DNA repair processes occur through multiple pathways. One of the common processes is termed homologous recombination. This procedure takes place when there arises a repair need for serious damage done to a DNA molecule, for example, if both the strands break simultaneously. A protein named Rad51 is the main component of this homologous recombination process. Also, it gets support from other proteins which are scientifically known as ‘helper proteins’.

There is also another type of DNA repair and mismatch repair where the protein involved is responsible for identifying the mismatch and developing a complex with other helper proteins. The primary protein associated with mismatch repair is MutS while the common helper proteins are MutL and MutH. Now in the next part of this article, we shall discuss the major mechanisms of DNA repair.

The 4 types of DNA repair

Before understanding the 4 types of DNA repair it is important to identify the agents that lead to DNA damage. Ionizing radiations like X-rays and gamma radiation along with harmful UV rays that penetrate through the Ozone holes get strongly absorbed by the DNA molecules. This leads to long-term damage. Other than this oxygen radicals that are highly reactive affect the DNA molecules when they are generated at the time of cellular respiration and also in times of other biochemical pathways. Chemicals like hydrocarbons that are present in the form of environmental toxins as well as chemicals released at the time of chemotherapy contribute to DNA damage to a considerable extent. The 4 types of DNA repair are intended to solve specific problems through biochemical pathways.

The first damage type is cited when each of the four bases namely A, T, G, and C of the DNA get overwritten at different positions. The most common type of damage that can be noticed under this category is the elimination of amino groups or deamination for instance sometimes the C base gets changed to U.

Mismatches are another form of DNA damage that is cured through one of the 4 types of DNA repair. This anomaly takes place at the time of DNA replication. The next prevalent form of damage is noticed as breaks in the structure of DNA. In this type, a single DNA strand breaks, or sometimes there is a condition of double-stranded break or DSB. Chemical exposure or ionizing rays may cause this type of damage which can be healed through direct chemical reversal – one of the 4 types of DNA repair. The last type of damage is cited as crosslinks where covalent links are produced between bases of DNA. Chemotherapeutic dosage is recommended against crosslink DNAs that can lead to cancer.

Explanation of the 4 types of DNA repair

Hereby we enlist the major mechanisms of DNA repair.

• Direct Chemical Reversal: This DNA repair occurs when the base of a DNA is affected. The most common form of damage repaired through this pathway is the healing of mutation of C bases to T. Glycosylase enzymes are responsible for most DNA repair. It restores the correct C base by removing the T. The whole process takes place seamlessly without hampering the DNA backbone.
• Basic Excision Repair: Glycosylase enzymes are responsible for most DNA repair that is required more than 20,000 times on a single day for each constituent cell of the human body. This repair mechanism facilitates the removal of deoxyribose phosphate from the backbone. Then the replacement takes place with the ideal nucleotide. Two enzymes are secreted to execute this process and they are backed up by ATP – the source of energy.
• Mismatch Repair: This repair process restores the mismatches involving the DNA bases. Mismatch repair ensures that the DNA backbone is structured according to the Watson-Crick pairing. This repair mechanism prevents mutations that can otherwise lead to colon cancer – a hereditary disorder. The protein involved in mismatch repair is MLH1.
• Nucleotide Excision Repair: After the damage is identified by several enzymes or indicators, the DNA unwinds to form a bubble. The transcription system IIH, TFIIH is responsible for this step. The repair mechanism makes cuts adjacent to the damaged zone to remove the affected tracts. Consequently, a fresh burst of DNA ligase is inserted into the DNA backbone.

Conclusion

DNA repair mechanisms are responsible for maintaining the genetic expression of cells. It is a crucial biochemical phenomenon that governs the overall health of a person. Mutations are prevented through DNA repair systems which otherwise can be the root cause of cancer. Thus for successful replication, DNA molecules must be repaired after they get damaged from ionizing radiation or chemical exposure. There are four types of DNA repair mechanisms that fulfill the purpose.