Jenis kerusakan DNA yang umum: - Modifikasi basa
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Jenis kerusakan DNA yang umum: - Modifikasi basa: methilasi dan oksidasi nukleotide oleh ROS (reactive oxygen species) yang diproduksi oleh metabolisme normal ataupun lingkungan yang tidak mendukung seperti merokok, bahan kimia oksidan, atau radiasi yang mengionisasi - Mispair (salah berpasangan): kesalahan dalam sintesis DNA - Cross-linked nucleotides: ikatan kovalen inter dan intra untai - Kerusakan double-stranded DNA: (A) deamination adenine, cytosine, dan guanine (B) depurination (kehilangan basa purin) karena lepasnya ikatan antara basa purin dan deoksiribosa, menyisakan bagian apurinic (AP) pada DNA. dGMP = deoxyguanosine monophosphate. Perbaikan DNA: - - BER: base excision repair– mengganti basa yang rusak pada kode DNA satu untai NER: nucleotide excision repair– mengganti rangkaian basa jika satu atau lebih basa rusak NHEJ: non-homologous end joining–memperbaiki kerusakan dua untai pada DNA double-helix HR: homologous repair- memperbaiki kerusakan dua untai di dalam dan antara cross link untai DNA MMR: mismatch repair – memperbaiki kesalahan berpasangan dalam urutan basa di DNA DNA repair: base excision repair pathway (BER) / basis perbaikan eksisi Salah satu kerusakan yang dapat terjadi adalah kerusakan basa atau bagian dari untai tunggal DNA. Kerusakan ini tidak dapat diperbaiki secara langsung oleh ligase tanpa diproses lebih lanjut. Kerusakan residu nukleotida ini dapat diperbaiki melalui proses BER. In this pathway damaged bases are removed by one of at least 10 DNA glycosylases, the resulting apurinic/ apyrimidinic (AP) sites are processed first by the Ape1 AP endonuclease, leaving a 5_ deoxyribose phosphate; then by anAPlyase activity leaving a 3_-elimination product. SSBs are then filled in by a DNApolymerase, such as theTLSpolymerase beta (Pol_), either with a single nucleotide or with a longer repair patch. This is then followed by ligation (a) salah satu DNA glycosylase mengenali basa yang rusak dan memisahkan antara basa dan deoksiribosa (b) AP endonuclease melepas phosphodiester sekitar AP. (c) DNA polymerase I mensitesis perbaikan dari bagian 3’ OH, menghilangkan bagian dari untai yang rusak (dengan menggunakan 5’3’ exonuclease) dan menggantinya dengan DNA yang tidak rusak (d) DNA ligase menyambung untai ganda. (a) DNA glycosylase recognizes a damaged base and cleaves between the base and deoxyribose in the backbone. (b) An AP endonuclease cleaves the phosphodiester backbone near the AP site. (c) DNA polymerase I initiates repair synthesis from the free 3’ OH at the nick, removing a portion of the damaged strand (with its 5’3’ exonuclease activity) and replacing it with undamaged DNA. (d) The nick remaining after DNA polymerase I has dissociated is sealed by DNA ligase. Excision Repair Although direct repair is an efficient way of dealing with particular types of DNA damage, excision repair is a more general means of repairing a wide variety of chemical alterations to DNA. Consequently, the various types of excision repair are the most important DNA repair mechanisms in both prokaryotic and eukaryotic cells. In excision repair, the damaged DNA is recognized and removed, either as free bases or as nucleotides. The resulting gap is then filled in by synthesis of a new DNA strand, using the undamaged complementary strand as a template. Three types of excision repair—baseexcision repair, nucleotide-excision repair, and mismatch repair—enable cells to cope with a variety of different kinds of DNA damage. The repair of uracil-containing DNA is a good example of base-excision repair, in which single damaged bases are recognized and removed from the DNA molecule (Figure 5.23). Uracil can arise in DNA by two mechanisms: (1) Uracil (as dUTP [deoxyuridine triphosphate]) is occasionally incorporated in place of thymine during DNA synthesis, and (2) uracil can be formed in DNA by the deamination of cytosine (see Figure 5.19A). The second mechanism is of much greater biological significance because it alters the normal pattern of complementary base pairing and thus represents a mutagenic event. The excision of uracil in DNA is catalyzed by DNA glycosylase, an enzyme that cleaves the bond linking the base (uracil) to the deoxyribose of the DNA backbone. This reaction yields free uracil and an apyrimidinic site—a sugar with no base attached. DNA glycosylases also recognize and remove other abnormal bases, including hypoxanthine formed by the deamination of adenine, pyrimidine dimers, alkylated purines other than O6-alkylguanine, and bases damaged by oxidation or ionizing radiation. Figure 5.23 Base-excision repair. In this example, uracil (U) has been formed by deamination of cytosine (C) and is therefore opposite a guanine (G) in the complementary strand of DNA. The bond between uracil and the deoxyribose is cleaved by a DNA glycosylase, leaving (more...) The result of DNA glycosylase action is the formation of an apyridiminic or apurinic site (generally called an AP site) in DNA. Similar AP sites are formed as the result of the spontaneous loss of purine bases (see Figure 5.19B), which occurs at a significant rate under normal cellular conditions. For example, each cell in the human body is estimated to lose several thousand purine bases daily. These sites are repaired by AP endonuclease, which cleaves adjacent to the AP site (see Figure 5.23). The remaining deoxyribose moiety is then removed, and the resulting single-base gap is filled by DNA polymerase and ligase. Whereas DNA glycosylases recognize only specific forms of damaged bases, other excision repair systems recognize a wide variety of damaged bases that distort the DNA molecule, including UV-induced pyrimidine dimers and bulky groups added to DNA bases as a result of the reaction of many carcinogens with DNA (see Figure 5.20C). This widespread form of DNA repair is known as nucleotide-excision repair, because the damaged bases (e.g., a thyminedimer) are removed as part of an oligonucleotide containing the lesion SUMBER: http://jco.ascopubs.org/content/24/23/3799.full.pdf http://www.ncbi.nlm.nih.gov/books/NBK9900/