DNA repair

Through mechanisms of DNA repair cells can eliminate changes in their DNA structure. Such damage in the DNA can be caused spontaneously during DNA replication or by exposure to mutagenic agents, extreme heat or ionizing radiation.

DNA damage can lead to DNA replication for mitosis important chromosome regions are cleaved by double-strand breaks done incorrectly, proteins no longer synthesized or wrong or.

Bring the complex repair mechanisms of the cell does not work, so accumulate in growing and quiescent somatic cells so many errors that the normal cell functions are disturbed. In a germ cell, the daughter cells would no longer be viable, leading to inactivation of the cell line: the cell or the second or third succeeding generation loses its ability to divide and dies. In the course of cell cycle control proteins can control a cell or its DNA seen as defective and initiate a cycle arrest ( G0 phase ) or programmed cell death ( apoptosis).

Individual DNA repair enzymes have since followed with PAL microscopy in their work in a bacterium and the corresponding parameters are determined. For example, take into E. coli base excision repair a good two seconds.

• 3.1 single-strand damage repair 3.1.1 base excision repair (BER )
• 3.1.2 nucleotide excision repair (NER )
• 3.1.3 Proofreading by DNA polymerase ( base mismatch repair, mismatch repair )

Causes DNA damage

Possible causes include metabolic processes, chemical substances or ionizing radiation, such as UV radiation, electrons or protons.

Both DNA damage and replication errors and other cellular processes can lead to mutations. As for mutations own repair mechanisms exist, they can be considered here is similar to DNA damage.

Metabolic processes

A cell, a system at steady state. You will continually molecules, processes it, synthesized materials needed, and are in turn certain substances to the environment. During normal cellular metabolism, reactive oxygen species (ROS, including oxygen free radicals ) are formed, which cause a significant amount of oxidative damage. Most commonly, this single-strand breaks and base damage, less than 0.5% are double-stranded breaks, which are also distributed relatively uniformly throughout the DNA. The probability endogenously induced damage clusters and so - difficult to repairing - frequent lesions (complex lesions ), as otherwise ionizing radiation occur due to the non-homogeneous energy output is very low. Too high proton density and / or temperature can trigger depurinations or Depyrimidierung.

UV - radiation

By UV radiation, it may direct changes (mutations ) of the DNA are, in particular wherein said UV -B radiation is absorbed. Single-stranded DNA shows its absorption maximum at 280 nm both UV- B and UV-A can indirectly damage DNA by the formation of reactive oxygen radicals that cause the formation of oxidative DNA lesions, which in turn lead to mutations. These are probably responsible for the formation of the UV-A -induced tumors.

Types of DNA damage

• Base modifications Pyrimidine dimers usually 6-4 photoproducts (6- 4PPS ) cyclobutane pyrimidine dimers or ( CPDs )
• Oxidized bases such as 8 -oxo -7 ,8- Dihydroguanin (8- oxoG ) and 8 -oxo -7 ,8- Dihydroadenin (8- oxoA )
• Alkylated bases ( for example, base methylations )
• Other bulky lesions ( bulky base changes )

Treatment with a Gray X-rays generated per cell is about

Repair of DNA damage

For the different types of DNA damage are different, specialized repair mechanisms. Several mechanisms have specialized, for example, in the repair of damage in DNA single-strand, others on the repair of DNA double- strand breaks.

There are also differences between prokaryotes and eukaryotes, which have different DNA polymerases.

Single-strand damage repair

Base excision repair (BER )

The base excision repair defects in the form of oxidized, alkylated or deaminierter individual bases are fixed. Thereby damage to the bases to be detected by a specific DNA - glycosylase and each cut (excision ). This travels along the minor groove and folds the individual bases in their catalytic center. A damaged base is removed from the DNA - glycosylase, and then is introduced through an AP endonuclease ( apurinic / apyrimidinic endonuclease ), a nick in the sugar - phosphate backbone. DNA polymerase synthesizes a function of the complementary base on the error-free strand, the correct base. There are here two variants of BER: short patch repair (a single base is replaced ) and long patch repair (2-20 nucleotides are replaced). In humans, DNA polymerase β (Pol β ), the main responsible polymerase. A DNA ligase, the new base in the DNA strand, whereby the error is corrected.

