Fig. 1.

Schematic representation of balance between ROS production and antioxidant defense. ROS can be produced by endo- or exogenous sources. The oxidative damage is prevented by enzymatic or non-enzymatic proteins and low molecular weight antioxidants. Under a physiological cellular state, the level of ROS is stable in a dynamic equilibrium balanced by oxidants and antioxidants and the physiological levels of ROS modulate the signalling pathways. When the levels of oxidants and antioxidants are imbalanced, the ROS increase and produce different types of DNA damage ranging from base modifications and transversions to genomic instability.