, 2013). .
However, oxidative damage affects replication and transcription of mitochondrial DNA (mtDNA) and results in a reduction of mt function, which renders more ROS and further damage to mtDNA (Cui et al., 2011). It is important to note that ageing is a very complex process, which probably cannot be explained fully by current theories. Nevertheless, the purpose of this paper is to describe the proximal relationship of mitochondria and ROS and to explain how mtDNA mutations and ROS add to the development of ageing and diseases such as Alzheimer's. .
The Effects of Free Radicals on Biomolecules.
Free radicals are responsible for most of the oxidized and cross-linked proteins. This is due to the fact that the amino acid residues of proteins are especially sensitive to oxidative attack. This, in turn, results in close to exponential damage in growth of cells and tissues as time passes and organisms age (Wickens, 2001). It has been reported that oxidized proteins in an old animal may represent up to 50% of total protein in cells. In principle, this could be the main explanation why the enzymatic function of various proteins is reduced as organisms become older (Stadtman, 2004). .
Another type of molecule that is highly susceptible to ROS diminishing effects is DNA. The reaction of an oxygen radical with DNA can result in loss of bases, or cause a strand breakage, with a likelihood of a harmful or deadly outcome (Klaunig et al., 2010). What is more, the deleterious effect caused by free radicals appears to be significant as shown by approximations of 10,000 oxidative hits to DNA in an average human cell/day (Lu et al. 2001). Randerath et al., 1995 state that even though much of the damage is removed by repair systems, oxidative DNA lesions nevertheless accumulate with age, and this probably plays an important role in the aging process (cited by Wickens, 2001).
ROS Exerted Effect on Mitochondria.