DNA repair.

. The name DNA
repair is a reaction of a cell to DNA damage that may restore the DNA structure to its original function; Perhaps the damage that survives if this is not fully repaired will be shown under the right conditions (e.g. cell cancer, etc.), but if the cell does not have this repair function, it will not be able to cope with the frequent events of DNA damage and will not survive. Therefore, the study of DNA repair is also an important aspect of exploring life, and is closely related to military medicine, oncology and so on. Cells can have different repair reactions to different DNA damage.(i) reply repair This is a simpler way to repair, generally can repair DNA as is.. 1. Light repair This is the first DNA repair method found. Repair is done by DNA photolyase (photolyase) in bacteria, which specifically identifies and binds to the
nucleic acid
chain of adjacent niacin co-price binding, which does not require light; Similar repair enzymes were later found to be widely found in plants and animals, as well as in human cells.. 2. Single-stranded re-connecting DNA single-stranded fractures are common injuries, some of which can be completely repaired with the participation of DNA connective enzymes (ligase). This enzyme is common in all kinds of biological cells, and the repair reaction is easy to carry out. But the double-stranded break is almost impossible to repair.16-21 Removal and repair of damaged DNA 3. The direct insertion of the base to the DNA strands results in the loss of radon-free bits, which can be identified and combined by DNA-insertase, Under the conditions of the presence of K-plus, catalytic free-fallon or deoxygen nucleosides are inserted to generate glycoside bonds, and the catalytic inserted base is highly exclusive and strictly paired with the base on the other chain, allowing DNA to fully recover.. 4. Alkyl metastasis An O6 methyl ostrich methyl
transferase
is found in cells that directly transfers methyl from the DNA chain to the cysteine residue of
protein
to repair damaged DNA. The repair capacity of this enzyme is not very strong, but it can induce the repair activity of this enzyme under the action of low-dose alkylation agent.(ii) excision repair is the most common way to repair DNA damage, which can play a repair role in a variety of DNA damage, including base shedding formation of baseless base points, niacin dimer, base alkylation, single-stranded fracture, etc. This type of repair is common in various biological cells and is the main DNA repair mechanism for human cells. The repair process requires a series of effects of a variety of enzymes, the basic steps shown in Figure 16?1, (1) first by the nuclease to identify the DNA damage site, in the damage site of 5' side cut phosphate phosphate bonds. Different DNA damage requires different special nucleic acid endoenzymes to identify and cut. (2) DNA fragments → be removed by 5' and 3' nucleic acid excision enzymes. (3) Under the catalysis of DNA polymerase, the DNA chain is used as a template with a complete complementary chain, and the DNA chain is pressed in the direction of 5'→3' to fill the removed gap. (4) The newly synthesized DNA fragments are connected to the original DNA strands by DNA connectivity enzymes. The repair, which was done in this way, restores the DNA to its original structure.Figure 16-22 Recombination Repair after DNA damage(iii) Recombination RepairThe above-mentioned excision repair is done after the removal of the injury paragraph is based on the original correct complementary chain as a template for the synthesis of new paragraphs. However, in some cases there are no complementary strands that can be used directly, such as DNA damage while DNA replication is taking place, when the two strands of DNA are separated and can be repaired using the DNA recombination method shown in Figure 16?2: (1) When the damaged DNA strands are copied, the resulting child DNA has a gap in the corresponding part of the damage. (2) Another maternal chain DNA is recombined and exchanged with the gaping subchain DNA, filling the corresponding fragments on the parent chain DNA at the subchain gap, while the female chain DNA is in the gap. (3) Using another subchain DNA as a template, a new DNA fragment is catalyzed by DNA polymerase to fill the gap in the female strand DNA, and finally connected by DNA connecting enzymes to complete the repair.recombination repair can not completely remove the damage, the damaged DNA paragraph is still retained in the progenito DNA strands, but the recombinant repair of the synthetic DNA molecules are not damaged, but after repeated replication, the damage is "diluted", only one cell in the child cells is with damaged DNA.(IV) SOS repair"SOS" is an internationally used emergency call for help. SOS repair refers to a DNA repair method induced when DNA is severely damaged and the cell is in a critical state, and the result of the repair is only to maintain the integrity of the
gene
group and improve the cell generation rate, but leaves more errors, so it is also known as error tendency repair, so that the cell has a higher
mutation
rate.
16-23 SOS Fix . As shown in Figure 16-23, when the damage of the two strands of DNA is adjacent, the damage can not be removed to repair or recombine repair, then under the role of the nucleic acid incision enzyme, excision enzyme caused by the DNA chain vacancy, and then by the injury induced by a set of special DNA polymerase OS repair enzymes, catalytic vacancy site DNA synthesis, when the added
nucleotides
is almost random, but still maintain the integrity of the DNA double strands. But this repair gives cells a high mutation rate.It should be said that little is known about the type of response to DNA repair in eunic cells, the enzymes involved in repair, and the mechanisms of repair, but DNA damage repair is closely related to mutations, lifespan, aging, tumor occurrence, radiation effects, and the role of certain poisons. More than 4,000 human genetic diseases have been identified, many of them related to DNA repair defects in cells that show increased sensitivity to radiation and carcinogens. For example, colorative dry skin disease (xeroderma pigmentosum) is the first to be found DNA repair defect genetic disease, the patient's skin and eyes are very sensitive to sunlight, especially ultraviolet light, the skin of the body's violent light
dry



..