Nucleotide excision repair (NER )

In contrast to the base excision repair by the nucleotide excision repair (NER ) are mainly so-called "bulky lesions" detected, that is, places that generate a kind of " hump" in the DNA molecule and thereby disrupt the helical structure. It can be either to pyrimidine dimers and 6,4 photoproducts generated by UV radiation.

The nucleotide excision divided into lesion recognition, incision, excision of a 25-30 base long DNA segment, the de novo synthesis of this section and the subsequent ligation.

NER is found in both pro-and eukaryotes, however, the mechanisms and enzymes involved differ. Whereas in prokaryotes such as Escherichia coli Uvr proteins and DNA polymerase I are involved, there are proteins in eukaryotes, have their name from the inherited diseases xeroderma pigmentosum and Cockayne's syndrome, for example, XPA and CSA. Among the participating polymerases include DNA polymerases δ, ε and / or κ.

In eukaryotes, there are two routes of nucleotide. For a Global genome repair ( PFR ), which removes defects in transcriptionally inactive regions of the DNA and the other, the so-called Transcription Coupled Repair ( TCR), which corrects the current losses to be transcribed DNA. These two forms differ only in the damage detection. In GGR, the DNA lesion is recognized by the protein complex XPC/HHR23B. In contrast, this complex does not play a role in TCR. In the TCR, it is important that the blocked by the damage to RNA polymerase II is removed, so as to allow the proteins access to the TCR DNA damage. This removal of the RNA polymerase II is made possible by CSA and CSB. The remaining steps are identical for both repair pathways. XPA and RPA serve to further DNA damage recognition and direct the helicases XPB and XPD to the lesion, which unwind the DNA in the immediate vicinity of the injury. Endonucleases XPG and XPF - ERCC1 cut the DNA strand at two positions 3 'and 5' ( dual incision ), so that a 30 base oligonucleotide is released comprehensive containing the injury. Now the polymerization of the missing section of DNA by DNA polymerase and other factors follows. Finally, the ligation of the synthesized portion of DNA ligase I and flap endonuclease 1 or ligase III XRCC1 complex occurs.

Mutations that the CSA / B affect genes that lead to the formation of the disease Cockayne syndrome. Mutations on the XPA - XPG family lead to the development of the disease xeroderma pigmentosum. In xeroderma pigmentosum skin cancer risk is increased, which indicates the importance of a functioning DNA repair following UV - irradiation.

Proofreading by DNA polymerase ( base mismatch repair, mismatch repair )

The charge for copying of the DNA binding protein DNA polymerase has the ability, the new DNA strand during synthesis or to check and comparing it with the original strand. However, this function is inaccurate and without further control by DNA mismatch repair proteins, the number of spontaneous mutations would increase to 1000-fold. The bacterium Escherichia coli can also distinguish the resulting replication defective daughter strand of the DNA depreciated based on the methylation status. The new strand is slightly later than the female ( the parent strand ) is methylated at the adenine residues of the sequence GATC. A defect in the Mismatchreparatur causes a form of colon cancer: Hereditary non- polyposis colorectal cancer (HNPCC, hereditary nonpolyposis colorectal cancer ).

Photoreactivation

Photolyases are able to dissolve formed by ultraviolet radiation in the DNA cyclobutane rings and the (6-4) photoproduct. They have a so-called antenna complex with which they absorb blue or ultraviolet light and by using this energy from the cofactor FAD two electrons transferred to the DNA damage -bound in the active site of the enzyme. The summit splits in the sequence. The photolyase provides so without cutting out and pasting bases the native structure of DNA restored. To date, Photolyases could be detected in many organisms from prokaryotes about mushrooms and plants to marsupials. Despite their undoubted beneficial properties for these organisms the photolyases have gone several times lost in the course of evolution. Also the higher mammals to which man is one, have no repair active variants of these proteins more. The reasons for this are not yet fully understood.

Repair of double-strand breaks ( recombination repair )

In a double-strand break, there is the possibility that the sister chromosome transmits the missing information for this DNA segment.

• Not homolog Endjoining
• Homologous recombination
• Single beach annealing

A disturbance of these repair systems often clinically manifests as chromosome breakage syndrome, such as the Nijmegen breakage syndrome.