A Non-homologous end-joining factor (XLF) prepares the link to be ligated (connected together) by an enzyme called DNA ligase IV. Then an endonuclease (Artemis) cleans up the break by removing neighbouring nucleotides. In NHEJ, a KU heterodimer binds to the broken ends of the DNA and helps the two strands line up properly. NHEJ is the main way breaks caused by ionizing radiation are repaired. Rearrangements like these are associated with some cancers, such as chronic myeloid leukemia.ĭSBs are repaired through one of two mechanisms: non-homologous end-joining (NHEJ) and homologous recombination (HR). This occurs when a piece of one chromosome becomes attached to another chromosome. They can also lead to chromosomal rearrangements. They can interfere with DNA replication and protein synthesis. Unlike SSB, DSBs can be extremely harmful to cells. The process of single-strand break repair (Source: Let’s Talk Science based on an image by LadyofHats via Wikimedia Commons). This process is called Single-Strand Break Repair (SSBR). The gap is joined together again by DNA ligase. Polynucleotide kinase (PNK) then cleans up the broken ends of the phosphate backbone, while DNA polymerase fills the gap until only a tiny gap remains. This enzyme signals other repair proteins to come to the break. That’s the job of an enzyme called poly (ADP-ribose) polymerase (PARP). This causes either single-strand breaks (SSB) or double-strand breaks (DSB).įor SSBs, the first step is detection of the break. High-energy ionizing radiation can blast through the sugar-phosphate backbone. How are breaks in the sugar-phosphate backbone repaired? This process is called Nucleotide Excision Repair (NER). Instead, they will cut out the entire damaged segment of the DNA strand and replace it. In this case, the enzymes won’t just remove and replace a single base. Finally, DNA ligase rejoins the sugar-phosphate backbone. Next, DNA polymerase assembles the missing base. Then an enzyme called AP endonuclease finds the gap in the sugar-phosphate backbone and removes neighbouring nucleotides. This is the process of cutting out and replacing a single nucleotide.įirst, an enzyme called DNA glycosylase extracts the damaged part. In this case, the cell uses a mechanism called Base Excision Repair (BER). This is because the DNA sequence on the complementary strand can serve as a template during DNA replication. It is fairly easy to repair damage like this if the complementary (or opposite) strand is undamaged. This can occur chemically when a hydroxyl (OH-) free radical reacts with a hydrogen atom and forms hydrogen peroxide (H 2O 2). Sometimes, radiation damages a single base on one strand of DNA. Base excision repair is one of the simplest forms of DNA repair (Source: Let’s Talk Science based on an image by LadyofHats via Wikimedia Commons).
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