Basic excision repair

DNA repair is the elimination of covalent DNA modifications and the correction of base mismatches. There are six basic repair categories direct repair, base excision repair, nucleotide excision repair, recombination, cross-link repair, and mismatch repair. 

Figure 6 Nucleotide excision repair in (A) E. coli and (B) humans. There are five basic steps of nucieotide excision repair (1) damage recognition, (2) dual incisions, (3) release of the excised oligomer, (4) repair synthesis to fill in the gap, and (5) ligation.

Fig. 1. Schematic illustration of nucleotide excision repair in prokaryotes and eukaryotes. The basic steps are conserved damage recognition and dual incisions to excise DNA damage, helicase activity to displace excised oligomer and repair factors, and resynthesis/ligation to restore the integrity of the DNA molecule. (See Color Insert.)...

In E. coli, dual incisions are accomphshed by three proteins (UvrA, UvrB, and UvrC), and in humans 15 polypeptides in six repair factors carry out the same task. The properties of the E. coU and human excision repair factors are summarized in Tables I and II, respectively. Of significance, in contrast to all other repair systems, the prokaryotic and eukaryotic excision repair factors are evolutionarily not related and show no sequence homology to one another. However, the basic strategies for the prokaryotic and eukaryotic excision nucleases are similar. First, damage is recognized by an ATP-independent mechanism to form an unstable DNA-protein complex. Then this complex is converted to a stable preincision form by ATPase subunits that hydrolyze ATP and unwind the... 

The NER pathway is also present in prokaryotic organisms, such as Escherichia coli, as well as in eukaryotes from yeast to mammals. Many of the basic steps of NER are evolutionarily conserved, including damage recognition and dual incisions to excise the lesion, helicase activity to displace the excised oligonucleotide and repair factors, and synthesis and ligation enzymes to seal the nick [19, 20], Nevertheless, the biochemical features in prokaryotes and eukaryotes are distinct. 

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