop the DNA or RNA polymerases involved in these processes from continuing along the DNA helix. A second topological challenge results from the linking or tangling of DNA during replication. Left unresolved, links between replicated DNA will impede cell division. The DNA topoisomerases prevent and correct these types of topological problems. They do this by binding to DNA and cutting the sugar-phosphate backbone of either one or both of the DNA strands. This transient break allows the DNA to be untangled or unwound, and, at the end of these processes, the DNA backbone is resealed. Since the overall chemical composition and connectivity of the DNA do not change, the DNA substrate and product are chemical isomers, differing only in their topology.

DNA primase is an enzyme involved in the replication of DNA and is a type of RNA polymerase. Primase catalyzes the synthesis of a short RNA segment called a primer complementary to a ssDNA template. After this elongation, the RNA piece is removed by a 5' to 3' exonuclease and refilled with DNA.

<span class="mw-page-title-main">Nuclease</span> Class of enzymes which cleave nucleic acids

In biochemistry, a nuclease is an enzyme capable of cleaving the phosphodiester bonds between nucleotides of nucleic acids. Nucleases variously effect single and double stranded breaks in their target molecules. In living organisms, they are essential machinery for many aspects of DNA repair. Defects in certain nucleases can cause genetic instability or immunodeficiency. Nucleases are also extensively used in molecular cloning.

<span class="mw-page-title-main">DNA polymerase I</span> Family of enzymes

DNA polymerase I is an enzyme that participates in the process of prokaryotic DNA replication. Discovered by Arthur Kornberg in 1956, it was the first known DNA polymerase. It was initially characterized in E. coli and is ubiquitous in prokaryotes. In E. coli and many other bacteria, the gene that encodes Pol I is known as polA. The E. coli Pol I enzyme is composed of 928 amino acids, and is an example of a processive enzyme — it can sequentially catalyze multiple polymerisation steps without releasing the single-stranded template. The physiological function of Pol I is mainly to support repair of damaged DNA, but it also contributes to connecting Okazaki fragments by deleting RNA primers and replacing the ribonucleotides with DNA.

<span class="mw-page-title-main">DNA polymerase III holoenzyme</span> Primary enzyme complex involved in prokaryotic DNA replication

DNA polymerase III holoenzyme is the primary enzyme complex involved in prokaryotic DNA replication. It was discovered by Thomas Kornberg and Malcolm Gefter in 1970. The complex has high processivity and, specifically referring to the replication of the E.coli genome, works in conjunction with four other DNA polymerases. Being the primary holoenzyme involved in replication activity, the DNA Pol III holoenzyme also has proofreading capabilities that corrects replication mistakes by means of exonuclease activity reading 3'→5' and synthesizing 5'→3'. DNA Pol III is a component of the replisome, which is located at the replication fork.

<span class="mw-page-title-main">Okazaki fragments</span> Parts of lagging strand in DNA replication

Okazaki fragments are short sequences of DNA nucleotides which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication. They were discovered in the 1960s by the Japanese molecular biologists Reiji and Tsuneko Okazaki, along with the help of some of their colleagues.

<span class="mw-page-title-main">DNA repair</span> Cellular mechanism

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.

In molecular biology, biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. This process often consists of metabolic pathways. Some of these biosynthetic pathways are located within a single cellular organelle, while others involve enzymes that are located within multiple cellular organelles. Examples of these biosynthetic pathways include the production of lipid membrane components and nucleotides. Biosynthesis is usually synonymous with anabolism.

<span class="mw-page-title-main">DNA polymerase II</span> Class of enzymes

DNA polymerase II is a prokaryotic DNA-dependent DNA polymerase encoded by the PolB gene.

<span class="mw-page-title-main">Replisome</span> Molecular complex

The replisome is a complex molecular machine that carries out replication of DNA. The replisome first unwinds double stranded DNA into two single strands. For each of the resulting single strands, a new complementary sequence of DNA is synthesized. The total result is formation of two new double stranded DNA sequences that are exact copies of the original double stranded DNA sequence.

UvrABC endonuclease is a multienzyme complex in bacteria involved in DNA repair by nucleotide excision repair, and it is, therefore, sometimes called an excinuclease. This UvrABC repair process, sometimes called the short-patch process, involves the removal of twelve nucleotides where a genetic mutation has occurred followed by a DNA polymerase, replacing these aberrant nucleotides with the correct nucleotides and completing the DNA repair. The subunits for this enzyme are encoded in the uvrA, uvrB, and uvrC genes. This enzyme complex is able to repair many different types of damage, including cyclobutyl dimer formation.

