Unrepaired mutations in DNA can lead to diseases such as cancer.
DNA can become damaged due to environmental conditions such as UV radiation from the sun or through normal metabolic processes. Cells are capable of identifying and correcting damage that occurs within the DNA molecule. These repair mechanisms are categorized by the way they operate. Direct correction reverses the damage while other mechanisms cut out the damaged areas of DNA and fix the gap by the synthesis of new DNA. In all single strand repair mechanisms damage is repaired by reading the information on the intact strand.
Polymerase Proofreading
During DNA replication, an enzyme identifies and corrects any mistakes made by the DNA polymerase. This direct correction occurs automatically when an incorrect nucleotide is inserted. The synthesis process is halted until the incorrect nucleotide is replaced with the correct one.
Nucleotide Excision Repair (NER)
UV damage is repaired through a process involving four steps. First the damage is detected, then the section of DNA containing the error is cut out. This is followed by the synthesis of a new strand by DNA polymerase and then the gap is sealed.
Base Excision Repair (BER)
Damage caused by metabolic processes and free radicals are corrected by base excision repair. During this process mutated bases are released from the sugar-phosphate backbone through cleavage of the chemical bonds. The space is filled in by a repair polymerase and then sealed.
Nonhomologous End Joining (NHEJ)
Radiation can cause breaks in the double strands which can interfere with transcription and replication and also instigate the occurrence of hybrid proteins or the activation of the improper genes. In one mechanism of repair for double-strand breaks, an enzyme uses the overhanging portion of DNA next to the break to fill in the gap. This type of repair can lead to further errors as sequences of nucleotides can be left out if the ends are not compatible.
Homologous Recombination Repair (HRR)
This mechanism for correcting a double-strand break repairs the damage by using the homologous chromosome to synthesize the correct sequence of nucleotides. This mechanism is more reliable as it uses an identical chromosome for a template restoring the damaged double-strand to its original state.
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