Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

DNA Repair
Excision Repair

During excision repair, damaged segments are first removed from the DNA strand (the term "excision repair" comes from the English word excision), after which the original Introduction/20.html">DNA Structure is restored. There are two types of excision repair. Option 1.

Special Enzymes called glycosylases play a crucial role in this type of excision repair. Glycosylases can recognize damaged bases and cleave the glycosidic bonds between the modified nitrogenous base and deoxyribose. Their action results in The formation of AP sites. At the next stage, the AP site is recognized by an AP endonuclease, which makes a break in the DNA strand. Next, phosphodiesterase comes into play. This enzyme removes the sugar-phosphate group from which the glycosylase has removed the base. DNA polymerase inserts the missing nucleotide, complementary to the nitrogenous base on the opposite strand, into the resulting single-nucleotide gap in the DNA strand. Finally, DNA ligase seals the nick in the DNA strand (Fig. 3.6). As a result, the damage is repaired.

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Fig. 3.6. DNA Repair involving glycosylase

Option 2.

This variant of excision repair involves the removal of extended segments of DNA. The process can be divided into four stages (Fig. 3.7):

1) recognition of the damaged DNA region;

2) introduction of single-stranded breaks in the DNA on both sides of the damaged region and its removal;

3) Filling of the resulting gap by DNA polymerase;

4) ligation of the single-stranded DNA break.

Fig. 3.7. Excision repair involving the removal of extended DNA segments