DNA damage can be caused by a variety of causes such as UV radiation. This damage is then repaired by a variety of DNA repair mechanisms so that subsequent protein biosynthesis, which is necessary fϋr all processes in the body, can proceed smoothly.
What is DNA repair?
DNA consists of a double strand and is continuously multiplied. This process is called DNA replication. During this process, errors can occur that need to be repaired. However, this is only one reason for possible DNA damage. Damage to DNA can also be caused by external factors such as UV radiation. This then leads to mutations that affect the proteins produced. They lose their function or become too active, they can no longer reach their destination in the cell or they can no longer be degraded by the cell when the protein is no longer needed. There are different DNA repair mechanisms. Which mechanism comes into effect depends on the type of DNA damage. It can be the repair of a single-strand break or a double-strand break, as well as the repair of individual bases. The repair is carried out by enzymes that reassemble the DNA when it breaks. These are ligases. The exchange of bases is carried out by recombinases and polymerases. DNA helicases are used to unwind DNA. They prepare the affected DNA segments for repair.
Function and task
When DNA is broken, various repair mechanisms can go into effect. These mechanisms are called homologous or nonhomologous recombination. Recombination occurs not only in cases of DNA damage, but also during reproduction, when recombination of DNA from both partners occurs and the embryo is formed. This recombination is then called sexual recombination. In homologous recombination for the elimination of DNA damage, the juxtaposition of two similar, homologous DNA strands occurs. Subsequently, pairing of the DNA strands occurs and a specific DNA segment is exchanged between the two strands. Meanwhile, the so-called “Holliday structure” of the DNA is formed. This process of exchange is carried out by special enzymes called recombinases. A break can also occur by the direct linking of two DNA ends. In this case, there is no homologous sequence, which means that a gap in the DNA between two ends must be filled to create the missing homologous region. This is called “synthesis dependent strand-annealing” and DNA polymerases fϋll out the Lϋ corners. Another repair option is to shorten two ends until they can be rejoined so that the regions match. This is called “single strand annealing.” This results in the loss of short regions of DNA. This repair is performed by the nucleotide excision repair system. Non-homologous repair processes are carried out independently of matching DNA sequences. Two main repairs are distinguished here. Non-homologous end joining directly joins two DNA double strands by the enzyme ligase. Compared to the other processes mentioned, this repair does not require a homologous sequence, which serves as a guide to ensure that as few errors as possible occur in the DNA after repair. Another sequence of DNA repair is “microhomology-mediated end-joining.” This involves deletion, the removal of DNA regions. Here, too, no Fϋguidance is used. This repair is considered highly error-prone and is often the reason fϋr the development of mutations.
Diseases and disorders
Defective DNA repair produces a variety of diseases, the specific expression of which depends on which DNA region and which genes are affected by these defects. One group of such diseases is called chromosome breakage syndrome. In this case, Brϋche in the DNA, which are packaged in chromosomes, are not repaired properly and these Brϋche also occur more frequently than in normal cases. This type of disease is hereditary. A well-known disease of this group is Werner syndrome. This is an autosomal recessive disease, i.e. the mutation causing this disease is located on one of the autosomes, one of the chromosomes (except the sex chromosomes).It is recessive, it is less likely to have an effect on the phenotype than a dominant gene mutation. Werner syndrome mainly affects the mesodermal tissue. There is increased aging of the affected person after puberty. Another disorder from the category of chromosome break syndromes is Louis Bar syndrome. It is also an autosomal recessive disorder. In this disease there is a high number of different symptoms. These can be explained by the fact that a gene is affected which recognizes DNA damage by UV radiation and is also involved in the regulation of DNA repair. Neurological defects occur, as well as an impairment of the immune system. This results in a number of other diseases such as pneumonia. Furthermore, the disease xeroderma pigmentosum is a disease that can be counted to this class. It is a disease of the skin. The affected individuals are also called moonlight children. Genes coding for enzymes of the DNA repair mechanism are affected by defects. The skin is affected by UV radiation, which leads to the development of skin tumors. Affected individuals must avoid daylight, which affects the entire rhythm of life.