<span class="mw-page-title-main">Prokaryotic DNA replication</span> DNA Replication in prokaryotes

Prokaryotic DNA Replication is the process by which a prokaryote duplicates its DNA into another copy that is passed on to daughter cells. Although it is often studied in the model organism E. coli, other bacteria show many similarities. Replication is bi-directional and originates at a single origin of replication (OriC). It consists of three steps: Initiation, elongation, and termination.

<span class="mw-page-title-main">T7 DNA polymerase</span> Enzyme

T7 DNA polymerase is an enzyme used during the DNA replication of the T7 bacteriophage. During this process, the DNA polymerase “reads” existing DNA strands and creates two new strands that match the existing ones. The T7 DNA polymerase requires a host factor, E. coli thioredoxin, in order to carry out its function. This helps stabilize the binding of the necessary protein to the primer-template to improve processivity by more than 100-fold, which is a feature unique to this enzyme. It is a member of the Family A DNA polymerases, which include E. coli DNA polymerase I and Taq DNA polymerase.

<span class="mw-page-title-main">Circular chromosome</span> Type of chromosome

A circular chromosome is a chromosome in bacteria, archaea, mitochondria, and chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromosome of most eukaryotes.

<span class="mw-page-title-main">Ligation (molecular biology)</span> Technique for joining nucleic acid fragments

Ligation is the joining of two nucleic acid fragments through the action of an enzyme. It is an essential laboratory procedure in the molecular cloning of DNA, whereby DNA fragments are joined to create recombinant DNA molecules (such as when a foreign DNA fragment is inserted into a plasmid). The ends of DNA fragments are joined by the formation of phosphodiester bonds between the 3'-hydroxyl of one DNA terminus with the 5'-phosphoryl of another. RNA may also be ligated similarly. A co-factor is generally involved in the reaction, and this is usually ATP or NAD+. Eukaryotic cells ligases belong to ATP type, and NAD+ - dependent are found in bacteria (e.g. E. coli).

<span class="mw-page-title-main">Ribose-seq</span> Genetic mapping technique

Ribose-seq is a mapping technique used in genetics research to determine the full profile of embedded ribonucleotides, specifically ribonucleoside monophosphates (rNMPs), in genomic DNA. Embedded ribonucleotides are thought to be the most common alteration to DNA in cells, and their presence in genomic DNA can affect genome stability. As recent studies have suggested that ribonucleotides in mouse DNA may affect disease pathology, ribonucleotide incorporation in genomic DNA has become an important target of medical genetics research. Ribose-seq allows scientists to determine the precise location and type of ribonucleotides that have been incorporated into eukaryotic or prokaryotic DNA.

