The double-strand break (DSB) repair pathway is a crucial element in ensuring the stability of DNA during meiosis. To understand this repair pathway, we must begin with DSB. DSBs occur when DNA is exposed to ionising radiation— namely, alpha particles, beta particles, positrons, gamma rays, and X-rays. The double-strand DNA is cut in half during the process and nucleotides are lost. Here, the DSB repair pathway assists cells in reconstructing the strand through two main methods: homologous recombination (HR) and non-homologous end-joining (NHEJ).
Note that there are many different ways DNA can suffer mutation, such as translocation or frameshift mutation. Likewise, there are corresponding, and equally as abundant, repair enzymes or pathways for each mutation. However, this article will only discuss DSBs and its repair pathways.
Author: Ethan Oh
Editor: Youhyun (Spany) Noh
Homologous Recombination (HR)
In HR, Mre11, Rad50 and Nbs1 proteins, otherwise known as the MRN complex, binds to the 5’ ends of the DSB and undergoes genetic resection, generating two single strand DNAs (ssDNA) which are coated by ssDNA binding protein complex RPA (Huertas, 2010). RPA prevents the ssDNA from winding and creating a secondary structure, allowing the ssDNA to undergo strand invasion, in which the homologous DNA is opened and forms a double cruciform structure known as the Holliday junction (Liberi). Here, the ssDNAs are elongated, using the homologous structure as a model. After the ssDNAs are paired with nucleotides, the HR process enters its termination sequence. There are two major outcomes: crossover and non-crossover gene conservation products. The non-crossover product would look identical to the homologous DNA (Raphael). However, the crossover product leads to the recombination of genes, famously known as chiasma discovered by Thomas Morgan.
Non-homologous End-joining (NHEJ)
The NHEJ process is distinct from that of HR in what way? Short summary. To begin, the Ku 70 and 80 proteins bind to the DBS. Ku proteins have high affinities for DNA ends, and provide docking sites for other proteins (Featherstone). This property allows the ensuing recruitment of protein DNA-dependent protein kinase, catalytic subunit, or DNA-PKcs (Huertas). DNA-PKcs, along with Ataxia-Telangiectasia Mutated (ATM), and ATM and RAD3 related (ATR), which all belong to the phosphatidylinositol 3-kinase related kinase (PIKK) family, induces the phosphorylation of these protein complexes in order to undergo DNA resection. This confrontational change results in the creation of a blunt end on the DSB, the detachment of these proteins, as well as the ligation of the broken DNA strands (Blackford). Given that no chiasma is created in this process, the topic of crossover products need not be discussed in NHEJ. However, it is important to keep in mind that NHEJ may result in a frameshift mutation, as the loss of nucleotides, unlike HR, is not compensated (Heidenreich).
Conclusion
The DSB repair pathway is a complex series of biochemical reactions that maintains DNA stability during meiosis. Although it is not the only monitoring mechanism during the cell cycle, it still remains of great importance as it prevents some of the deadliest diseases caused by gene mutation, especially DSBs. In the DSB repair pathway, the HR product is much more preservative compared to NHEJ, given that the replication model is the homologous structure. This repair pathway is also tightly regulated, and by whence individual pathways are used follows strict regulations.
Works Cited
Blackford, A. N., & Jackson, S. P. (2017). ATM, ATR, and DNA-PK: the trinity at the heart of the DNA damage response. Molecular cell, 66(6), 801-817.
Heidenreich, E., Novotny, R., Kneidinger, B., Holzmann, V., & Wintersberger, U. (2003). Non-homologous end joining as an important mutagenic process in cell cycle-arrested cells. The EMBO journal, 22(9), 2274–2283. https://doi.org/10.1093/emboj/cdg203
Huertas P. (2010). DNA resection in eukaryotes: deciding how to fix the break. Nature structural & molecular biology, 17(1), 11–16. https://doi.org/10.1038/nsmb.1710
Khanna, K., Jackson, S. DNA double-strand breaks: signaling, repair and the cancer connection. Nat Genet 27, 247–254 (2001). https://doi.org/10.1038/85798
Liberi, G., Foiani, M. The double life of Hollidtay junctions. Cell Res 20, 611–613 (2010). https://doi.org/10.1038/cr.2010.73
Raphael Ceccaldi, Beatrice Rondinelli, Alan D. D’Andrea, Repair Pathway Choices and Consequences at the Double-Strand Break, Trends in Cell Biology, Volume 26, Issue 1, 2016, Pages 52-64, ISSN 0962-8924, https://doi.org/10.1016/j.tcb.2015.07.009.
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