Oxidative attack on DNA generates both altered bases and damaged sugars that undergo fragmentation and lead to DNA strand breaks. DSB are often a consequence of ROS attack on DNA and are a very severe type of DNA damage that can be rapidly repaired in cells by two mechanisms, HR and NHEJ. While HR creates covalent linkages between DNA in regions of highly similar or identical sequence, the DNA ends of severed chains are joined directly in NHEJ, and deletions or insertions may then occur (Puchta 2005). It has been suggested that higher plants possess a more efficient repair of DSB than mammalian cells due to a higher tolerance for ionizing radiation (Yokota et al. 2005). Again, Arabidopsis has been the main plant studied. AtRad50 and AtMre11, which are involved in homologous and nonhomologous DNA repair processes, as well as in DNA damage sensing and signaling, have been isolated and investigated (Gallego et al. 2001; Daoudal-Cotterell et al. 2002; Puizina et al. 2004). In rice and in Arabidopsis, the HR proteins AtRad51, OsRad51, AtRadA, and OsRadA showed a high degree of homology with mammalian Rad51 and RadA/Sms, a eubacterial homolog of RecA. AtKu70, AtKu80, DNA ligase IV, and AtXRCC4 are all involved in NHEJ and have been isolated and analyzed in plants (West et al. 2002). Interestingly two NER-related genes, AtXPF and AtERCC1, have also been shown to be involved in plant DSB repair (Hefner et al. 2003; Dubest et al. 2004). As demonstrated by Cuypers et al. (1999), each heavy metal induces different oxidative stress mechanisms and different responses to the exposure that are determined by the chemical properties of each metal. As a transition element, Ni (nickel) can participate in the Haber-Weiss reactions, generating free radicals directly. Metals such as Cd and aluminum (Al) have only one oxidation state (2+ and 3+, respectively) and are not transition elements. Cd is often treated as one, at least with regard to the criterion of having an unpaired electron in its valence orbital. These properties influence the mechanisms by which each metal produces oxidative stress.
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