References

  1. 1 2 3 Williams JS, Kunkel TA (July 2014). "Ribonucleotides in DNA: origins, repair and consequences". DNA Repair (Amst). 19: 27–37. doi:10.1016/j.dnarep.2014.03.029. PMC   4065383 . PMID   24794402.
  2. 1 2 3 Ji, Chao; Zhang, Lingyun; Wang, Pengye (2015). "Configuration Transitions of Free Circular DNA System Induced by Nicks". Journal of Nanomaterials. 2015 546851: 1–7. doi: 10.1155/2015/546851 .
  3. 1 2 Aymami, J.; Coll, M.; Marel, G. A. van der; Boom, J. H. van; Wang, A. H.; Rich, A. (1990). "Molecular structure of nicked DNA: a substrate for DNA repair enzymes". Proceedings of the National Academy of Sciences. 87 (7): 2526–30. Bibcode:1990PNAS...87.2526A. doi: 10.1073/pnas.87.7.2526 . PMC   53722 . PMID   2320572.
  4. 1 2 3 4 Timson, David J; Singleton, Martin R; Wigley, Dale B (2000). "DNA ligases in the repair and replication of DNA". Mutation Research/DNA Repair. 460 (3–4): 301–318. doi:10.1016/S0921-8777(00)00033-1. PMID   10946235.
  5. Ellenberger T, Tomkinson AE (2008). "Eukaryotic DNA ligases: structural and functional insights". Annu. Rev. Biochem. 77: 313–38. doi:10.1146/annurev.biochem.77.061306.123941. PMC   2933818 . PMID   18518823.
  6. Sriskanda V, Shuman S (January 1998). "Chlorella virus DNA ligase: nick recognition and mutational analysis". Nucleic Acids Res. 26 (2): 525–31. doi:10.1093/nar/26.2.525 (inactive 2024-07-02). PMC   147278 . PMID   9421510.{{cite journal}}: CS1 maint: DOI inactive as of July 2024 (link)
  7. Nandakumar, Jayakrishnan; Nair, Pravin A.; Shuman, Stewart (2007-04-27). "Last stop on the road to repair: structure of E. coli DNA ligase bound to nicked DNA-adenylate". Molecular Cell. 26 (2): 257–271. doi: 10.1016/j.molcel.2007.02.026 . ISSN   1097-2765. PMID   17466627.
  8. Odell, Mark; Sriskanda, Verl; Shuman, Stewart; Nikolov, Dimitar B. (2000). "Crystal Structure of Eukaryotic DNA Ligase–Adenylate Illuminates the Mechanism of Nick Sensing and Strand Joining". Molecular Cell. 6 (5): 1183–93. doi: 10.1016/S1097-2765(00)00115-5 . PMID   11106756.
  9. Pascal, John M.; O'Brien, Patrick J.; Tomkinson, Alan E.; Ellenberger, Tom (2004). "Human DNA ligase I completely encircles and partially unwinds nicked DNA". Nature. 432 (7016): 473–8. Bibcode:2004Natur.432..473P. doi:10.1038/nature03082. PMID   15565146. S2CID   3105417.
  10. Morris, James (2013). "14. Mutations and DNA Repair". Biology: How Life Works. W.H. Freeman. ISBN   978-1-319-05691-9.
  11. Fukui, Kenji (2010). "DNA Mismatch Repair in Eukaryotes and Bacteria". Journal of Nucleic Acids. 2010: 1–16. doi: 10.4061/2010/260512 . PMC   2915661 . PMID   20725617.
  12. 1 2 Huang, Shar-Yin Naomi; Ghosh, Sanchari; Pommier, Yves (2015-05-29). "Topoisomerase I alone is sufficient to produce short DNA deletions and can also reverse nicks at ribonucleotide sites". The Journal of Biological Chemistry. 290 (22): 14068–76. doi: 10.1074/jbc.M115.653345 . ISSN   1083-351X. PMC   4447978 . PMID   25887397.
  13. Racko, Dusan; Benedetti, Fabrizio; Dorier, Julien; Burnier, Yannis; Stasiak, Andrzej (2015). "Generation of supercoils in nicked and gapped DNA drives DNA unknotting and postreplicative decatenation". Nucleic Acids Research. 43 (15): 7229–36. doi:10.1093/nar/gkv683. PMC   4551925 . PMID   26150424.
  14. 1 2 Hays, J. B.; Zimm, B. H. (1970-03-14). "Flexibility and stiffness in nicked DNA". Journal of Molecular Biology. 48 (2): 297–317. doi: 10.1016/0022-2836(70)90162-2 . ISSN   0022-2836. PMID   5448592.
  15. 1 2 3 4 Lanka E, Wilkins BM (1995). "DNA processing reactions in bacterial conjugation". Annu Rev Biochem. 64: 141–69. doi:10.1146/annurev.bi.64.070195.001041. PMID   7574478.
  16. Matson SW, Nelson WC, Morton BS (May 1993). "Characterization of the reaction product of the oriT nicking reaction catalyzed by Escherichia coli DNA helicase I". J Bacteriol. 175 (9): 2599–606. doi:10.1128/jb.175.9.2599-2606.1993. PMC   204561 . PMID   8386720.
  17. Grohmann E, Muth G, Espinosa M (June 2003). "Conjugative plasmid transfer in gram-positive bacteria". Microbiol Mol Biol Rev. 67 (2): 277–301. doi:10.1128/MMBR.67.2.277-301.2003. PMC   156469 . PMID   12794193.
  18. Gioia M, Payero L, Salim S, Fajish VG, Farnaz AF, Pannafino G, Chen JJ, Ajith VP, Momoh S, Scotland M, Raghavan V, Manhart CM, Shinohara A, Nishant KT, Alani E (April 2023). "Exo1 protects DNA nicks from ligation to promote crossover formation during meiosis". PLOS Biol. 21 (4): e3002085. doi: 10.1371/journal.pbio.3002085 . PMC   10153752 . PMID   37079643.